How to Calculate Attic Vent Area Needed When Adding Soffit Vents

Can a 8″x 16″ be install between each rafter and both side of roof.

Milt,
The rule of thumb on the amount of vent space needed is approximately 1 sq. ft. of vent space for every 150 sq. ft. of attic area. So if your attic is 1,500 sq. ft., you divide that by 150 to get 10 sq. ft. of vent area. Ideally, half the vents should be located in the soffit at the bottom of the roof and half in gable or ridge vents near the top. Divide the vent area by the opening in each vent to come up with the number of vents needed. For example a vent with a 6”x12” opening would equal 0.5 sq. ft. Space soffit vents evenly around the bottom of the low sides of your roof.

I am ready to do this plus add styrofoam ventilation channels. I am wondering if the ventilation channels go only where the soffit vents are or must I do all the roof rafters with the channel vents?

My house is 22×60 the attic is about 4ft from attic floor to the peak not a walk up attic i want to install 4in soffit round holes how many do i need. thanks

Bob,
Using the formula above, you would need 8.8 square feet (or 1,267 square inches) of vent space in your attic. Since each 4” diameter hole would have an area of 12.56 square inches (3.14 x r²), you would divide 1,267 by 12.56 to come up with 101 vents. That’s a lot of holes to cut! If half the vent space is near the top of the roof in ridge or gable vents, that would reduce the number of soffit vents at the bottom to 50.

Does it matter whether the vent holes face
away from the house into the wind or should they face away from the wind toward the side
of the house so they dont clog?

I have a 28 X 24 garage with 24″ centers of rafters.

I went with a plywood eave. The ceiling is insulated, and it is cold up here, 25 degrees f for a few months of the year.

I was going to put an 8″ x 16″ soffit vent on every second rafter in the front, starting with the outer most ‘space’, between rafter 1 and 2. On the rear I was going every second one, but alternate by starting between rafter 2 and 3.

I thought that would give better ventilation that mirroring front to back.

Also, do I need them in the ends of the garage, where the peak is?

Thanks.

Just a quick comment in regards to Ben Erikson’s response to Bob dated December 27, 2007. Apparently, Mr. Erikson did not double-check his math. According to the formula, Bob’s attic area would be 1320 sq ft not 1267 sq ft. Dividing 1320 by 150 gives a vent area of approximately 9 sq ft which would interpert into 9 4″ soffit vents or 10 if you like even numbers.

Chris, I think you’re confusing sq feet and sq inches. They’re not the same thing. For instance you say he needs a vent area of 9 sq ft and that 9 four inch soffit vents would do. But 9 four inch holes does not equal 9 square feet.

My mistake and thanks for the clarification.

My garage floor shows a crack in it. It appears to come from the outside wall about half way in the garage. Also, lots of water comes off the trucks wheels from melted snow or just rain water. What is the best thing to do in this case?

We live in a 50’s style ranch and over the years the soffit vents have been painted over while painting the overhang. We would like to replace or restore these, but haven’t been able to find the size we need to replace them. Any ideas would be appreciated.

Rick,

It doesn’t have to be exact. Just get one that is larger then your existing as they just nail or screw over the cut out opening.

Ben,
I used your formula to do my house (38×60), and had planned on using soffit vents with three 2″ round holes on them-until I realized that I would need a crazy amount of holes. So here’s my question-if i modify the hole to make it bigger, is there any such thing as too much ventilation when it comes to attics?

Paul,
While you can have too little venting in your attic, you can’t really have too much. Keep in mind that it’s best if half the venting is in the soffit at the bottom of the roof and the other half at the top in gable or ridge vents to allow outside air to flow through the attic. Depending on local building codes, the venting may be reduced to 1 sq. ft. of vents for every 300 sq. ft. of attic area if the house has an adequate vapor barrier and the venting is balanced between the top and bottom of the attic.

I remember hearing or reading somewhere that soffit vents should not be installed directly above windows (and doors also?), though the picture here shows one squarely above a window 🙂 I’m adding soffit vents to my home, and the ones I’ve got are currently evenly placed, except for above windows. Any truth behind this?

Thanks!

Hi Adam,
I see no problem with locating a soffit vent above a window or door. Many houses have continuous soffit vents which go all the way across the house. The only reason I could think of not to put a vent over a window would be in the unlikely instance that a downdraft caused hot air to come out of the vent and in an open window. However, since hot air rises, under most circumstances the soffit vent would be drawing cool air in rather than letting hot air out.

Danny:

I have a run the numbers on my vents and I am a little short on the total vent opening space, I think. I have about 3,400′ of floor space in the attic and 26 soffit vents that are about 8″ X 16″ and maybe 60% open (the efficiency is eaten up by loovers and screens. Then on the roof I have six wind turbins that are about 12″ (when i measure the inside diameter they seem like they don’t quite measure 12″ but are more than 10″. If I did the numbers right I have enough soffit vent but need a little more venting higher up. I live in Texas and when the outside temp is 100F the attic temp is arond 135F. I had read once that if the attic were properly ventilated the attic temp should be around 105 if the outside temp is 95. What are your comments on this? Do I need more vent space and if so, where? Is it really reasonable to expect to get the attic temp down to 105 with an outside temp of 95 with natural ventilation?

Would it be a problem to install soffit vents near or over an A/C? Also, when I install my bath vent out through the soffits, will that interfere with the attic soffit vents? Should I vent thebath out the roof? I have a roof with a ridge vent and gable vents, but no soffit vents. I can install some easily on one side, but the other side has the problems outlined above. This side also has half the soffit length because of an addition with a torch-down roof. Maybe I can get some venting through the torch-down roof soffit by cutting a hole in the roof to allow the air to flow.

Thanks for your help

Gary,
Since AC units expel quite a bit of heat, putting a soffit vent directly above one wouldn’t be the best place for it. Rather than taking up one of your existing soffit vents with a bath vent, perhaps you could cut a separate hole in the soffit for it, or run it out the gable if it’s not too far away.

where can i purchase that vent cover? i have 3 2″ holes inline every 2 feet all around the house – most of the screens are missing & need to cover them. thank you.

When installing soffit vents, do you install them with the vents facing the house or with the vents facing away from the house?

fyi regarding direction of vents

ATTIC VENTILATION FOR HOMES
B. R. Stewart
Agricultural engineer-environmental control ~ Agricultural Extension Service ~ Texas A&M University

http://www.factsfacts.com/MyHomeRepair/ventilation.htm

per http://www.diynetwork.com/diy/he_home_insulation/article/0,,diy_13895_2275822,00.html

vents point toward the house

I have a house where the second floor is a loft. There are triangular crawlspaces to the sides of the room and a triangular attic space above the ceiling. There is an access door to the northern crawlspace and no access to the attic space.

The crawlspaces to the sides are not connected to the attic, but there are soffit vents and ridge vents. Since the crawlspace is not connected to the attic there can be no airflow from the soffit vents to the ridge vents.

The problem is that in the winter we often get strong, cold north winds. This sends lots of cold air into the crawlspace via the soffit vents, and then directly into the house via the crawlspace access door and most of the electrical outlets on that side of the house. Sometimes large areas of the loft will be 25 degrees colder than the first floor, and frequently the plumbing in the loft bathroom freezes (the cold air seems to run under the tile floor (burr!) and chills the sink plumbing. Have to put hot water in the tub and wait for it all to thaw out).

So, what’s the correct way to do this? Should the crawlspace have gable vents? This would allow air to flow up the soffit vents and then out the ends through the gable vents. I’d still want to seal the leaks into the living spaces, but hopefully the added vents would reduce the air pressure in the crawlspace so air wouldn’t be forced into the house.

Can there be too much soffit open area ventilation

We are redoing our soffits and cutting out the hard board above the soffits. Is this ok?

Is there an answer to B. hanson 11/8/07 re: how many baffle vents per soffit?

Your calc’s for # of required soffits is very helpful. Wondering if baffles are needed between all rafters or only those rafters over soffits.

Install soffits on ends and in middle for best results? Ie: need or 3 for 13 rafters.

Thank you.

Kevin,
Baffles are only needed in the rafter/joist spaces where soffit vents are located. If you have continuous soffit vents, install baffles in enough (or all) the spaces to give you the needed square footage of vent area. Space the soffit vents out so they divide the attic area into equal areas. In your case, I would put one in the center, then come in a couple of rafters on each end for the other two.

This home was built around 1929 near Schenectady, NY. Uninsulated & no soffit vents. The roof rafters are 24″ on center 5.5″ deep. i want to finish the attic space, but insulate rafters and vent soffits in the same project. Do I need to channel soffit venting to the ridge venting(pre-existing)before insulating or not even channel the venting? Should I install vapor barrier before finishing attic ceilng? Is styrofoam insulation available in higher r-rating density than fiberglass? Thanks for any advice on my first major upgrade!
dan s

Dan,
You calculate, for lack of a better word, gross area of needed vents. But manufacturers report Net Free Area on their vents (the actual amount of air space in the vent, not it’s overall dimension). My problem – I have ridge vents rated in Net Free Area per lineal foot; how do I equate that to a gross vent area as used in your example above, so I can see if my total ridge vent area is adequate? Thanks.

I have a 2500 sq-ft attic floor, ten 8″ x 16″ soffit vents, 2 wind turbines and radiant barrier roof decking. 7 of the soffit vents are on the north side of the house, only 3 are on the south (hotter) side and there are none on the west (front) and east (backyard) sides. How do these impact the rule-of-thumb of 1 sq-ft vent area per 150 sq-ft of attic floor? Following the formula, it seems like I need at least 6 more soffit vents. Thank you!

Hi Bing,
While it wouldn’t hurt to add more soffit vents on the east and west sides of your house to give a more even flow of air through the attic, you already have about half the amount of venting needed in soffit vents, so where you should really put your efforts is in more venting near the peak of your attic roof with either more turbines, gable vents, or a vented ridge.

I have an attic area of 1176sq.ft. Based on the formula I need 7.84sq.ft. of total vent area. My current ventilation is 2.17sq.ft. from gables and 6.66sq.ft. from ridge vent. Do I need to add soffitt vents? If so, how many sq,ft.?

I LIVE IN SOUTH TX;RGV.MY HOUSE WAS BUILT IN 1929 WITH FLAT A ROOF AND NO CRAWL SPACE.I AM INSTALLING NEW WINDOWS AND SIDING.THE SOFFATS ALL AROUND MY HOUSE ARE 4 FT.DO I NEED TO HAVE VENT IN THE SOFFATS TO PREVENT MOLD AND MOISTURE .

My colonial (built in 1930’s) has NO soffit vents and only a continuous ridge vent 50′ in length. The attic has two finished rooms on both sides of a center-hall staircase. The front of the house faces directly South and both rooms get extremely hot in the summer. Since I am not sure if there are baffles located between the roof and the interior walls and dont know how far each joist is located from one another, how do I pick the proper location to install my soffit vents? The way I see it, I have 1000sq ft of attic space so need (~7sq ft of total vent area/2) 3.5sq ft of Soffit vent space. Id like to use 6×12″ soffits… Any advice?

sorry..forgot one part..should I locate all soffit vents on the south facing side (disregarding the North face) or should I do both equally?

Ben, I really appreciate it. Should I be concerned that I dont have Baffles?

Sandy,
Baffles are usually installed before insulation is blown in to prevent it from clogging up the vents. Since insulation doesn’t move around once it’s in place, if you can see or feel a clear path for the air to vent up between the joists/rafters, it should be fine.

When figuring out soffit vent requirements for an attic–and calculating total vent area needed–does height of the attic come into play?

Hi Ben,
I have an older ranch style home which I recently had re-roofed and a ridgevent installed.I would like to install soffit venting to take full advantage of the ridge vent.My soffits are aluminum covered with the wood soffit under that.I drilled a small pilot hole (2″)and find that there is a space of about an inch or so between the aluminum and wood soffits.Must I be sure to get deep vents that push through both the aluminium and wood soffit and into the attic?Or is it ok to just cut through both the aluminum and wood and only place a surface vent over the aluminum?Thank you for your help.

I have 1000 square feet of attic space and have 50 soffit vents. 20 in front, 20 in back and 10 on the attached garage. Seems like plenty but the roofer that’s quoting my job said it’s not because they are the perforated type and don’t allow much air to enter. Other roofers said it’s plenty because the soffits aren’t air tight and draw air through the holes and surrounding gaps.

Any idea if the perforated type soffit vents are ok to use and if I have enough vents?

One of my attics has both a turbine and power fan, and seems to have the minimum needed soffit vents. The other attic has adequate soffit vents but only a turbine. I don’t really want to cut holes in my year old roof so I was considering adding a powered fan to exit thru the soffit. My idea was to run a couple lenths of clothes dryer duct to the top of the attic area, thru a fan, then out thru the soffit in an area where there are no intake vents. Does this sound workable?

I had soffit vents installed several years ago. Now, I’m at a point where the house needs a paint job. As I walked around my house inspecting things, I noticed that the openings in the soffit vents were pointed toward the house and the air it vented left a dirty area on the brick moulding below the eaves. Is there any reason I can’t just take those soffit vents off and turn them 180 degrees so that the openings of the vents point away from the house?

Why should soffit vent openings point toward the house? Is it because windblown rain could possibly get in the attic if vent openings are pointed away from the house. It would seem that you would get better air flow if the vent openings were pointed away from the house.

My home is a colonial built in the in the 1930’s. It was about 3 years ago. The roof is about 15 year old and will need to be replaced in the not too distant future. However, in the interim, we would like to address the ventilation issue. When the previous owner had the roof replaced, a ridge vent was installed and soffit vents installed. The problem is that the soffit vents do not have visible openings in the attic. The attic floor is covered with tongue and groove boards, part of the original construction. Cellulose insulation was added some time in the past as well. What is your suggestion for connecting the soffit vents to the attic area?

Ben,

I was thinking it might be possible that the air current might move up the house to the soffit then outwards? Then
again I have never seen an air current analysis on a roof.

I need to install soffit vents in our home; several years ago we had 12 inches of insulation blown into the attic. Will this complicate installation of the soffit vents? The attic is small and difficult to work in especially considering the blown in insulation, is it necessary to be in the attic in order to install the vents or could this all be done from outside?

I need air flow in my attic badly. I took off two soffit vents only to find batt insulation from the vent back up to the inside of the attic. I don’t see a path for air flow? I have about three feet of slopped ceilings in each room that mirror the roof slope and the insulation is between the roof and the room ceiling.
Can I pull out the insulation, put in baffles and then blow in insulation arouind the baffel? I am also putting Radiant barrier on the underside of the attic roof.

I have a ranch house 90×30. I installed a new roof last year with a ridge vent; took out the old powered roof vents (ugly!). I have two gable vents at opposite ends of the house; about 4sq/ft each. There is perforated vinyl soffit material running the full length of 90ft under the soffit on the north side of my house. This covers the soffit board which has two 2″ diameter holes drilled between each joist. There are no soffit vents on the south side of my house. Three questions; first, are the 2 inch holes enough ventilation or should I remove the perforated vinyl soffit material and cut larger holes? Second, should I remove the perforated product completely because it it prevents the passage of air and is not efficient enough? Third, must there be a equal amount of soffit venting on both sides of the house? Thank you for any help!

hi, 16 yr old house. 7 roof vents, soffit vents all around. notice shingles curling above 1 room. attic is very hot.. how do i check to see if my roof and soffit vents are plugged? have had strong wind storms in past, one time attic trap was sucked up into opening. could this have disturbed the insulation in attic enough to cause plugged vents.?

I have a seasonal unheated cottege with a vaulted ceiling-the roof pitch is @5in 12(10″eaves)-at present there is no venting at all. I want to know if 1 gable vent on the back wall as high up as I can install it would be enough for this 400 sq.ft. building built on blocks

When I did research on this I focused on net free area. I wonder why this article fails to mention net free. Based on your Danny’s example of using vent size alone, you will not let enough air in since the grills/screen take up space. An 6*12 soffit vent is not really 72 sq.” of ventilation but is only 27sq.” of net free area. This can make a big if you put in 10 vents but really need 30 vents. I am not a professional but would it makes more sense to me…

I hate to repeat someone else’s question, but I cant seem to find or access the answer from B. Hanson’s question on
November 8th, 2007 at 1:19 pm (AND I need the same answer)
I am ready to do this plus add styrofoam ventilation channels. I am wondering if the ventilation channels go only where the soffit vents are or must I do all the roof rafters with the channel vents?

Never mind – I see it now –

Kevin Says:
January 24th, 2009 at 2:42 pm
Is there an answer to B. hanson 11/8/07 re: how many baffle vents per soffit?

Your calc’s for # of required soffits is very helpful. Wondering if baffles are needed between all rafters or only those rafters over soffits.

Install soffits on ends and in middle for best results? Ie: need or 3 for 13 rafters.

Thank you.

Ben Erickson Says:
January 26th, 2009 at 9:38 am
Kevin,
Baffles are only needed in the rafter/joist spaces where soffit vents are located. If you have continuous soffit vents, install baffles in enough (or all) the spaces to give you the needed square footage of vent area. Space the soffit vents out so they divide the attic area into equal areas. In your case, I would put one in the center, then come in a couple of rafters on each end for the other two.

I have a 1700 sqft ranch, when the roof was redone aprox. 10 years ago, we had a continous ridge vent installed, the 3 foot deep plywood soffits were removed and a continous vented vinyl soffit was installed the entire length of bothe the north and south sides,there are also 2 gable vents on opposite ends east & west sides each 2’H x 14″W. we have over the years had difficulty keep the house warm during the cold michigan winters. and have experienced outrageous energy bills. during the summer months we notice that the house also seems to go through a heat soak in the evenings that overwelms the air conditioned living space. Is it possible that too much ventilation could be causing incresed energy use, and causing erradic temperture fluctations. Oh I might add that on windy days it is impossible to keep any heat. The house always feels cold even when it is warm outside.

Also in response to an earilier post about vents above windows and doors, I was told that this should not be done, because in the unlikely event that a fire should occur the air currents would draw the flames into the attic causing more damage.

Thanks for your reply Ben. I should have stated that the attic has an origianal r13 layer then I added a r30 layer a few years ago. furnace is 92% efficient installed in 1998 and all the windows have been replaced within the last two years. We do have a whole house attic fan that gets covered in the winter time. It seems the more I try to be efficient the higher the bills go.

Randy,
I want to install attic baffles before blowing in insulation. The area I need to install baffles the roof trusses and joists are at almost a 30 angle from each other (below this area is cathedral ceiling. There’s no way I can get down to the soffit from the attic to install the baffles. Is there any way to remove the soffit (outside) and somehow slide the baffles in from there? Is there something else that could be done? Thanks, Craig

I am going to add blown-in insulation in my attic. I know I need to install soffit baffles in order to avoid the insulation from blocking the soffit vent. The challenge is my attic is very low and installation of the baffles is almost impossible. Any suggestion?

I currenlty have a powered attic fan near the peak and gabled louvers (though not directly accross from each other.. it’s an oddly shaped ranch). I need to replace some rotten soffits. I currently have no soffit vents or roof vents. As I replace the soffits, should I hadd vents.. or does the fact of having no roof vents impact this?

My house has ridge vents at three different levels as the roof profile creates three different ridges. I was wondering if for the calculation mentioned above the 50% venting on the ridge needed to be at the highest level or if it can be split between the three levels. I am assuming if it can be split between the three levels, you would do three separate calculations, one for each amount of square footage the ridge covers. Is this correct? Thanks.

I live in Colorado (arid environment) in a small house and have a small attic (approx. 800 sq. feet) with an access panel. I do not have any soffit vents (or an eave for that matter) and just two vents on the side of the house near the top of the attic. I have blown in cellulouse insulation on the floor of the attic. I’d like to add pull down access and create conditioned storage (a floor, knee walls, replace the vents with windows – essentially an insulated box in the attic). I’m still just in the conceptual stages, but my main concern is ventilation in the non-conditioned area (i.e. behind the knee walls and above the future ceiling along the ridge). Should I add small vents above the new windows that can effectively vent the area behind the knee walls that would be connected to the small area above the ridge? Or should I just create my room in the middle (that includes all space up to the ridge), effectively separating the attic into three parts – area behind the kneewall on one side, conditioned area, area behind the other kneewall – then just add four vents (two on either side of the house at each kneewall)? I could by way off base with what is even possible here. Any insight is appreciated (I also realize this fairly in-depth, so if anyone had a suggestion on a good book on the subject that would be great too).

Many thanks-

I have heard that if you have soffit vents and a ridge vent that you don’t need gable vents. In fact if you have all three, flow of air through the attic will not be adequate. Is there any truth to this? I have all three vent types and currently have a mold problem in my attic that I am remediating and am wondering if I should block off the gable vents and just go with the soffit and ridge vents. Any ideas?

Hi,
I had aluminum placed over my soffit for appearance, but mostly for additional venting. They drilled 2″ holes in the soffit every 8″-12″ or so. The aluminum is perforated for 10″ every 36″. When I figure out the area of the venting, do I include the area of the holes that aren’t at the preforated sections of the aluminum? Granted the aluminum is not pressed up tight to the soffit so air should be able to enter through the other holes, but not sure.

Thanks,
Tim

Actually I did some poking around and this is what I found…they drilled (3)-1″ holes every 46″ (8 sections of venting) or so where the aluminum is vented. If the vented sections are 12″ * 6.5″ and the manufacturer has the free air space to be 12 sq inches/LF, do you have a handy calculation of what the soffit vents provide for air flow? I know how to figure the area of all the circles they drilled, but what I don’t know is how much air flow will actually be usable since it’s going through the vents.

Thanks for the response. The lesser would be the area of the (3)-1″ circles is 2.34 sq inches * 8 vented sections= 18.8 sq inches of incoming air. Yikes. That is low for what is required in a 21’x34′ attic correct? Based on the calculation above, I need 342 sq inches. Even if I cut the entire soffit and install the vented aluminum I would only be getting 221 sq inches. Any suggestions? I guess that explains why that room is so hot.

Our attic is about 1700 square feet. The only vents now are small 1 inch round louvres. They’ve been painted over so many times they hardly let air in. Should I replace each of them? I counted 20 total. Is this even enough vents? Or should I add 8×16 vents instead? Or both? How many would I need?

would drilling more holes and making the entire side perforated let too much cold air in the winter time? I have been reading the horror stories about icles and mold in the attic.

It just the blowm in insulation between the joists on top of the dry wall. I have 10″ of insulation and no vapor barrier. What’s the best way to add a vapor barrier?

I just had this argument with my roofing contractor who has the family busines for 60 years installing roofing.

I told him specifically about the airflow, didn’t appear to get the air flow idea, just told me hot air will rise throug the ridge vent and gable vents.

I am in the north east in Ontario, The house is dual brick contruction built about 60 years ago. There bricks go right to the top of the underside of the roof. So putting soffit vents won’t help, it’s already sealed. At lease one quote I received and the most epensive the he said lower vents on the roof would help to try and get airflow from the soffit would be costly, though others say that rain and snow could get in, some suggest knocking out some the bricks to allow air in through the soffits.

Never seen any icicles in the attic or outside with my old room, surpisingly. I’ll need to see how hot it is in the summer.

Any suggestions how to deal with this, or is this being sealed ok?

Mr Erikson,
I’ve got a house that has higher than normal ceilings in a few places. In all these places (at least 1/3 – 2/5 of the usable soffit area) it isn’t possible to place soffit venting because of batting insulation that needs to be there.(otherwise there would be no insulation above the ceiling there) So my concern is that it will be hard if not impossible to get even venting all the way around the house without removing this insulation, which in Texas I’m not sure I want to do. And to top of it off it’s in the hottest corner of the house. I’m in the process of calculating out the needed ridge and soffit venting and would like to know what would be the best course of action in this situation?

The reason to point the vents toward the house on soffit vents is so that you can not see in and they are less noticable. Just like when you install wall return vents for your forced air Heating system.[if they are low returns you face the vents down, if they are high returns, you face them up]

Danny,
Are you saying that you have sloped [cethedral] type ceilings?

I’m not sure that’s exactly what you can call my ceilings. It only slopes up the rafters for about 2 ft, to add about 1ft of height to the ceiling. If i removed this insulation, that would be 2ft all the way across the living room, master bed, and office that would remain uninsulated.

I was on the roof the other day and decided to take a closer look at my ridge vent. I noticed that on the shorter ends (perpendicular to the vent run) that some ends had a foam piece in them, some had some folded shingles in them and others had nothing. Should all of the ends have something in them, maybe to keep rain out? Or should they all be open to allow better airflow? Thanks.

I am changing from standard “turtle” vents to a ridge vent. I also have a powered exhaust vent that turns on when the temp in the attic becomes hot. Some questions:
a. If I add a ridge vent should I remove the power vent?
b. I plan on adding some soffit vents so the Sq footage of the soffit venting matches the ridge venting. The soffit vents I plan on using are metal with finned intake set at an angle. Question. Should the soffit vent have the slotted angles point towards the house, or away from the house. It appears there may be a correct or incorrect method.

I have a ranch style house with a hip roof on one section and am going to have the whole roof replaced. A ridge vent will be added to the main peak, however I would also like to add a ridge vent to the hip portion of the roof since that section is quite a bit lower than the rest of the roof, with no ventilation. I have heard that this may not be a good idea because water running down the main section of the roof may get under the ridge vent where the two meet. Any comment?

OUR HOUSE DOESN’T HAVE THE SPACE FOR SOFFIT VENTS SO WE PUT IN ATTIC FANS AND A RIDGE VENT. IS THERE ANY MORE WE CAN DO?

I have just finished the attic (July 2010)of my 85 year old house into a usable living space. Its rather hot in the summer, NJ area. I am in the process of insalling soffit vents, but I am quite afraid that there will not be proper air flow because I do not have a ridge to incorporate a ridge vent. The top of the roof is flat (about 7′ x 12′), sloping on all four sides– (no place for air to exit at the top). The actual size of the house/attic is about 30’x 26′. There are also two dormers on opposite ends of the south and north. What do I do? The contractor do not seem to have an answer that makes me comfortable for how the air will be circulated. He says that the air will move up from the soffit through the baffles and back down to the soffit. Please let me know if this strategy is reliable, because it makes me nervous since every article I read on venting an attic properly says that the air needs to exit at the top of the roof. Should I consider installing some type of vent maybe two on each side of the east and west side of the roof that will draw air through one of the soffits? I also need to verify how many soffit vents I will need on each side of the house(using 8″ x 16″ vents). I think that maybe I need 3 or 4 vents on each of the four sides of the house. I try to calculate based on your instructions and came up with 3 vents– but is that total of 3 vents total around the house, or 3 vents on each side of the house? Plus I may need to place 2 vents on each of the dormers. The dormers are about 6’W x 5’D. The contractor is coming tomorrow 7/22/10 to install soffit vents (and I need to determine how many vents (using 8″ x 16″ vents) do I actually need. Can you help me please.Your quick response will be greatly appreciated.

Also, the soffit that is being installed is done by drilling three 3″ holes through the solid wooden soffit. Is this strategy good, or is it better to cut out the soffit in a rectangle slightly smaller thanthe vent. I also have 4″ x 16″ soffit vents for use on the dormer soffit, can I also use the same size on the larger area of the house instead of the 8″ x 16″? The current soffit depth is 20″.

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I’m putting soffit vents under my eaves and am not sure how far I should space them apart. Does it matter if I spread them out evenly or not?

I just gutted out an upstairs room in my Cape style house. I was amazed to see that the vinyl siding / soffit I had install a year earlier was merely placed over the wood soffit leaving zero ventilation. I could not drill though the wood from inside. What’s the best way to install soffit vents from teh outside? I found ventilation screens that are flat, but that will not work with the soffit siding since it is baffled / ridged. Should I remove the siding to install the vents into the wood, or do they make vents with a sleeve that will not allow bugs and debri?

I’m located in upstate NY and after reading nearly all comments I find myself a little confused. I have a 1760 sq ft attic space and a smaller cathedral ceiling. There is no venting for the cathedral ceiling at all which spans two stories of the house. The attic space has only two gable vents one of which is mostly blocked by a new chimney. I’ve added 16 temperature sensors to the house in various rooms includng the attic which I monitored and logged on my computer. On a sunny day the temperature in the attic has reached 133 degrees F (and that wasn’t a particularly warm day). With no ceiling insulation in the rooms below the attic (this is a project house), these rooms have got as high as 110 degrees F which really shows the need for good attic ventilation. I have researched attic ventilation a fair amount on the internet and come to the following conclusions. I intend to add 6″ round soffit vents between every rafter (round to simplify installation). These vents are mesh only (no louvres) which gretaly improves their Net Free Area (NFA), in this particular case the NFA is 17 sq in per vent. In total I’ll need 72 of these. I’m also adding a ridge vent, there are many variations of this too, I’ve chosen the Omniridge OR-4 as it has a NFA of 18 sq in per lin ft and a few other features to help improve air flow. I’m also adding styrofoam duravent channels bewteen every rafter spanning the complete roof sheathing from soffit vent to ridge vent. Baffles will be added to every soffit vent between rafters to prevent any air flow blockages from ceiling insulation and to prevent air leakage into the attic area. I plan on conditioning the attic area (at least half of it) for storage space. I will be adding fiberglass batts between every rafter over the styrofoam channels followed by a vapor barrier of radiant foil insulation (this hold the fiberglass batts in place and contain the fiberglass fibers. Apologies at this point for the long winded description but I’d like to explain why and make certain I’m on the right track. In summer the sun is responsible for heating the roof shingles and therefore the attic to extremely high temperatures. In winter, warm moist air from inside the home eventually rises to the attic roof where it condenses with the freezing cold roof sheathing causing wet, rot and mold. Therefore to me, the most important aspect of attic ventilation is to keep the roof sheathing between ALL rafters ventilated. This also keeps the roof cooler in summer and therefore increases roof shingle life. The ventilation will keep the roof sheathing dry it in winter. I can’t understand why venting with only a few soffit vents and therefore between only a few rafter is adequate ventilation even though it meets the required farmula calculation. It also doesn’t make sense to me to vent the whole attic as opposed to using the styrofoam channeling.

Here’s a tip for the folks asking about whether they need to install baffles along with soffit vents to an attic with existing insulation. After cutting your first vent if there is no obvious blockage go into the attic, close the attic stairs and turn the lights off. If you can clearly see the glow from your vent, then you’re all good! Otherwise a baffle and some digging through the existing insulation will be necessary for each vent.

Ben,

First, my house is a ranch style built in 1974. It has about 1400 sq ft of attic space and a standard pitch roof (which means getting over to the soffit is a chore (I just did it today to find out this info).

I need proper ventilation in my attic as it is about 135F on a 95F day. I have crawled up into the attic and see part of the problem. It appears that I only have 6″ of rolled fiberglass insulation which I hope to correct soon. The other issue is that when they ran the insulation they ran it all the way out to the roof line and stapled the moisture barrier to the rafters.

This doesn’t appear to be allowing the air to flow through the soffit vents up to the roof vents(24 of them 8″x16″ approx). The soffit is vinyl and the vents do go up into the attic (I checked with a coat hanger).

Is my only option to take apart the soffit so that I can gain access to the vents and cut the insulation back a bit so that I can install some rafter vents or what should I do?

Thanks

Florida house here built in 1997. I cant seem to find much info on this venting method. Soffit runs continuiously around house, no upper vents (ie: ridge,etc.) Do I really need upper vents or is soffit only ok. Also thinking of a radiant barrier and/or more insulation (currently R19) Any info would help, Thanks.

My mom’s house was built in the 20s. The house has no soffits… All you see when looking up into where the soffits should be are the rafters and the support boards for the roofing material. She has ice buildup most winters so we are looking to try to stop that. Our thoughts are to install some vents at the very top of the sides of the house, and then install some rigid type of insulation in the attic as the rafters are all open. Basically creating a cold space from the bottom of the rafters and the roof. I’m not sure if the peak of the roof is vented or not… Do you have any other ideas that would be better?

I have a 24ft(L)x 26ft(W) attic with venting across the peak of the new roof. You can stand up in the attic which is abou 8 ft tall. I believe the siding contractor is going to be putting in 3″ round holes before installing the vinyl soffits tomorrow – 9/20. How many total holes should he drill? Thanks!

Every formula for attic ventilation I’ve seen says 1 sq ft of ventilation is required for every 300 sq ft of attic floor space. That number is divided by 2 so that the ventilation is equally distributed between the soffit and the ridge.

I have a new roof, with ridge vent all the way down the peak. My soffits are solid, with no vent holes or perfs. Can I just drill a bunch of small holes in the existing soffitt? I dont want to tear it all down and replace.Thanks!

I am having new vinyl siding, soffitt and new gutters installed on my home. I had soffitt ventilation before they started residing my home. I have have NONE. I understand the recommendations, but is there a required code on having soffitt ventilation?? Would you suggest I insist my installers install soffitt ventilation??

I have a 1959 bungalow in Ottawa. My attic space is about 1000 square feet. I have continuous openings in my eaves around the circumference of my home covered with perforated aluminum soffit. I have 5 roof vents up top that are covering 9 X 9 inch holes cut in my roof. I am on the verge of getting 10 to 12 inches of fiberglass blown in and my insulator has told me that he will be installing soffit vent channels or baffles in every single 14.5 inch space between my trusses that he can get to. My estimate is that will work out to somewhere around 70 channels/baffles. My concern is that I’m reading here about the importance of striking a balance between soffit venting and upper roof venting to get proper air flow and I’m worried that this potential imbalance between the soffit venting being proposed by my likely insulator and the upper roof vents could cause some sort of air flow issue? Could you please clarify this for me?

A couple of people have asked my question–which direction should the vents face–and the answer seems to be that the openings should face toward the house. Carlton asked the same question, and Ben answered by saying that he had not found a definitive reason for that answer, and Tony answered the same question from Glen by posting a link to diynetwork, which also gave no explanation. The pros to point them inward would be in case of blowing dust or extreme rain, there would be less entry into the attic. However, when the attic is venting out, it seems to me that you want the vented air to be directed away from the house, otherwise you would be contributing to the heating/cooling problem. In addition, if the vented air is moist or has mold or other contaminants, you would want that air to be directed away. My inclination is that they should be pointed out, but I am not an expert in any construction or engineering field. So I would appreciate a debate here. Just because people have been doing it that way doesn’t mean that it’s right; which way you do it may depend on the environment. I do not know, and I would like to hear other thoughts on the issue.

Thanks for the answer. The reason I asked is that I am getting repairs on a house that I inherited, and I should have mentioned that there is no attic power vent. Since it is hotter in the attic than it is outside, it would seem that the expansion of the air in the attic (following what, in chemistry, is called the ideal gas law, pv=nrt) would force the flow to the outside, and when it cools, the air would then be sucked in. In other words, without a power vent, it would have to go both ways, depending. There are 12 soffit vents downstairs, and 8 upstairs, and they are the original ones. The house was built in 1986. I am replacing all of them–they are unbelievably filthy–and I am using hard-to-find ss screws. I have another question. Ideally, when painting, what I would do, if I were the painter, is remove all the vents and light fixtures, sand, then paint and reinstall all the vents and fixtures. Looking at the evidence here, I note that many painters don’t even bother to tape off the fixtures these days, and they just paint all over the soffits and let it glob up here and there. I know it’s a lot of trouble to do it right, since I have done it myself decades ago. But I am now wondering, having observed that they just merrily paint over the soffits, if I should attach the new soffits and let them paint the new soffits along with the eaves, using the most expensive Behr product, the one that does not need a primer, or if I should remove all the vents but leave the vents open until they paint, then reattach them, and let the soffits be just naked metal on the eaves. I haven’t looked at a lot of houses, so I am wondering if the naked metal look would mark me as some kind of decorating idiot or what. Are soffits almost always painted in situ or what? It strikes me that the metal surface, if it is decent stainless, is easier to clean and doesn’t trap mold, etc., as easily as paint–or is there something I haven’t considered?

I am re-doing my attic insulation and venting. I have added baffels in every cavity that was possible (just about everyone) I have done the math and my venting seems to be spot on. My house is pretty much a rectangle and only has soffit vents on the sides and across the rear not 1 vent on the front of the house. Seems to me like you would want vents on all sides. Should I add vents to the front?

I find this ridiculous. My house was built in 1956, I have a ridge vent at both peaks of my attic. Front to back above garage and left to right above center of house (a 1340sqft ranch). Anyway, after my own thorough inspection I have found not a single soffit vent! Not even perforated vents!!! No wonder I have gutters full of ice and 2 to 4 foot icecicles and high cooling/heating bills (along with only 3.5in of insulation in attic. I’ll be installing plenty of soffit vents and getting my attic up to spec with at least R-49 insulation (I live in Ohio). Home ownership is a *bleep*!

My current soffits are aluminum panels that snap together with U-shaped gaps between each one. they’re about 4in wide by 18in (from gutter to house). How would I add rectangular soffit vents or should I put in round ones or replace the old snap-together soffits with perforated ones? I need some help here.

My attic measures 24’x36′ with a 30″ soffit running the two longer sides, minus 12′ on one side. A previous addition eliminated 12’of soffit from one side making the two sides uneven. Currently, I have 3 mushroom vents, 1 gables vent, and only 24′ of ridge vent out of 36′ total ridge. The mushroom vents and gables vent are older, but the ridge vent must have been installed in only one section to combat a moisture/ventilation issue. There are no soffit vents, and two of the mushroom vents are just below the section of ridge vent. I have water dripping from condensation that accumulates on the ridge vent and mold growing on the lower 1/3 of that section of 2″x8″ decking. Needless to say, soon I will need a new roof, despite the shingles being only in the first half of their life.

In order to properly address the problem, I’m afraid I would lose money best put towards the new roof I will soon have to buy. Currently there is about 600 sq. inches of ventilation loaded in the top of my attic. I will need almost 16 (42 sq. in) soffit vents to match. My instinct is to leave the top venting alone and simply add the 16 soffit vents and observe the changes, hopefully curbing the condensation issue until we re-roof. If you agree with this course of action, then I only have one question – With uneven length of soffit on each side (24′ on one side and 36′ on the other), is it okay to have uneven numbers of venting per side (10 and 6), or would you try to keep the numbers even (8 per side). Otherwise, what other course of action would you recommend to address the combination of issues?

I have an issue with how hot my attic gets here in AZ during the summer and it just cooks the 2nd story of my home and the AC can’t keep up. Looking in my attic (40×25) I have just two gable vents at the corners. I want to add soffit vents but I can only access the soffits at the back of the house due to the angles of the roof at the front. My eaves are open so the soffit vents will mount right to the bird soffits that keep the rafters from twisting. How big of a problem will this be with air flow? I also was thinking of installing a gable vent fan to help with pulling the hot air out. Last year my attic hit 155 degrees, I am trying to avoid that this year.

I have a cape cod with 3 1/2 in soffits What to use for vents.

Hello,

I am currently adding insulation to my attic and installing rafter baffles into the existing soffits. My problem is the sub-attic over the bedroom & garage area – I have experienced a mold issue primarily due to living space air leakage and a lack of proper ventilation. This small roof space has a ridge vent, and I have recently installed baffles to also direct air to the upper attic area (and to keep the roof cold in the winter).

However, the perforated soffit vents around this section are not visible or accessible from the interior of the attic – the roof sheathing just runs right to the edge of the floor or walls with no eave open to the soffit vents. Is there a way to open these access to these vents, or do I need to add intake vents on the lower sides of the house walls (I was thinking small gable-type vents might work).

We also have a Cape Cod with narrow space for soffit vents and a solid board on the inside attic space so we can not install soffit vents. We have ridge vents and gable vents and wonder about installing an attic exhaust fan. Question is where would it pull the air from w/o soffit vents?

Home inspector recommended improvement of the attic ventilation for my new farmhouse build in the 1970s. The roof has a ridge vent and gables in two side of the roof. My roof contractor closed the gables and installed soffit vents. Since the soffit area for my old house is small he used 3″ diameter round soffit vents. For a roof area of 1100 SF he used 12 soffits vent. The ridge vent is about 25 feet long. Is 12 round soffit vents with 3″ diameter sufficient?

This is great information. I have wondered if I have enough of them around my house, now I have a calculation I can use to determine it. Thanks for your advice.

I am not sure, if 12 round soffit vents with 3″ diameter sufficient for 1100 SF attic area?

I am in the process of getting a new roof put on. It currently has 2 roofs on it…I already have an attic fan in my attic and my roofer said that you don’t need a cobra ridge vent since I already have a fan. Is that true? Could I have an attic fan and have him install a cobra ridge vent along with soffit vents? By the way, my attic is around 500 square feet and I can stand in it. Its highest peak(6 ft high)…Any advice would be greatly appreciated.

I have 3- 12″ worley-birds (2 attic 1 garage attic) and a ridge vent on the garage.. live in north texas, hot hot summers.. i noticed only 3-4 soffit vents around my whole house.. seem easy enough to add them myself but my question is.. can i have too many? 1750 sq ft house.

I have a 1950’s ranch house with very wide eaves and unvented wood soffit underneath. I haven’t measured the soffit width yet, but would guess it’s 36″ or more. The paint peels off and I know it’s time to replace with something vented. The home building materials online stores only list soffit at 16″ wide. Does wider soffit panel exist, or how do I deal with this? We don’t have a ridge vent but do have several installed in the roof plus a solar-powered vent fan. Thank you!

I’m dealing with a moisture issue in my garage (air very moist and appears to be affecting roofing), and am thinking that I need more ventillation. The soffit vents seem covered up with some insullation (approx half inch) sitting on top of the soffit surface… could be that the new soffits were simply put over old wooden ones which never had any vents, I can`t tell unfortunately only can feel with my hand. The garage is not otherwise insulated, it is attached to side of house but not heated and not finished (i.e. rafters are visible and not covered from below). I live in southern Ontario, so climate similar to upper state NY. Any good reason for the soffit vents to be coveredÉ

I am replacing my damaged r-7 insullation in my knee wall storage space. Although i have gable vents in my small cape with sloped ceilings upstairs and a very small attic peak, i noticed i dont have soffit vents. The builder at least left a small space for venting between the insulation and the roof slope so heat can rise to the peak…my question is this…i am adding insulation to this crawl space to eliminate some of the winter ice damming ive been getting…i will be adding soffit vents to EVERY OTHER joist space….should i be adding a vent to each rafter space,,,im concerned about the bays that wont have their own vent…although i never had any before. i dont want to have a 3″ round vent every 12″ if i dont have to. ive installed styrofoam spacer vents to maintain airflow up to the peak and im putting an r-13 in place of the old r-7, as well as better insulating the storage space to keep the house heat from escaping. Thank you everyone for all the good info in this forum.

Last year we had a new roof put on our two story home. Previously we had a shake roof which not require soffit vents. With the new roof, they were added. The second floor of our house is really cold. We had a contractor add insulation between the attic floor and the ceiling of the second floor. It made NO difference. Please can you help?
Thank you.

Hi,
Can someone help me to understand need of soffit vent in NJ area? I have a Bilevel house with attic about size of 80×25 ft. I have a large industrial type exhaust fan on one end and a intake ventilation on the other end (on walls), also ridge vent in the roof. Soffit is only in the front of the house – no Soffit hang on the back of the house. Currently there are no soffit vent installed. I was told by a roofer that I should install Soffit vent but no one can tell how many. Above calculation can help but i am not sure if i really need it installed since i have an attic fan already. also, in NJ it gets very cold in winter – am I going to have any problem if I add more ventilation? Thank you in advance for your help…

Danny,
Where can I find nice cast(iron, bronze, etc.) soffit vent covers for my 1911 house?
Thanks,
Keith

I have had blown in insulation added to my attic. I live in a ranch style home with a walk out basement. We have asphalt shingles, 2 gable vents and a few roof vents. We are going to have soffit vents added. Will the blown-in insulation be a problem getting into the vents?

I want to install a tougue and groove cedar in my soffit area and have found a wooden soffit vent on-line at Whitehaven Wood Products, they look pretty nice, could you give me your opinion if these would work for me? The site states that they would provide sufficent ventalation. Your thoughts ?

We have a 28×48 split level home. We just opened soffit vents all the way along the back of the house and we are going to do the front also. we have a ridge vent at the top of the house.We live in New England. after reading your page I’m afraid we did too much.

I am thinking of replacing plastic oval 3″x 12″ vents on my soffit to bigger 8″ x 16″ ( cutting bigger vent holes ) Good idea or not? I thought the more air in the attic the better. I was going to do this before painting so I have to remove them anyway. Do you agree?

I am about to add insulation to my attic. I own a single gable home with a large attic space (6/12 pitch). In my prep I ran a 12″ wide 1/4″ R1 foam baffle the width of the house where the joists meet the rafters with staples and I used pieces of batting in between each joist so I will have a clean flow of air between each rafter – I have a continuous soffit vent, ridge venting, add a gable vent on each end of the home. I need to blow 11″ of insulation to get the correct R rating and I don’t want insulation to spill into the soffit area so I made this ‘baffle’. When I went to begin prep to the other side of the house, which faces the east, I realized the previous owner jammed batting where the rafters and joists meet completing blocking air flow from the soffit vents the entire length of the house. This has to be wrong, correct? Also, is my ‘baffle’ system a sound plan?

Adding more soffit ventilation – hidden vent vinyl soffit currently installed ?

I need to improve attic ventilation at my house and need to install more intake soffits (plywood soffit under vinyl) Hidden vent vinyl soffit currently installed ? What is the best way efficent and appearance wise to accomplish this. BTW- the eaves are at the 2 and 3 story level of course. – Thanks

Owens Corning says their 3″ round vent has a vent area of 1.36 square inches, which is a far cry from simply calculating the area of a 3″ hole which is closer to 7 square inches. It makes sence when you see how the vent is mere slots in the face.

Can you have too may sofit vents? I used this formula to get the right number of vents. But when I did the calculations I came up with only 6 vents for a 28X56 size attic. Did I do that right? My vent size is 8″x16″ making it 0.8. So would it hurt if I did 8 vent instead of the 6? Thanks in advance for you help.

John

Help. I don’t have soffit vents. My roof started to leak- it is 6 years old. The roofers that gave me estimates told me the roofer that installed the roof did not install a ridge vent or any venting on the roof. I have a split level and there is an attic fan. 2 roofers giving estimates for repairing leaks disagree on what roof vents should be installed. One said to install a ridge vent on the lower roof and just leave the attic fan on the higher section. The other wants to install 4 louvre type roof vents, 2 on each roof for intake and outtake purposes. Which is correct?

My house has continuous soffit vents with roof ridge vent system. The soffit vents had been painted over a number of times and the vents are clogged and I can not wire brush them as the paint is like cement. Any suggestions to open? Will paint solvent help soften? Drilling?
Thanks.

How are functional soffit vents installed when the existing ones are blocked presumably by an earlier siding installation?

How much reduction in FFA is there from no. 4 mesh to no. 8 mesh.
All the vents show values for no. 4 mesh .
No. 8 mesh is required in high fire hazard areas..
Can the vents be ordered with no. 8 mesh?

I am assessing the new roof on a house I just bought. It has a steel stone coated roof system on battens. It appears the installer put the underlayment right over the old static vent holes on the upper back side of the roof and did not cut the ridge vent holes in the sheathing. When asked the installer simply says I don’t need to worry about attic ventilation. Any input?

I live in a small ranch. Over the bedrooms is a crawl space in the attic. Over the kitchen, living room, and dining room is a cathedral ceiling. I want to add soffit vents, do i need to install them in between each rafter in the cathedral section both sides of the roof?

Ice damming and curling new shingles are proof that I need to determine that the soffits are vented. Ridge vents appear to be in place. The soffits are covered in perforated metal so I can not see the actual holes. The attic is very low and moving around up there is quite difficult. Any suggestions?

I live in a small ranch in nj, how many 1.5 round holes would I need to vent holes would I need to put in the soffits for 48x25x5 attic space ?

I have a front to back split ranch and recently had my roof replaced. My home has a ridge vet and 1 gable vent. I had them onstall soffit vents and they installed the 2 1/4″ round soffit vents. I am having ice dam issues in my gutters now much worse than before. My questions are the following:

1. Should I cover the gable vent since I now have soffit vents?

2. Are the round puck vents adequate or should they have used a continuous perforated soffit?

3. How can I fix this for the future? Add insulation to attic?

My attic area is approx. 1150 sq. ft. I have one 10″ turbine in the center top of the roof and two gable vents approx. 1 sq. ft each. I will be drilling 45- 3″ holes in each of the 2 soffits and covering the holes with continuous 8 ft. White Under Eave Aluminum Soffit Vents. I have fiberglass batts in the attic and will also be installing a radiant barrier under the roof. My question is – do I need baffles ( or should I just make sure there is a small gap over the soffit vents) and if so, should the baffles be positioned below or above the radiant barrier.
Thanks

My roof is 22ft x 60ft x 2 = 2640 sqft. basic square roof.
your calculator Is telling me I need 24 vents???
I only have about 10 on there now. old roof….

how far apart do I put them ? really 24 vents?
please advise

thanks,
gary

Calculation from author is not correct. See IRC-2012 R806.1 which says you can use 1:300 if you have rafter/attic vents in the space no more than 3-feet below the roof line. So, for a 2400-sf attic space (covered by roof), you only need 8-sf of net free vent area to comply with the building code. If you do not have upper vents within 3-ft of the ridge, then use 1:150. Check with your building official for current code and a copy of the section regarding roof ventilation.

I have a roof that measures 42’9″ x 30’7″. I will be installing 2 Maximum roof vents model #301. Using the formulas above I calculated (rounded up to 43 x 31) 1333 square feet of attic floorspace. Divided by 150 is 8.87 square feet of total vent space…divided by 2 leaves me with 4.44 square feet of soffit vent area. My soffit area is currently covered with plywood, so my two options are to remove all plywood and leave the eave open with a screen and cover it with soffit. The other option is to cut holes to install 16″ x 8″ under eave vents…but I’m having trouble calculating because I’ve read that it’s ok to have more venting at the soffit than out through the roof vents but not the reverse. The specs state that these vents have 65 square inches of of NFA. If I just cut out holes for vents how many would I be looking at? Or would it be best to remove the plywood and just have standard soffit. Would I still require vents? Really scratching my head here.

I have a furnace in the attic that puts off a lot of heat resulting in ice dams. How much additional air flow do I need to keep the attic relatively cool? The attic is 26×26.
Thx…

I would suggest anyone doing roof vent calculations actual get the net free area of the vent from the manufacture. For example, if I use a soffit vent that is 8″x16″ that would make the area 128 sq. in., but that is not the net free area of the vent it is actually 64 sq. in. (actually half the size of the vent). If one assumed the 128 sq. in. as the net free area they would be deficient by half the net free area required. I’ve looked up other vents from manufactures website or places like Home Depot and I’ve found some net free areas of soffit vents are actually less than 50% the size of the vent.
Thanks, Alex

Ben,
I am a contractor in CT. I got a call to look at a mold/mildew problem in an attic. Upon inspection and testing it is revealed they have a substansial mold problem due to no ventilation. My initial inspection showed the soffits were blocked with insulation, and there appears to be an active ridge vent. I can mitigate the mold problem, but whats a solution to vent the attic space properly, I can send photos and dimensions, but using your calculations above will that ensure the mold doesnt come back? I have a turbine fan, but the manufacturer states to not use it in that application to vent an attic, which I do not understand. Any feedback would be appreciated so I can properly take care of this for my customer, thank you sir!

A circular vent has a radius of 6 inches how much material is needed to cover the vent? Use 3.14=22/7

what type of vents do you use on a flat roof? typical sizes?

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Hello just had a roof put on by one of the box store, we live in southern MO.. We went with the 3 products of corning, and went with the ridge vent and they said we don’t need any other venting now. My attic has been as high as 126 degrees up there. They did not address the soft venting. In a 30 x 50 area of stick there are only 6 old sofit vents. Is that enough to make the ridge vent work right? Thank you for your time, cindy

Thank you for this outstanding information. I have a question about our vent size.
The house was wood siding with 19 – 4” x 4” soffit vents. 10 years ago, the house was sided with aluminum and the 4 x 4 openings were coved with 10” by 22” soffit grates. Our attic is 1,596’. We have 5 roof vents.
In 2015, we blew in extra insulation including over the uninsulated, unheated garage. Since then, we have noticed mold growth on the roof rafters over the north side of the garage. I suspect we do not have enough ventilation? Should we enlarge our soffit vents? Should each soffit vent be the same size? Do we need a vapor barrier over an unheated garage
Thank you for your help!

@Kim Holeton,
I’m not an expert but I would guess your blow in insulation has blocked the soffits. You may need to install roof vent baffles to stop the blockage.