How to Insulate Under Floors in a Basement or Crawlspace

Insulating the space between the joists in a basement or crawlspace can help reduce your energy bills. Here’s how to insulate under a floor:

  1. Cut the insulation to length using a square and sharp utility knife.
  2. In older homes, where the spacing of the floor joists varies, you may need to cut the insulation to width as well.
  3. Install the insulation with the vapor barrier facing up toward the heated living area of the house. If both floors of the house are heated, use unfaced insulation.
  4. Don’t compress the insulation when installing it, but allow it to expand to fill the cavity.
  5. Hold the insulation in place using support wires beneath it. Cut to length if needed using wire cutters.


  1. Will isolating the basement by insulating the ceiling cause the basement to become colder? If I have to now open the heat run in the basement that I currently keep closed, that seems like it will offset any savings I get in the living area. Not sure.

  2. installing roll insulation in ceiling of basement which is under floors of living space my basement is heated but a smaller amount than up in living space (usually 66-68 upstairs and 58-60 at the same time in basement) my question is about the kraft paper facing, I think it should face the basement because of any moisture in basement. It has concrete floors (not covered with carpet or anything other flooring)it’s 22 degrees outside and the hardwood floors upstairs are pretty cool. Before the floors in the living space were warmer because I was heating with baseboard heaters in each room but this year the electric company raised the price 2 cents a kwh and I got a $300.00 electric bill so now I’ve turned on the natural gas furnace.

  3. in a crawlspace if the insulation is facing out it will retain
    moister and mold and eventually fall out it has to have some kind of barrier to prevent this from happening??????????

  4. I want to put insulation on the floor under the house I watched your video and want to know if it would be alright to do what you did only I would like to put foil bubble wrap on last to hold the insulation up. is this ok or not please let me know. Thanks carl

  5. We have an old house and want to insulate between the floor joists but there are wires and pipes running everywhere. I don’t see addressed anywhere what to do when encounter these wires and pipes in the space we are trying to insulate. Do we cover them with the insulation, insulate around them? I don’t want to create more problems.

  6. Hello
    We have an old house and want to insulate between the floor joists but there are wires and pipes running everywhere. Do we cover them with the insulation, insulate around them? I don’t want to create more problems. It’s very tricky to do

  7. It is my understanding the insulation goes on the walls of the crawl space as well as the floor of crawl space. NO insulation on the ceiling of the crawl space.

    Please clarify.

  8. Excellent video. My house has an x board support in the floor joists for support. Also electric wires. Do you cover up the electric wires? Do you run insulation up to the x support or cover that up with insulation? Putting the paper so it faces the basement floor makes it look a lot neater and complete, but we are not to do it that way? The paper side goes up to the floor and insulation shows? Please advise. Thanks for your help.

  9. Is it okay to place fiberglass insulation between joists that are on a concrete slab with osb subfloor above? This will be heated living space. About 6 -7 inches between the slab & subfloor, with always dry, well drained conditions?

    • Hi, Clayton,

      That should be no problem at all. However, you might consider mineral wool insulation, which is a lot more resistant to moisture and mold/mildew. Rockwool is the brand of insulation that we use.

      Good luck!

  10. How far apart to I put the wire spacers when holding up the newly installed insulation in the crawlspace under my house? And do I buy the spacers at the same place where I buy the insulation? Thanks for your reply . . .

  11. How to insulate the top of the basement foundation and space between the floor joyces (spelling?) Will the backing be toward the inside of the basement or against the outside wall.

    • Hi, B.,
      Danny says, “I would suggest using Rockwool insulation (unfaced), which is also known as mineral wool insulation.
      Thanks for your question!”

    In a basement space that is not heated and has a “double” joice (1890 home) I want to do two layers of insulation. The sewer pipe and bathroom plumbing is in the second joice level (or below the first Joyce level). Each joice level is running opposite of each day other. I put the first layer with the paper up. But do I place the second layer with paper down or up?


  13. I am building a new home. On the subfloor I will be using a foiled plywood facing down. I will be insulating between the floor joists. I will be installing a radiant heat floor as well. My question is can I cover the bottom of the floor joists to give a better seal? Will this cause any moisture problem?

  14. I have insulated my basement like the video above. If I dont have a finished basement, should I cover the insulation with plastic between the studs so particles dont get loose, or is it fine to leave exposed as the video?

  15. people also must remember that not all crawl spaces are built the same. my home was built in 1958. the joists are on 40″ centers. trying to put up insulation was very difficult. had to use plastic mesh.

  16. I have encapsulated my crawl space. The installers s took out the 2 year old fiberglass batting. My floors are still cold. Can I put R-19 batting back in to make the floors warmer? Paper side up to the floor? Thanks

    • Hi, Bruce,
      This should be OK to do if there is a dehumidifier in the crawl space. The only reason we could see for removing insulation after sealing the crawl space would be to avoid trapping moisture in the crawl space.
      A dehumidifier fixes that — if there isn’t one, it could be added.
      And yes, the paper side should be up, or toward the living space.
      Good luck!

  17. I have a older home 1955. We removed the carpet to refinish the original floors. We have a walk in basement with plenty of air gaps in the floor . So I spray foam to fill in the gaps. My question do I still put the insulation with craft paper still up or down. Next question should I block off the the air that brings in the fresh air. Thank You for you time. P.S. just found your show.

    • Hi, Mark!
      Thanks for watching Today’s Homeowner. 🙂
      Can you please clarify these things so we can better understand the situation:
      -Where exactly are you installing the insulation?
      -You wondered whether to ‘block off the air that brings in the fresh air,’ and we’re not following. Can you please rephrase that?

  18. Clarification for March 23rd e mail. First insulation will be going between the floor joist, I only filled gaps with foam. Second I have foundation vent all around the house, should the remain unblock or do the need to be covered.

  19. My son has an 80 yr old home , on piers (no subfloor) . Inside, floor is tongue and groove directly on top of floor joists. Will this same process work in his situation? Air seeps through the floors creating high utility bills. He will do the work himself to save on expenses , so, detailed information would be very helpful.
    Thank you!

  20. Hello,

    We’re building a treehouse atop a raised platform. I’ve installed R-13 fiberglass insulation between the joists and rim plates. I would like to enclose the bottom of the joists with 1/4″ plywood, sealed and painted. My question is whether or not I need to vent this space? There is about 4″ of dead space, basically like an unfinished basement, but I want to close it off since it’s visible and to prevent moisture underneath.

    • Hi, Phillip,
      For this treehouse, we’re wondering if you meant 4 feet, as opposed to inches.
      That said, there’s no need to vent if plywood is going under joists, but if there are only 4 inches below the joists, it would be difficult to install the plywood — for anything that close to the ground, there will surely be moisture problems!
      Either way, good luck with your project.

  21. Howdy do,

    I am wanting to insulate my crawl space (ceiling); which is below my cold living room. Can I use Faced insulation-with paper facing up and those “spring stays” to keep the insulation up?
    Would I also need plastic up; going across the joists?

    I also want to insulate the rest of my basement but leave it unfinished, until a later date. Having allergies, I’m worried about the fiberglass floating around the basement. Would I want to use faced or unfaced insulation? Then plastic across the joists?

    Any information will be greatly appreciated.

    Thanks tons and well wishes,



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