How to Stop Condensation in the Crawl Space Under Your Home

The crawlspace under my house is wet with sweat from condensation. I have insulation between the floor joists with plastic stapled underneath. The house has five vents, three on the west side and two on the east. What could be causing the moisture problem under my house? -Eve

Hi Eve,

From your description, I see several problems that could be causing excess moisture in the crawl space under your house:

  • Not Enough Vent Space: You may need more ventilation in the crawl space. The rule of thumb is to have at least one square foot of vent area for every 150 square feet of crawl space.
  • Inadequate Cross Ventilation: You also need to have adequate cross ventilation with vents on all sides of your house, not just two.
  • No Plastic Ground Cover: The plastic sheeting should be covering the ground of the crawl space, to keep moisture from coming up through the soil, not stapled beneath the insulation.
  • Leaky or Uninsulated Ductwork: If you have ductwork for your HVAC system under your house, have it inspected for any air leaks, and to make sure it has enough insulation to prevent condensation on the outside of the ducts when the air conditioner is running during the summer.
  • Add Fan Venting: If you’re still having problems, have a small fan installed on one of the vents with a humidistat set to turn the fan on at 70% relative humidity to remove excess moisture from your crawl space.

Good luck with your project,


Further Information


  1. Hello Danny,

    I put some plastic on the dirt crawl space in 2008 after purchasing my newly build house. I overlapped the sheets but did not tape them. I also did not run the plastic in up front of the block walls. I do have vented openings. However, now I have condensation under the plastic and mold in several places. The plastic is probably too thin. My question is this: Can I lay heavier polyethylene plastic over the sheets that are on the floor or do you suggest that I take out them out of the crawl space and let the dirt floor dry before laying the heavier plastic?
    Thank you for your help.


  2. Hi, I really appreciate you guys first of all! Now, my crawl space, I read many of your helpful tips and know what I need to do, but one thing I need a little extra tip on, my furnace/air ductwork runs thru the space. I often look in and when the air is on in the summer months I see condensation. Can I wrap that with some type of insulation-tape? To help solve that? I am about to finish with the plastic in there, I have two vents and this is the last piece of the puzzle and I thank you in advance for your help. Tim in Grove City Ohio

  3. Hello every one. I need some advice. I’m having problem with my side walk. In winter the tiles of it are pumping up. Why is that happen and what can I do to fix it? Please help me.
    Thank you

  4. I have had new plastic layed under my house in a dirt floor crawl space. Now it is creating moisture under the plastic. Any suggestion for a remedy would be appreciated.

  5. I have a conditioned crawl space which is only 6 yrs. old, the HVAC unit and the trunk lines are covered in condensation. There trunk line and the branches are all insulated as well as the walls and there is a vapor barrier on the ground which is covered with gravel dust under the vapor barrier. It is as cool under there as the living area. Why do I have this condensation?

  6. i have condensation under the plastic flooring that covers up the pea gravel in the crawl space where is it coming from and how do i get rid of it thanx

  7. Can I put plastic over damp dirt? We have tried to control the water that comesIn when I rains. Water flow has beat us. We have sump pumps but Iit still gets wet under there. The crawl space I about 5 ft tall. Thanks for your help.

  8. Vented crawl spaces in hot humid environments allow hot humid air to enter into a cooler area such that moisture condenses. No amount of additional air from more vents or fans will help this problem. Your solutions are wrong.

  9. I have a house that is close to the ground,can not get under to have plastic between floor and ground.would it be wise to block vents so damp and moisture can not get under house.What should I do?
    Norm, Tampa Fl

  10. i have excessive moisture on the under side of my subfloor, wet with mold starting. I have 16 wall vents evenly spaced around the foundation. The ac lines are all flex insulated and they all condense, the wood in my subfloor is wet around the ac vents which appear to be spreading out. My thoughts was to turn my ac to a higher reading, close my vents, seal the walls down to the vapor barrier on the ground and add a dehumidifier and monitor humidity from there. Will deal with mold after I’m sure moisture is eliminated. What do you think?.

    • Hi, Alan,
      Danny says, “I think your approach is a good one; it definitely is moisture related; the dehumidifier will solve that problem. Thanks for your question, and good luck!”

  11. Hello Danny,

    One of your suggestions is to “Add Fan Venting: If you’re still having problems, have a small fan installed on one of the vents with a humidistat set to turn the fan on at 70% relative humidity to remove excess moisture from your crawl space” is this fan under the house? I live in LA and my 60 yr old wood floors are now buckling. In LA the humidity rarely gets BELOW 70% so this fan would run constantly. Would a fan of this type in the attic help at all? Do you have any other suggestions why after 60 years the floors are buckling?



    • Hi, Deborah,
      Adding a fan in the crawl space will help reduce moisture, and adding a vapor barrier (such as plastic sheeting) to the floor of the crawl space will help prevent moisture from the ground from rising up to affect the underside of the wood floors.
      Hope this helps!


Please enter your comment!
Please enter your name here