DIY crawlspace encapsulation

5 Replies

Bought my first rental about 4 years ago.    When we had our initial termite inspection, they recommended we install gravel and drains in the crawlspace along with a sump pump.    That helped, but, this being South Carolina, it is still to moist.   RH in the crawlspace was 94%, outside is 77%.  He also said moisture readings in some joists were in the 20-30% range.

 So now I am looking at encapsulating and a dehumidifier.   Been looking on, and I think we can handle the job.   Question, is it important to insulate the foundation walls?   It rarely gets below freezing here and we are talking about 18-24 inches of wall, and have to leave 3" for termite inspection.   Any tips or things to watch out for is appreciated.   Thanks in advance...


@Rich Riesmeyer

Yes I would insulation the crawl space.  This will put the dew point to inside the insulation and not on the face of the wall where it is now.  This condensation then turns into added RH in the space. I think part of your problem is you don't have enough ventilation to the space.   If you encapsulate the space then you are required to mechanically ventilate this space.   If you look at IBC 2012 section 1203 Ventilation will give you the requirements to meet.  I don't have the residential code section handy but I believe its the same for very similar.

Depending on how wet it is in the crawlspace, I might or might not use a sump pump. I would probably put plastic down to keep moisture in the ground in general.

I would also consider closed cell foam insulation on the joists and underside of the subfloor. Basically seal and insulate the wood. If foam is too expensive, maybe just spray with primer to keep the wood from absorbing moisture.

If the foundation walls are mostly above grade, I doubt they are a big source of moisture compared to the ground.


I researched this for one of our properties.  I still have to do it.

Do you have vents to the outside?  If so, close them and encapsulate the crawlspace to include it as an extension of the conditioned space above. 

Make sure you don't get water infiltration from the floor. Is the moisture coming from vents or ground water? 

Use mold foggers with mold control 2 and 3. Concrobium should do. Borax on the floor may be a good option. 

Floor: use reinforced heavy duty plastic,  heavy duty tape, and garden stakes. Bring plastic to 12 inches above grade. Seal all gaps. 

Walls: foam board to avoid condensation. Secure with adhesive and 1x3 treated wood. 

Do not insulate floor joists as the space will be part of the conditioned interior. 

Good luck and send pictures, 


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