Moisture in the Crawl Space

December 18th 2003
Moisture in the Crawl Space

Q: We have an 18 year old split level home. It has a large 5 foot high crawlspace. The floor in the crawlspace is not proper concrete but seems to just have a light pour that is uneven and rough. When we moved in there was a torn and tattered vapour barrier in place. We replaced the vapour barrier with new 8mm barrier and tuck tape. We were told to place 2 x 4's around the outside perimeter.

The crawlspace has three outside air vents that were all closed off and covered with insulation. There is one furnace vent into the crawlspace that was also closed. There has always been a very strong musty smell emanating from the crawlspace. We have recently had our furnace replaced and the HVAC guy said we had an excessive moisture problem in the crawlspace.

There is no visible moisture on the walls but we can see a lot under the vapour barrier. He suggested that we open two of the outside vents as well as the furnace vent and to run the furnace fan continuously. We have done this and it has improved to some degree. He also suggested sealing the vapour barrier with a silicone sealer around the foundation walls. It has also been suggested that we put a new floor down there which sounds expensive. Any help would be appreciated.

Heidi, Coquitlam

A: You make no mention of insulation on the concrete walls or between the joists on top of the concrete walls in the crawlspace. If there is none in place I would suggest that you apply 1 1/2 " rigid foam insulation using a foam adhesive. Then apply floor 8 mil poly up onto the foam and seal with acoustical adhesive. Don't worry about the moisture under the poly; it is doing its job. Crawlspace vents should be open in the summer and closed in the winter. I would not recommend placing more concrete over the existing floor.