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Question DetailsAsked on 2/25/2014

How to get rid of mold in the crawl space that is too shallow. Then how to insulate the crawl space and water proof

Cabin built on two rows of cinder blocks. The crawl space was not properly prepared and therefore has shallow points that are causing the floor joists to have white surface mold which is affecting the integrity of the wood. I have jacked the cabin and now would like to dig out the uneven parts of the soil under neath in the crawl space, use bleach to rid the mold and then insulate and shield from further moisture issues.

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Voted Best Answer

Is the cabin conditioned year round or at all?

Are there any grading or moisture issues that are allowing bulk moisture into the crawl?

Conditioning the crawl is usually the best answer but if it is sporadic use and/or makes better sense to leave as a vented space, you need to do the following for optimum performance:

- Install a vapor barrier across the floor. Seal all piers and penetrations as well as seal to the stem wall.

- Insulate the underside of the floor and ideally thermally break the floor joists from the earth.

- This is best accomplished by covering the floor joist with a rigid foam and sealing all the seams.

Answered 6 years ago by WoWHomeSolutions


1) Digging out the soil near the timber and ensuring there is no wood path to the house is important, especially if in carpenter ant or termite country. Of course, if timber was sitting on the ground for foundation support you have to replace the dirt with concreteblocks or piers. There should be no less than 3-4 inches of concrete between soil and wood - preferably more like 6", and any posts or piers should be concrete or treated wood.

2) The foam board under joists idea by WowHome Solutions is an easy way, but you need to do something about carpenter ants and some other insects in that case, as they LOVE to build their tunnels and nests in foam board. That is why code requires a minimum of 4 inches of compacted soil cover over foundation insulation board in most areas. You can spray it with insect killer, which would have to be redone every couple of years. Another thing that works but is NOT legal from a code standpoint is spraying or smearing it with 100% coverage of asphaltic foundation bitumastic or asphaltic automotive undercoating. Not legal because it leaves a flammable material exposed to ignition.

3) Also, if you seal the joints, any water leak from above will pool and rot your joists rather than leak away, so there is a potential downside there to foam encapsulation. And of course, full bottom side foam means you cannot inspect or repair piping or wiring without destroying part of it. I would say more people probably use spray insulation on the underside of the sheathing, not thick enough to encapsulate wires and pipes running laterally under the floor.

4) One other risk of any type of foam insulation in a crawlspace, and the reason exposed foam is prohibited in many ocde areas, is pipe thawing operations can easily set fire to it. The above reasons are why fiberglass insulation is probably the most common for this use.

5) Be sure to screen all openings to the outside, so your insulation does not become an insect or rodent haven.

Answered 6 years ago by LCD

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