DIY Doctor

Vapour barrier in crawl space?

Postby Jemster » Tue Sep 15, 2020 7:49 pm

We live in a 1920s house with an unfinished crawl space about 3-5 feet high under the entire house. All except one room has a dirt floor, the last one being concrete floored.

The dirt floor is damp, salts have risen on it leaving white on the earth, there’s a variety of rubbish in there built up over the years, everything from a pram wheel to old lead plumbing and loads of old snail shells...

Of course there’s a light smell of decomposition that is detectable in the house, the floor timbers are sound, I suspect it’s just from the damp soil.

I was going to clean out, skim the top soil layer, remove everything I could find... then I started wondering is it a good idea to add a vapour barrier to the floor? Not a thoroughly sealed solution, but up to the walls and overlapped everywhere possible.. or would this make the situation worse? We have already increased air vents down there.

Seems really hard to find any info on crawl spaces that isn’t American... any experts in the U.K.?
Jemster
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Postby stoneyboy » Wed Sep 16, 2020 8:40 pm

Hi jemster
Good idea to skim the top surface, suggest you then cover with 50mm concrete mix - use a ballast with smaller stones in it.
With your increased ventilation the musty smell should reduce over time but it may be years.
Regards S
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Postby Jemster » Thu Sep 17, 2020 8:25 am

Hi stoneyboy,

Thanks for the reply - the dirt floor is very uneven - it's pushed up to the walls at the edges and has random surface variations that I don't think could be levelled out too easily. Makes me think if I attempted a concrete screed over it, it would be in no way flat.

The house next door has concrete in their crawlspace, but from time to time they get groundwater pushing up through it. I have seen a small puddle in the middle of our crawlspace after very very heavy rain once so not sure how good concrete would be in this situation?

I was thinking a vapour barrier would just help keep that moisture down in the ground, but has it a negative effect, preventing the ground from drying out? Not sure.... and I don't want to make the situation worse by trying to help it :)
Jemster
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Postby stoneyboy » Thu Sep 17, 2020 8:21 pm

Hi jemster
I don't think covering the sub-floor with a vapour barrier will have a positive effect, it will trap in moisture and possibly create fetid areas below the barrier.
A layer of concrete does not need to be flat, it will be porous (provided you use a dryish mix) and will help disperse any moisture even if this is causes by rising ground water.
Regards S
stoneyboy
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Posts: 3540
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