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MoreCowbell 08-29-2012 10:44 PM

Vapor Barrier and Crawlspace Retaining Wall
1 Attachment(s)
Hello. I am approaching the point of my crawlspace-to-basement dig out project where I will be building my retaining walls. The retaining walls are what will be giving me extra ceiling height by allowing the floor to extend below the level of the foundation footings.

Naturally I want to have a good effective vapor barrier under the concrete slab. My question is how do I work a vapor barrier into the retaining wall. It seems like it should be run directly behind it and uder the upper slab (depicted by the red line in the sketch). However, is it really wise to run plastic between the wall and its footing? I'll have rebar tying it all together but still I wonder.

Any thoughts? Thanks.
Attachment 4225

P.S. The tan part between the retaining wall and foundation is my trying to represent gravel. I'll also have gravel under the floor slab and vapor barrier but I forgot to draw that in the picture.

nealtw 08-29-2012 10:59 PM

If you are hand digging the footing trench, you likely won't need forms, include the footing with your vapour bearier and if you are digging the whole thing by hand and it is undesturbed soil I wouldn't dig it deeper to put gravel in. I would put drain in the gravel at the level of the lower floor.

BridgeMan 08-30-2012 12:14 AM

A gravel base under the floor provides several advantages to not having one. For starters, it provides for more freedom of movement (called capillary wicking) for any moisture that gets under the slab, enabling it to migrate more readily to the drain system you install. Another reason is the ease of leveling the slab base, to make the slab a uniform thickness--spreading gravel with a rake before compaction is far easier than trying to uniformly redistribute undisturbed soil. Third reason would be gravel providing a buffer between the slab and soil having possible high sulfate or organic content, resulting in a slab less likely to deteriorate from the bottom up. Vapor barrier alone won't do it, as it won't be completely air-tight and is prone to puncture from fractured faces of the concrete's coarse aggregate during consolidation operations.

MoreCowbell 09-01-2012 06:28 PM

BridgeMan, I was planning to put in a gravel base under the slab for moisture reasons, but I never thought of the sulphate issue. Thanks for the tip.

Nealtw, we have very thick clay in this area of NC so I will definitely be able to cut my footing "forms" directly into the soil. That's what I did with my column footings and it worked great. Getting back to how the vapor barrier traverses the retaining wall, are you saying that I could just run it under the footing? That would seem to make a lot of sense in that it would not require a break between the footing and the wall, but my only concern is that I would probably need to be more careful digging my forms since any low spots would couse stretching of the plastic - although probably not enough to break a hole in it unless I get very sloppy.

Here is something else. I'd like to have footing drainage on both sides of the footing. Is it better to have separate pipes, running to daylight, or have a single drain pipe on either the inside or outside combined with a series of smaller pipes formed into the footing that lets water flow through the footing? I'm thinking two are better since I can install cleanouts would make maintenance of the drain system easier over the years. What do you guys think?

Thanks again.

nealtw 09-03-2012 08:44 PM

I would run two seperate pipes to day light. The one under the floor should never have water in it and would be a good indicator of problem with the other one. You could smooth your trench with sand so as not to dammage the poly. I would also use sand rather than grave under the slab for the same reason.

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