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Question DetailsAsked on 4/12/2016

Should sand be applied prior to a Vapor barrier?

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Assuming you mean under a basement slab or crawlspace vapor barrier - then yes if your soil or base has any sharp stones or coarse sand in it. On pure clay or sandy/silty soil without material that could penetrate the vapor barrior a sand overlay is not necessary, though my practice is to grade the soil to drain to a point where it can easily be recovered - a corner sump or existing sump pump pit, so you don't get stagnant water smell. On impervious silty/clayey soil a sand layer over the top can promote that drainage.

Rake the soil to remove larger material and sharp stones, put down 1/4-1/2" mason's sand (NOT beach sand or coarse crushed traction sand - organic odor risk / sharpness issues respectively), and if going under concrete slab compact with plate compactor before putting down vapor barrier and slab. If open crawlspace, especially if route for access to HVAC/plumbing, I leave 6-12" extra vapor barrier each way so it is not pulled tight - makes it less likely to tear when crawled on.

And remember vapor barrier has to be fastened to the foundation wall to be effective - can use caulk at wall, VB, then overlying wood firring strip to hold the VB to the wall. There are also polyurethane sealants designed to seal VB to concrete/block, though I would not count on that alone - I use 2x2 cedar strips to hold it to the wall.

If a crawlspace VB and you have buried lines that might have to be dug up in the future under there, use wall anchors like screws into drywall anchor inserts that can be removed so the VB can be pulled back if needed in the future. If that possibility, fasten the VB with the caulked line under the top edge of the wood strip, so when it is removed the VB can be cut at the bottom of the caulked edge to loosen it up, then when work is done VB can be reapplied to the cut line with a new caulk line and firring strip can go same place a second or even third time if needed, without drilling new holes in the foundation for the screws. Another reason for the slack in the VB also.

If crawlspace/concrete slab underdrainage might run through the sand, be sure to put a permanant (not garden type) filter fabric and fine gravel drain at the downflow end BEFORE it gets to the sump pump sump, so the sump pump is not pumping sand.

Answered 4 years ago by LCD

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