Comments on: Passive Project Under Slab Air Sealing

By: Jerry Vanek

Jerry Vanek — Tue, 25 May 2010 17:28:59 +0000

The poly sheet should be set on a level “building layer”, usually sand. You can also put down a layer of rigid foam insulation board instead of the sand, and set the poly sheet on top of that.

By: Slab Leaks

Slab Leaks — Sun, 24 Jan 2010 11:59:34 +0000

Great post , thanks for sharing

By: Cliff Kornegay

Cliff Kornegay — Fri, 15 Jan 2010 19:23:42 +0000

I will be digging out my basement in a DC rowhome at some point this year. I am concerned about pouring concrete on the layer of poly and having holes poked in the sheet. Anybody considered whether or not a thin layer of stone dust would be sufficient?

By: Carol

Carol — Mon, 29 Jun 2009 10:44:59 +0000

I had never heard about Air Sealing before so was very interesting when I came across this informative article; thank you for sharing this and your excellent photos.

By: lavardera

lavardera — Sat, 27 Jun 2009 12:41:26 +0000

I have tremendous respect for building sciences inc, but in this case the assertions in this paper do not hold up. The sand layer will dry out with the slab curing. The only way it can become a resovior for water is if there is some external source of water – high ground water? Faulty placement of irrigation systems? Interior slab continuous to exterior? Those problems will put moisture into your slab sand or not. Although the sand makes a place for more water, the sand is not the problem there. The problem is the low elevation realtive to ground water, or an edge detail that exposes the slab edge to adjacent soils. The paper is weak in this regard as they never qualify the source of this water. They make it sound like it will always get wet, when the truth is the wetting would be due to some other problem. They also say the damage caused by concrete placement is not an issue, and I would agree that if you are not taping the seems a few small holes hardly matter.

By: Jesse Thompson

Jesse Thompson — Sat, 27 Jun 2009 01:10:40 +0000

I don’t think anyone is worried about air flowing through the concrete, it’s the pipe penetrations and slab edge details through the concrete that allow air leakage. Since you have to have a vapor barrier under the slab to prevent moisture from wicking up through the concrete, you might as well tape it well so it can also serve as an air barrier.

On one of our recent tight houses (<1.0 ACH50) during a blower door test, the contractor was surprised to see air pulling up from under the slab edge where the rigid was turned up for a thermal break. The crushed stone that lets you pull radon out from under the slab also was letting lots of air leak up under pressure.

Greg, these days the Building Science folks are strongly recommending not to pour concrete on a sand layer, warning that the sand pulls water from the wet concrete and changes the mixture, and if the sand gets wet before the pour, it will trap water under the slab where it can later wick up and damage flooring, mildew, etc.

Link:

By: lavardera

lavardera — Thu, 25 Jun 2009 17:32:01 +0000

A vapor barrier should be standard fare on a slab on grade, but its typically 6 mil poly without taped seams. And unless the contractor lays down a protective sand layer before the slab is poured – seems they never do even when you call for it – sharp edges on the concrete’s gravel aggregate will often poke holes in the plastic when its ~~dropped~~ placed (which is why the plastic should be covered with sand. Also it typically is not returned up to the house walls which in theory makes the house vulnerable for air leaks where the wall framing or panels meet the foundation wall.

Now an air barrier is typically what you call the house wrap which goes on the cold side of a wall in the north east. It is supposed to stop air but pass vapor – so that water vapor is not trapped in your walls, where it can condense and cause rot. A vapor barrier goes on the warm side of the wall and prevents moist air from the interior from passing into the wall and condensing to water as it moves from warm to cold. A vapor barrier would block air too, but also the water vapor.

So this is on the warm side of the slab insulation, so its in the position where you would put a vapor barrier, not a vapor permeable air barrier.

So is this a better vapor barrier, or an air barrier? I think its a vapor barrier, but detailed and installed to make a continuous barrier with the one in the walls.

I should note that in our region – Phila – we build for the winter condition even though the vapor pressure situation reverses itself. I’m not sure where the breaking point is but in the south you would build for the summer condition with the vapor barrier on the exterior.

By: Kevin D

Kevin D — Thu, 25 Jun 2009 04:34:12 +0000

Air doesn’t flow through concrete, so “underslab air sealing” is a bit of a misnomer. Isn’t the goal to seal out water vapor? How many CFM of air could pass through a 4″ slab over 10″ of foam?

Why isn’t taping the seams of the sub-slab insulation good enough?