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Passive Project Under Slab Air Sealing

by Chad Ludeman on June 24, 2009 · 8 comments

in Uncategorized

We get asked a lot what the biggest difference is between the 100K House project and the Passive Project. The answer is always the same. Air Sealing.

Passive Project Foundation Crew Pouring

On a Passive House, we have to hit a very strict target on our blower door test of 0.6 Air Changes per Hour at 50 Pascals of pressure. This is no easy task. Every single seam must be caulked, or preferably taped, to hit this target. That all starts with the foundation. We’ll review a couple of the key details for the foundation air sealing below.

The basic concept is that we have to have a continuous air barrier around the entire home starting with the foundation. Last time we installed an air barrier, but did not bother too much with seams. This time we have to tape every seam with Tyvek Tape. The yellow poly you see is actually 15 mils thick to help prevent puncturing while we work on it.

Passive Project Foundation

The next thing to worry about is every electrical or plumbing penetration through the air barrier. You can see above that we eliminated the need to worry about our electrical penetrations in the image above by placing them in the concrete and above the poly.

The plumbing penetrations require a much more more thorough detail. We must encase every penetration through the air barrier and slab in PVC that is roughly 2″ larger in diameter than the penetration. After that is done, the poly air barrier is securely taped to the outside of the PVC. Once we have poured the slab, we then fill the inside of the PVC with spray foam. Any cracks that we don’t like are then sealed with caulk. The detail can be shown in the diagram below. Basically the poly is the air barrier until it reaches the penetration. Then the assembly shown must become the air barrier.

Passive House Slab Penetration Detail

Last but not least, we must wrap the poly up around the edges of the slab like shown below. You can also see the construction details in the previous post on slab insulation. When we set the SIPs walls, we will tape this poly to the inside layer of OSB on the SIPs that will then take over as our air barrier for the rest of the homes.

Passive Project Foundation

Questions, comments? This was a lot of work and coordination. If you look closely, you’ll even see our GC in the pics actually taping seams. The dedication. The commitment to extreme Passivity in home building. Fantastic.

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{ 8 comments… read them below or add one }

1 Kevin D June 25, 2009 at 12:34 am

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?

2 lavardera June 25, 2009 at 1:32 pm

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.

3 Jesse Thompson June 26, 2009 at 9:10 pm

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.


4 lavardera June 27, 2009 at 8:41 am

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.

5 Carol June 29, 2009 at 6:44 am

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.

6 Cliff Kornegay January 15, 2010 at 3:23 pm

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?

7 Slab Leaks January 24, 2010 at 7:59 am

Great post , thanks for sharing

8 Jerry Vanek May 25, 2010 at 1:28 pm

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.

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