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Air barrier continuity

I am working on total renovation of a 1950's ranch with a basement that walks out to grade. The roof and framed wall assemblies described below describe how the home will be renovated.

1) Roof assembly: 2x10 rafters, 1x4 board sheathing, peel-and-stick waterproof membrane, 8" reclaimed polyiso ((4) 2" layers), OSB, tar paper, standing seam metal roof
2)Exterior framed wall section: 2x4, OSB, tyvek drain wrap, (2) 2" reclaimed polyiso, 1x4 rainscreen, wood siding
3) Exterior CMU walls: 2x4 framed wall, 2" reclaimed polyiso, drainage mat, CMU, parging

The air barrier is not difficult to make continuous from the roof to the wall, as we have cut off all of the overhangs and will wrap the peel and stick membrane over the tyvek drainwrap. At the rim joist, we are applying peel and stick membrane to bridge the transition from framed wall/floor system to CMU (above grade). The membrane covers the top 3" of the CMU and then covers the rim joist and six inches above floor level. The tyvek drainwrap above will then over lap that membrane, all the way to the bottom of the rim joist. Because we have to insulate the basement from the interior I am wondering if I need an air barrier on the exterior above grade CMU wall? I am concerned with air passing through the pores of the block, getting into the block cavity (which is open at the top inside the basement) and making it's way into the house. My thought is I can either make my air barrier at the exterior face of the CMU or I will have to block the top of the block wall on the interior and spray foam the rim joist. My budget is tight and my gut says that an exterior air barrier would be more cost effective. Maybe a fluid applied air barrier? Thanks for any feedback / ideas.

Asked by Charles Chiampou
Posted Sat, 06/21/2014 - 10:01

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3 Answers

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1.
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Charles,
If I understand your question correctly, it concerns the question of whether it makes sense to cap the cavities at the top of a CMU wall. The answer is yes, you should definitely seal the top of these CMU cavities -- to prevent air leaks and to limit convective loops in the block wall.

The usual method is to stuff something in each cavity -- usually a scrap of fiberglass insulation -- and then to seal the top of the cavity with mortar, grout, or spray foam.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 06/22/2014 - 05:08

2.
Helpful? 0

Thanks Martin that makes sense.
I was also wondering if a fluid applied air barrier on the exterior of the above grade CMU would stop any air infiltration before it can even get to the cavities which would mean we wouldn't have to do the cavity sealing work on the interior. I am going to paint the CMU anyways and I have used some products in the past that air seal and can be tinted (by Sto Corp I think). Thought I might be able to kill (2) birds with one stone that way and not mess with the cavities.

Answered by Charles Chiampou
Posted Sun, 06/22/2014 - 05:21

3.
Helpful? 0

Charles,
Whether or not you decide to install an air-sealing product on the exterior side of your CMU wall, it's still a good idea to cap the top of the cavities.

There are a great many products available that might be used to seal the exterior side of a CMU foundation wall, and I haven't used any of them. I will leave it to other GBA readers to suggest the best products for this application.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 06/22/2014 - 05:32

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