On concrete walls that are fully exposed (above grade) for a new residence, should there be an air space?
We are constructing a new dwelling in Climate Zone 4. Site constraint will have one side of the house built in to a hillside, so the exterior wall is retaining, and will be poured concrete. The owner likes the modernist look of exterior concrete walls, so we are planning to use poured concrete walls on the other elevations as well. I am planning on dampproofing the exterior of the retaining sections of wall, and framing the interior with 2×6 studding and insulation, like a finished basement wall condition.
My question is, on concrete walls that are fully exposed (above grade) for a new residence, should there be an air space and interior moisture barrier behind the concrete walls, and therefore weeps in the concrete walls as well?
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Replies
Jeffrey,
First of all, I strongly urge you to use a waterproofing system, not a dampproofing system, on the below-grade concrete wall. A waterproofing system usually includes a full dimple mat from footing to grade.
Second, the below-grade wall should be insulated like a basement wall. Your insulation plan -- "framing the interior with 2x6 studding and insulation" -- sounds a little worrisome. If you are insulating these walls on the interior, you need to install rigid foam or closed-cell spray foam on the interior side of the concrete. For more information, see How to Insulate a Basement Wall.
Third, the below-grade concrete wall is likely to be cold and damp. These conditions will occur at the corners of the wall, and therefore the walls that are 90 degrees to the below-grade wall will share, to some extent, the temperature and moisture content of the below-grade wall (due to moisture wicking and conduction). For that reason, I strongly suggest that you insulate all four walls -- not just the below-grade wall -- on the interior with rigid foam or closed-cell spray foam. The foam insulation layer needs to be between the concrete walls and your interior 2x6 framing.
If you're already building structural concrete walls, why bother with the extra expense and fiddliness of building a separate 2x6 stud wall, thereby negating many of the advantages of building with a durable, fireproof, moldproof material? Consider pre-manufactured insulated concrete sandwich panels or shotcrete panels instead--these will give you concrete on the inside and outside, with a layer of rigid foam insulation between them in the middle. It'll certainly be faster to build than erecting two different walls with totally different construction methods and detailing. And you can do a fast skim coat of gypsum plaster on the interior walls instead of a full drywall job.
Jeffrey,
The concrete walls above grade can be exposed to the interior by insulating and cladding the exterior. This house on Bowen Island is detailed that way:
https://www.greenbuildingadvisor.com/articles/dept/green-building-blog/exploiting-elements-passive-design
Jeffrey,
If you take Malcolm's advice, just remember that all of the insulation (on all four walls) has to be on the exterior side of the concrete -- or, alternatively, all of the insulation (on all four walls) has to be on the interior side of the concrete.
You can't mix-and-match, with exterior insulation on some walls and interior insulation on other walls, or you will get horrendous thermal bridging at the corners.
Martin,
Looking more closely at the house i linked to, it doesn't appear they they addressed the thermal bridging where the concrete walls move from the exterior to the inside - although maybe I'm missing something.
If sticking with the original concept (rather than a concrete foam sandwich or SCIP), in a zone 4 climate with structural concrete as the exterior siding it's far better to meet code for wall-R with an R13+ 5 solution than 2x6/R20.
Using 1.5" of EPS or 1" polyiso against the concrete and R15 batts (if not blown fiber insulation ) in the 2x4 cavities, the foam becomes the necessary vapor retarder protecting the studwall from the high moisture drives of the concrete, and you don't need an interior vapor retarder to protect from wintertime moisture accumulation. It's a thinner, somewhat higher thermal performance and far more moisture resilient wall than using 2x6 framing, no matter what you have for cavity fill, and even if you built it with a gap between the concrete & studwall as a capillary break.