Should rigid foam extend the full depth of basement foundation?
alumniu
| Posted in Green Building Techniques on
I met with a builder and in his quote he accounted for exterior rigid foam extending 3′ deep below grade. He said that foam is not needed any deeper because the ground is a constant 55 degrees.
I’ve seen several posts on here about going full depth and insulating below basement slab, but I never thought about this.
I am in Northern Illinois climate zone 5 for reference.
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Replies
Adam,
Most building codes require basement walls to be insulated. As far as I understand the building code, the entire wall has to be insulated -- not just the upper half of the wall.
A subsoil temp of 55F is still a significant heat loss through the slab, and the design frost depth for N.IL is much deeper than 3', probably about 5', which means the soil in the yard will be substantially colder than 55F during the depths of winter on into the spring. Take it all the way down to the footing.
If that's the only basement insulation, continuous R10 would be required IRC 2012 code-min in zone 5. If insulated on the interior as well as the exterior you can use thinner exterior foam. R4 on the exterior with an interior side studwall with R13 batts would beat code-min, and would be sufficient exterior-R to limit condensation issues at the concrete/fiber interface for the above-grade section. The concrete can adsorb quite a bit of moisture and is not damaged by moisture. But a layer 1/4" fan-fold XPS between the studwall & concrete would be good insurance, since it would also limit the release rate of groundwater in the concrete into the studwall cavities, and block capillary wicking directly into the studs.
His facts are a bit off - in your area it would be about 50 degrees, and you have to go about 30' down before it's constant. Just 3' down might vary from 35 to 65 across seasons.
But even ignoring that, most people like to keep their houses warmer than 55 degrees, so it's still not a strong argument to omit insulation entirely, even if it means less of it is cost effective.
Thank you all so much for your insight. All very good points. I will discuss this with my builder.
When Dana says " R4 on the exterior with an interior side studwall with R13 batts would beat code-min, and would be sufficient exterior-R to limit condensation issues at the concrete/fiber interface for the above-grade section. " he means if you run a dehumidifier in the basement. I recommend avoiding a concrete/fiber interface so that you are less dependent on the dehumidifier, but I agree with him that a dehumidifier is a good idea in a basement regardless.
Thanks Charlie...From what I've read (and like you said), concrete to fiberglass in basement application is a big no-no, and I don't want to rely on a dehumidifier for protection against moisture damage. I am hoping to stick to the 5-10-20-40-60 rule for insulating (the 20 being the R20 for basement walls). And I really like the idea of doing all basement insulating on the exterior to reduce thermal bridging through concrete, increasing the thermal mass inside, and having the ability to easily monitor and inspect the condition of the concrete as the house ages. I am still in the process of getting bids, so I don't know how financially feasible this goal is at the moment.
You don't want fiber in direct contact with the concrete, since it can wick ground moisture, but it's fine to put 6mil poly sheeting between the concrete & fiber.
Dehumidification wasn't part of that calculation- I was referring only to wintertime moisture accumulation at the above-grade portion of the foundation. Keeping the cold side of the fiber insulation at a sufficiently high average temp with exterior foam mitigates mold conditions from being created by accumulating wintertime condensation inside the stud bays.
With R20 whole-wall performance basement walls and R10 under the slab you won't have summertime condensation issues to worry about. R20 all on the exterior is a lot of foam (5" of EPS), but if you build out the interior side with a 2x4 24" o.c.studwall w/R15 batts the studwall lay'ers performance will be R10-R12-ish, which means your exterior EPS could shrink to 2-2.5" and still get the R20 performance. In fact it would be a thermal mass-enhanced stackup, since about half the R would be on the exterior.
A typical insulated concrete form (ICF) has 2.5" of EPS on both sides, which comes in at about R22+ whole-wall, with the R-value of the concrete and interior gypsum factored in, and it may be cheaper/easier to go that route for the foundation.
But if you are insisting on bare-concrete on the interior for inspection purposes, you're pretty much looking at fat-foam on the exterior as your only R20-ish solution. It's buildable, but it adds some wrinkles to the details of what happens at the top of the foundation. You can cantilever the first floor out a few inches if needed to get the siding proud of the foundation foam layer- it's not hard, but has to be thought out and specified carefully for the framing crew to get it right.