Insulating basement foundation
Greetings,
I have read a lot on GBA and a few other trusted resources online and am still not clear. I have an unfinished basement below grade in zone 4 (Minnesota). House was built in 2015. Builder sprayed a black coating on the poured concrete foundation and then applied a 1.5″-2″ thick rigid foam insulation to the exterior of the foundation before backfilling (completely below grade basement).
As I go to finish the basement, should I be insulating the inside of the foundation also? From reading here is sounds like I need to keep a side uninsulated and without a vapor/water barrier to allow moisture to move one direction. Since the builder already insulated the exterior, sounds like I should not do anything beyond 2x4s and sheetrock along the interior foundation.
Thank you.
(of of many sources: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall )
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Concrete doesn't need to dry. It won't rot, so it doesn't matter if it's sandwiched between layers of foam.
Bob,
Whether or not you decide to add a layer of rigid foam on the interior depends on your R-value goals and your budget.
How much R-value is provided by the exterior rigid foam? Does the existing exterior rigid foam meet minimum code requirements?
Any location in MN is either US DOE climate zone 6 or zone 7, not zone 4 (what's that, a plant hardiness zone?)
Minnesota has very specific foundation insulation requirements that diverge (by quite a bit!) from the generic IRC 2015. The prior versions of MN code were so different that even insulated concrete forms were technically illegal (that has since been fixed). Conditioned basements are now required to have R15 continuous insulation from the footing on up, including above grade, at least R10 of that has to be on the exterior. Interior studwalls (insulated or not) may not be in direct contact with the foundation.
Start reading at R402.1.1.1 through R402.1.1.8, but also pay close attention to R402.2.8 :
https://codes.iccsafe.org/public/document/code/357/5875961
So if you have 1.5-2" of foam on the exterior it's not enough to meet the R10 exterior requirement, and if it stops at grade, it's an even bigger issue. Does it go all the way up to the foundation sill, or does it stop at grade?
In retrofit situations the inspectors tend to be lenient, but it's wise to come up with a plan, and negotiate it with the inspector first. I suspect most would allow you to use 1.5" interior foil faced polyiso or EPS trapped to the foundation by an R13 batt insulated studwall, which would exceed code min thermal performance, and would be relatively moisture safe as long as they don't demand an interior side polyethylene vapor barrier.
Clarifications:
I quoted the wrong climate zone. Correct climate zone is 6. Basement r-value 15 continues or 19 cavity.
The builder appears to be using 2" thick Polyisocyanurate foam. Yes it does extent above the grade up to the bottom edge of the siding (which starts about 10"-12" from grade).
The builder of the home (Cal Atlantic) is still building homes in the neighborhood. They seal and insulate the foundation on the exterior and when they finish a basement they toss up 2x4 walls with 1/2" sheetrock without any vapor barrier or stud cavity insulation. I will confirm with my city building department before starting, but I am confident not insulating the inside is likely fine for achieving building code compliance.
Also, I do not look to Cal Atlantic as any kind of good steward doing what is in the best interest of energy efficiency or long term maintenance. They pound out homes at a low price that achieve building code requirements.
What I am trying to identify is whether there is value to adding XPS insulation on the interior of the slab. I have it priced out at about $800 in materials (xps, anchors, adhesive, tape, etc) + labor. Is it worth adding 2" of XPS inside or is it a case of diminishing returns?
Bob,
If you are talking about the slab rather than the walls, the answer depends on whether there is any rigid foam insulation under the slab. If there isn't any insulation there, you definitely want to install a continuous layer of horizontal rigid foam (either EPS or recycled XPS) above the slab before installing your subfloor.
The main reason to add rigid foam above the slab is to address moisture and odors. Cold concrete slabs become condensing surfaces during the summer, and this leads to a musty odor.
Sorry I should not have used the word "slab". I meant the poured wall foundation. What I am trying to identify is whether there is value to adding XPS insulation on the interior (conditioned) side of the poured concrete foundation walls.
But since we have brought up foundation slabs...
If I do need to insulate the conditioned side of the slab, would I place the bottom plate of the stud wall on top of a foam insulation or go between the "cavities".
I am fairly certain there is a layer of foam insulation under (non-conditioned side) the basement floor slab.
First, polyiso should NEVER be installed with ground contact, since it's mildly hygroscopic and will load up with moisture over time losing a good part of it's thermal performance, and in your climate it will also become severely degrated by freeze/thaw damage once the moisture has penetrated the foam. The exterior insulation needs to be polystyrene, preferably Type-II EPS (or denser), which runs ~R4.2/inch.
If it's on the exterior side of the foundation let's hope the stuff they're installing is polystyrene, not polyisocyanurate!
XPS has a higher labeled R/inch, but that performance uptick is temporary, and artifact of it's climate-damaging HFC blowing agents. As the HFCs leak out over time XPS performance will drop to R4.2/inch over a few decades. From a design point of view, over the lifecycle of a house assume no more than R4.2/inch.
Assuming the exerior side foam is polystyrene, you're at about R8.4 (labeled) but closer to R9 for a wintertime performance average. That is sufficient R-value for dew point control on an interior side 2x4/R13 wall. MN code doesn't allow you to install the studs in contact with the concrete, so inserting a half-inch of UNFACED foam (any type) would allow you to go ahead with an insulated studwall to hit better-than-code total performance. With no studwall, 1.5" of unfaced EPS strapped to the wall with 1x4 furring through-screwed to the foundation with wallboard fastened to the furring takes it to code minimum, with some capacity for the foundation to dry toward the interior, thereby protecting the wood foundation sill and band joists. If there is a good capillary break between the foundation & foundation sill to block foundation moisture from getting into the sill ( such as an EPDM sill gasket or metal flashing over the entire top of the foundation) you can get to code min with a continuous 1" of foil faced polyiso on the interior side of the foundation.
Reclaimed roofing foam is pretty cheap, so 2" of unfaced EPS on the interior may be the right way to go. Going any thicker than that, or using XPS or polyiso would require verification of the capillary break at the top of the foundation. (A think pink foamy sill gasket is NOT good enough.)
I seal off the basement and dehumidify to 55%. The epoxy coated un-insulated basement slab (in MI) never gets close to condensing temperature. And no odor.
If I insulated above the slab, the slab surface would be near condensing temperatures (ie, very high %RH, which is what mold likes) - condensing without dehumidification. Spills from above* or water from below would also cause standing water. And covered with foam insulation, none of these would dry quickly.
* one upstairs overflowing toilet, two plumbing leaks, one AC condensate issue.
Bob,
Here's the short version of Dana's advice: Yes, it probably makes sense to boost the R-value of your basement walls by adding interior insulation. Now is a good time to do it. There are several possible approaches that can work. Whatever you decide to do, it can't hurt to check with your local code authority to make sure that your plan meets code requirements.
Thank you everyone. I really appreciate your feedback. You are helping me finish my basement out as a much more energy efficient basement. Spoke with some spray foam pros and will be doing top of rim joist all the way the way down the foundation. Stud wall will be space off the foundation and the spray foam will protrude about a 1/4" into the stud bay. Pretty excited.
Example: http://foundationhandbook.ornl.gov/handbook/section3-2-concrete-foam.shtml
This assembly meets the needs of draft blocking, but not technically fire blocking. My local building inspector has commented he will accept the draft blocking. I could add fire blocking from the stud wall to the sill plate, but that would create a thermal bridge and interrupt the "blanket" of spray foam.
Is the fire blocking worth compromising the continues spray foam?
Bob,
Are you planning to use open or closed cell foam? Will you install the stud wall before the foam goes in?
Closed cell (Spraytite by BASF). Stud walls installed before foam is sprayed.
Bob,
If you want to include the fire blocking, the detail below shows how it's done.
If your local inspector isn't insisting on it, the decision is yours. I wouldn't worry about the thermal penalty; whether it's worth the hassle depends on whether or not the dangers of house fires worry you enough to interrupt your sleep.
[Credit for illustration: Journal of Light Construction]
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