2×6 stem wall detail options
Hello everyone,
I’m in the early stages of designing my future home and ran into a situation where I’m looking for some input. I’m wondering what the best option for me would be at the intersection of the radiant floor slab, foundation stem wall and 2×6 exterior wall. I’ve attached the 3 options I’m considering.
Option A is what I had originally envisioned. I work in commercial construction, and this is close to what I’ve done on previous projects, such as a hangar I just built. But that hangar didn’t have hydronic heating in the slab and was only meant to be kept at about 55 degrees in winter for storage. So then that got me thinking that since this is a residence where I will be heating into the upper 60’s during the winter I may want to revisit this detail. After doing some research i started to be a little concerned about heat loss at the 12 inches of concrete stem wall. That’s where I came up with Options B & C. I like Option B because I don’t need to change any detailing on the exterior, but I have some concern’s whether the engineer will go for overhanging the 2×6 wall 1.5″ with it only sitting on a 4″ stem wall. Option C sounds better structurally in my opinion and it leaves me the option of continuing the 2″ XPS all the way up the exterior if I can find it in my budget, but I have major concerns on how to cover up this exterior insulation and protect it from damage and also make it look decent.
And for reference, this house has a garage and some living space on the slab level with the majority of the living space on the second floor. And I’m located in Maine, zone 6.
I’m wondering if my concern about heat loss in this area is valid or a none issue, and maybe the overhang in Option B is possible, I’ve just never seen it done before. Any help is appreciated, thanks!
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Replies
Sturgeon,
Wall detail A is clearly out, since the above-grade portion of the concrete stemwall is totally uninsulated.
Either Wall detail B or C is better, because you have made an attempt at insulating the above-grade portion of the stemwall. Overhanging a 2x6 plate by 1.5 inch is usually not a problem; I've done it several times myself. That said, it's always a good idea to run it by your engineer.
The basic problem with details B and C is that you have chosen a low level of insulation for the above-grade portion of your concrete wall. If you call the concrete stem wall a "mass wall," which as far as I know is permitted, the prescriptive table still requires that the wall have a minimum R-value of R-15 if you install exterior insulation, or a minimum R-value of R-20 if you install interior insulation. Two inches of XPS is only R-10.
Finally, I'm a strong advocate for the use of thick horizontal rigid foam under any rigid slab that includes hydronic tubing. In a cold climate, my usual advice for anyone designing a slab with radiant heating is to spring for a minimum of R-20 of horizontal rigid foam under the entire slab. For more information, see "Insulating a slab on grade."
Nowhere in the US does the IRC prescribe more than R15 for basement or crawlspace walls. (Even finished basements.) Only fully above grade walls would need to be treated as a "mass wall".
In zones 4C or zone 5 & higher (including zone 6 Maine) it's continuous R15, on either side of the wall. (Or 2x6/R19 )
In zone 6 for fully above grade conditioned space mass a continuous R20 required if insulated with more than half the R on the interior, but only R15 if at least half the R is on the exterior.
For slabs in zone 6 the IRC calls out R10 slab edge insulation to a depth of 4' below grade.
See TABLE N1102.1.2 :
https://codes.iccsafe.org/public/document/IRC2015/chapter-11-re-energy-efficiency
Dana,
It looks to me like the top 16 inches or so of this stem wall is above grade. Not only that, it's above the interior floor level -- meaning that if the wall is cold, it cold be a comfort issue.
Thanks for the input. And yes there would be roughly 12-16” of the concrete wall (mass wall or stem wall, not positive on the specific deference’s in terminology). What you’re saying makes sense given it’s a conditioned space that there should be a higher R-value. I was thinking back to all the basements I’ve seen around here where there is little to no insulation of the concrete above grade below the stud wall above, but those were unconditioned/unfinished basements. So maybe I’ll modify detail C to include R15 or 20. I’m still cautious of how to clad or protect this insulation above ground, but I’m sure there are good options out there. As for increasing the R-value below the slab to R-20, that’s an interesting idea. I will look at that as well when I tackle the issue of a thermal break at the garage doors and whether to do that or not. But that’s an issue for another day.
Sturg,
Here is a link to an article that provides a list of materials that can be used to protect rigid foam installed on the above-grade portion of a concrete foundation: How to Insulate a Basement Wall. (Scroll down to the section of the article below the heading, "If I insulate on the outside, how should I protect the above-grade foam?")
Sturg: I'd surely consider adding exterior foam to the walls. It looks like you plan a conventional 2x6 wall with insulation between the studs and nothing to mitigate thermal bridging. That's pretty meager for Maine. You can save a bundle by getting reclaimed foam instead of new xps. With better insulation and good air sealing, you can skip the radiant heat in the floor.
Sturg,
Option one has exposed concrete at the lower part of the wall with no insulation.
Option two would not meet our code because at 4" the stem wall is too narrow to extend up that far past the full width foundation.
Option three again has the concrete wall exposed at the floor level, and a thermal bypass through the wall to the interior.
Apart from aesthetic concerns, the exposed concrete makes detailing of the slab edge, and exterior doors, and transition to the drywall above fairly difficult.
Another concern for me is the height of exterior grade relative to the slab. It makes for a nice walkout transition between the inside and out, but means all water management has to be perfect or you will end up with moisture infiltrating to the interior.
Nobody has mentioned it yet, but...
XPS will lose performance over time as it's climate damaging HFC blowing agents diffuse out over time. At 20 years for 2" goods no vendors will guarantee more than R9, and at 5 decades you're probably in the R8.5-ish range.
EPS is blown with much more benign hydrocarbons (usually a variant of pentane, about 7x CO2), most of which leaves the foam (and is recaptured) at the factory, and has stable performance over time. Under a heated slab in Maine 4" of Type-VIII (1.25lbs per cubic foot density) or Type II (1.5lbs density) would run about R17 and make good lifecycle economic sense no matter what you're using for heating fuel, even for virgin-stock foam. If using reclaimed roofing foam (at less than 1/3 the cost of new foam) taking that up to 5-6" is a no-brainer. Most roofing foam is Type-VIII, which is just dense enough to be a "walkable" roof under flexible membranes, and is just fine under slabs, and is commonly available at foam reclaimers.
Newbies often get concerned (for no good reason) about the lower compressive specs for Type-VIII EPS compared to 1.5lb XPS (~15 psi rather than ~25 psi), but under a structural floor slab those numbers are completely irrelevant. Even 10 psi goods won't be overloaded under a slab, but would be a bit fragile for walking on before the slab is installed.
If 4" of exterior EPS is deemed too awkward to build, at 3" you'd be at R12.5+, and with 3.5" it would be R14.5+ which isn't quite R15, but close. You might still get to code compliance on the average U-factor of the wall in it's entirety, if the framed wall portion is sufficiently below the U0.045 max for zone 6:
https://up.codes/viewer/wyoming/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4
While 3" of XPS would get you to the bare minimum R15 from a labeled-R point of view, in 50 years it's not going to be more than R13, despite the "lifetime warranty" of at least 90% of labeled R from some vendors. (Go ahead- try to collect on that one- I dare you! :-) )
Of course you COULD pour the stemwall in a minimalist 2.25" + 2.25" Type-II EPS insulated concrete form (~R19), but you'd have to work out both the interior and exterior details. In some ways that is preferable, since it puts a thermal break between the interior, the slab edge, and the thermallyconductive stemwall's connection to the cold footing.
Sturgeon,
For more information on the issues raised by Dana, see "Choosing Rigid Foam."