Insulate under your exterior walls?
While working on designing my house, I thought about installing 3″ x 6″ x 8′ strips of 100 psi blue foam under my interior, exterior wall (double-stud wall) to minimize conduction through the bottom plate to the concrete. The compression numbers were not there, so I abandoned the idea; for a smaller house the numbers would work fine. Then I thought of two alternatives, and wonder if they would be worth the PITA to utilize either one. The first is a strip of plastic honeycomb, filled w/ foam, 2-3″ thick. Those would have to be invented, as far as I know, or custom made on the job in some way. The other is to use short pieces of PT 2×4 under each stud, filling the void between each when you dense pack cellulose for insulation. This would require using an exterior air barrier, like plywood on the outside of this interior, exterior wall. Is it worth even thinking about minimizing losses through the bottom plate?
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BTW; This would not apply to houses that have copious quantities of foam under and outside the edge of the slab.
John,
Not sure the exact details of your foundation/wall, but I liked the foundation of the Montague home.
http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/Plans.htm
No foam is outside the house, so no need to protect it from mechanical damage, UV degradation, ants or termites. There is 4-6" of foam between the edge of the slab and the foundation, plus the outer wall plate is discontinuous with the inner wall, so minimal conductivity.
Now I just need to find out whether a foundation contractor will cheerfully put a 4"x10" "rabbet" in the top of a stem wall.
Jack: My walls/foundation will likely be similar to Thorsten's Sunrise House. However, this is an interesting twist on things (moving the concrete to the outside) and that may be a more appropriate method for my situation than Thorsten's. Thanks.
Jack: Why even have the "rabbet"? Run the concrete straight down to the footer, and foam right beside it. I don't see any real advantage to that rabbet, except to hold up the foam, which gravel could do. Less hassle that way, it seems to me.
I didn't look at the link... what's the 4x10 rabbet for?
see the link; a way to foam outside the slab.
OK, overcame laziness and took a look. You could rip 9.5" strips of 4" foam, install them in the forms before the pour, and then just leave them there permanently. It wouldn't take much effort at all, assuming the rebar and anchor bolts were kept out of the way. I think John's idea is probably better, though, just lay the foam next to the stemwall after the pour.
The advantage I see to putting the foamed rabbet in the concrete foundation, rather than the easier solution of running the foam right next to the inner surface of the foundation, is that it allows the outer 4" wall (of a 12" double wall) to rest on the foundation without any need to overhang the foundation to cover exterior foam, yet allows the inner 3" or 4" wall to rest on the slab, with there still being room for 4" of foam between the edge of the slab and the stem wall to break thermal conduction between foundation and slab, and also breaks thermal conduction by not using a full-width plate. This wall's width is: 4" concrete wall (and outer wall of double wall) + 4" foam + 4" inner wall = 12" total
If you place 4" of vertical foam next to a typical 8" poured foundation, and if you want the outer stud wall to rest on the outer edge of the foundation, and if you want the inner wall to rest on the slab, then the total wall thickness will be 8" concrete (only 4" will be supporting outer stud wall) + 4" foam + 4" inner stud wall = 16" wall.
I'm interested in how David M would attach the 4"x9.5" foam strips to the concrete forms? The foam has to be attached firmly enough to withstand the pouring and flow and buoyancy of the concrete, yet has to easily release from the forms to stay with the concrete when the forms are stripped off the foundation.
I'd probably put scraps of 1x across the top of the forms to keep the foam from floating up, and I'd just physically push it against the form panel as the concrete was going in, to keep any concrete from getting behind it. We pour almost all foundations with a boom pump with a 3" hose end, so it would be easy for me to shoot the concrete down past the foam and keep the foam in place without mechanically attaching it.
I still wonder if the rabbet is really necessary. How thick is the stemwall at the bottom? At the top? It probably has to be 6" thick at the top, and might not have to be any thicker at the bottom, so, no rabbet.
I don't see any dimensions associated with the foundation drawings, but scaling them suggests they are fairly typical:
footer 1' high x 2' wide
stem wall 8" thick
rabbet 4" deep by 10" high
leaving a 4" thick by 10" high concrete fin or rib for the outer wall to set on.
I assume you'd want at least vertical and perhaps horizontal rebar in the fin to ensure strength.
http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/Montague%20Powerhouse%20As-Built%20Sheet%203.pdf
Roger that on the details, but I had this in mind, as I'll have thicker walls. I still feel there should be insulation under and around the edges of the foundation wall, but maybe with the concrete so far out now it does not make a huge difference. That gnaws at me. BTW: This is starting to look like other walls I have designed, before I looked at Thorsten's with all the foam on the outside of the stem wall and house walls.