Hi all,
I am planning on building a garage/shop building sometime this summer. My plan is to have this as a year-round heated building, so I will need some way to insulate the foundation of this. I’ll be placing radiant tubing in the slab. The site is sloping, with about a 3-4′ drop from the high point to the low point, and I can fill a little of that in, but not enough so that I can avoid a step in the footings. I was planning on doing a fairly standard frost wall, starting at 4′ on the high side and 6′ on the low side and be able to expose about 2′ of the wall on the low side. It is near my septic system leach field so I can’t fill more than about a foot before I start encroaching on the leach field with fill materials.
Anyway – I’d like to be able to have a stem wall above the slab so that my exterior siding doesn’t get wet too often, probably 8-12″ high. I need a way to insulate not only the stem wall and frost wall but also the slab as well. I don’t need a LOT of insulation, R-8 to R-10 should be fine. What I’d like to avoid is to have any of the insulation on the OUTSIDE of the wall, or at least have it exposed. I think I can deal with having it exposed on the inside, but I also am aware that it’s better to insulate the outside of a concrete wall than the inside.
If I were to go with exterior insulation (either polyisocyuranate or XPS) I’d need some way to protect that insulation from damage, and I also don’t want the insulation to stick out past the siding.
I haven’t decided yet on the actual garage construction yet – it will either be a timber frame to match my house, or ‘conventional’ construction. Either way, I plan on using either rigid foam or spray foam to insulate it.
Can anybody give me some ideas on how you’d do this? Anybody ever placed foam board vertically INSIDE a concrete form, centered on the form so that you have a concrete-foam-concrete sandwich? Maybe use an 10″ form and 2″ foam board so that you have a 4″ concrete wall on either face of the foam? I could go down 2′ in the form and then insulate the inside of the wall under the slab…..just thinking out loud, I guess.
Replies
I used 2" rigid foam (Dow PerformGuard) on the inside of the block foundation of DW studio bldg. Took it all the way down to the footing. I then used 2" board under the slab and poured the slab even with the top of the block foundation and slushed the block durring the pour. I was going to use radiant floor heat but got such an outrageous qoute I ended up doing forced air. My intent was to isolate the slab from the wall and subgrade. Heat loss at the edge of a slab is much greater than just 2' inside the slab and to the earth.
It is a little compromise from putting the foam on the outside of the foundation wall, but not a bank breaker in terms of energy loss.
you didn't give any dimensions
and...is the slab going to be stepped also ?
i see you live in Vermont... what is your design frost depth ?Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Edited 2/5/2009 6:49 am ET by MikeSmith
I'll give it a bump so the OP sees your questions.
Question for you.
Has your frost depth ever been changed in you local codes? Ours changed from 24" to 30" about 1980. We had several months of below freezing and sub zeroe weather durring the winter of 77/78. Frost penetrated to below the old 24" standard and froze a lot of water lines, so the code was modified to the current 30" depth. I spent a month or so of that winter working for a plumber fixing a ton of frozen water lines. It made such an impression on me that Ihave since used 36" as my personal frost depth.
Just wondered if the same thing has happened elswhere in the country.
i'm not sure ...but i think the old frost depth was 32"
can't remember when they changed it to 40"
but i do remember that 40" was a little arbitrary.....'if i recall, it was supposed to be 42"
and then some trade group pointed out that a 42" wall and a required 8" of exposed concrete from finish grade to the bottom of framing would mean 50" forms
so they backed it offMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
This area is typical frost depth of 48". Footing will be stepped but not the slab. The building is probably 24 x 32 but not set in stone yet. What other dimensions were you asking about?
well..... in your climate i'd be thinking of more than 2" of foam ( high density )
so maybe two layers of 1 1/2 or 2"..... and isolate the slab edge too.... you want the heat comming up... not down or outMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
My house has ICF basement walls, and I put that foam/bubblewrap/foam roll stuff under my basement slab. It's only about 1/4" thick but 'claims' an R-value of about 5. I honestly have no idea what the R-value is but the floor stays nice and warm. I do agree that isolating the slab is critical. I do want a stemwall though, and I really don't like the idea of putting insulation on the inside face of an exposed stemwall. Even if it's only a foot high.
last building on slab I did was 48" to the top of the footers (which is where you'll need to be I think), then 5' high walls on top of that, giving about 12" of concrete above grade (pretty typical). We backfilled and compacted, then put down 2" of blue board, then poured a 4" slab flush with the top of the foundation. PT mudsills flush with the outside of the foundation, and sheathing and siding overhanging about 1/2". I don't see the need to insulate your frost walls, you just need to insulate below the slab, 2" of foam beneath the slab should do it.
If it was mine, I'd go with the XPS on the outside (and under the slab). You can deal with the siding by hanging your plates the thickness of the foam. Or, since you spoke of siding the whole building with exterior foam, you could sheath the whole building with the same foam that covers your stem walls and your siding would be in the same plane (you would have to install some type of strapping to fasten you siding to).
Exterior foam near grad can be protected in a number of ways. Fastening metal lath and applying concrete stucco is probably the best. You can also use an EIFS type system. Or you can use coil stock, though this is fairly tacky looking IMO.
Your idea of making a foam and concrete sandwich has a number of problems. You would need some bombproof method of securing the foam, like some sort of modified snap tie that held it between the two wall forms (maybe you could devise something out of threaded rods). Pouring could be done, but you would always run the risk of smashing the foam and ruining the whole thing. You would wind up with two 4" walls which don't equal the strength of an 8" wall. Then there is the rebar to deal with. It's hard enough sometimes to get sufficient cover over the rebar with a conventional wall, let alone two four inch walls.