slab and foam

[raybob]

I'm wondering how one installs foam sheathing on studs on a slab foundation.  Specifically, do you set the sill plate in from the edge of the slab the thickness of the foam sheathing?  With 1" styrofoam, do you set the sill plate in 1" all around?

Bob

[PEG688]

  The foam goes burried outside of the cement foundation generally. We get a sheet metal flashing bent to bevel away from the wood and cover the foam .

 

 

Under that unpainted T1-11 you can see a bronze / brown metal flashing covering the foam and going up and under the T1-11.

The foam is to act as athermal break from the earth to the concrete wall and floor / slab.

The older part of the building does not have the foam or metal as it was built as  a shop , and was MAYBE not required by code at the time , I say MAYBE as some thing don't on this project by the org. builder where not done well , and or right It was a PITA straightening out those issues with the building dept. at the time we did the remodel.

Does that help?

G/L PEG

[John Raabe]

If you are insulating with foam on the outside of the wall framing you would normally set the sill plate flush with the foundation edge per a standard layout.

Your 1" of exterior foam will stick out beyond the foundation but you will most likely use 2" foam (Dow blueboard or equal) for the perimeter slab insulation --- so you will still need the flashing detail PEG mentions. In addition, you have to physically protect the foundation foam (stucco, PT ply, cement mill board) so the flashing must get to the outside of this as well.

If you move the wall inward of the slab edge you will likely screw up some critical dimensions somewhere (stair width, bathtub, etc.) so try to avoid this unless you oversize the foundation slab to make the framing come out right.

[raybob]

Thanks John.  That's what I was asking.  

I was wondering whether I had to size it to allow for the foam or not when I layout the slab forms.  I can see pros and cons for both methods.  Does oversizing the slab for an inch foam all the way around (so the siding would end up flush with the slab) end up with a water trap at the sill plate?

Bob

[raybob]

Okay.  Now I get the little house plans in and John says in there to put the foam board INSIDE the walls.  Argh.

I think there are 50 ways to do anything and, for each one, someone somewhere is shouting each one through a megaphone.  Does that about get it?

Bob

[John Raabe]

Oh, yes -- there are still about 35 other ways to do it --- see my book on Superinsulated Design and Construction .

Decide how much insulation you want in the slab first and then look at the walls.

Foam in a wall system can go either outside or inside the studs. When inside it gets sealed for the vapor/air barrier.

[raybob]

I ask because of termite paranoia... and (if it isn't obvious over the past few days) I'm trying to figure out how to build a termite free structure (slab foundation, all metal  framing, hardieboard, paperless gypsumboard, etc.).  From all of the prescriptive tables I've found for cold-formed steel framing, it seems pretty straight-forward to swap one for the other (with sufficient x-bracing and screws), if only I can figure out how to insulate them (no shinola, huh?).  The steel framing book I bought shows the foam on the outside.  

So the impermeable layer (vapor barrier) goes on the inside, the permeable barrier (such as building wrap) on the outside.  Are the foam boards air-permeable (do they breathe)?  If one had foam board on both the inside and outside of a 2x4 metal stud wall, one would have the (inside) vb between the foam and the gypsum board and (outside) building wrap/tar paper between the foam and siding?

I don't think I can have perimeter foundation insulation, as one must be able to see the cement to do a visual inspection of the foundation/slab.  However, termite shield flashing is pretty standard between the sill plate and the slab.

Bob

[MountainDon]

From all of the prescriptive tables I've found for cold-formed steel framing, it seems pretty straight-forward to swap one for the other (with sufficient x-bracing and screws), if only I can figure out how to insulate them (no shinola, huh?).  The steel framing book I bought shows the foam on the outside.  

Steel framing is a whole different ball game. All I know about it is it's imperative that insulation be placed on the exterior of the walls. I'm sure someone will have something more to add.