03-07-2012, 08:48 AM
I have few questions about basement insulation installation details:
1. How high the wall foam board should be? close to the ceiling joists or just as high as the concrete wall?
2. put the rigid foam board as tight as possible in all areas? do I need to leave gap for its expansion?
3. how about the plywood subfloor? does it need gap for expansion?
4. do I have to use tapcon to fix the frame bottom plate to the concrete floor? it will break the foam and ploy then. It is not enough to nail the top plate with the joists for the frame to stay?
5. The frame should be sit on top of the foam but not the plywood. Is it right?
1> If you are going to use XPS cut'n'cobbled to insulate the foundation sill and band joist, stop the wall foam at the top of the concrete to allow for the strip of 1.5" step to match the thickness of the foundation sill to make it easier to seal the seam. Then cut in another layer to cover the top of the foundation sill, then cut in the sections for the band joist. That way all of the seams in the foam are horizonal, and can easily be reached with a foam gun (or desposable can-foam nozzle.)
If you plan to use 2lb foam to fill in and insulate the band joist, take the wall foam up to the joists and use the top edge as your depth-gauge for the spray foam.
2> The rigid foam should be installed as tight as possible everywhere. Ship-lap or T& G versions are preferred if doing a single-layer, since over the next 2 decades there will be at least SOME shrinkage of the XPS.
3> It's probably wise to leave a tiny amount of room for hygric expansion/contraction, but not more than 1/8" at the seams, and use ship-lap or t & g.
4> It's OK to put tapcons through the foam & poly 24" o.c. to anchor the bottom plate of the studwall when building a studwall on the interior of a foamed wall. Unless it's mechanically secured both top & bottom it won't meet the letter of code on it's ignition-barrier function.
5> DO run the subfloor under the stud-plate snugged to the wall foam (no gap necessary), and tapcon right through the studplate/subloor/foam/poly into the slab. That way it it restricts the otherwise floating subfloor & flooring from ever rising against the wall that could otherwise occur with major humidity changes, and you don't have to double-up the bottom plate to have something to fasten the gypsum to at the bottom. (It's not a bad idea to tapcon each sheet of subfloor to the slab 24" o.c. too, to avoid any potential for future rapid humidity changes to cause it to warp & lift in the middle somewhere.)