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Here’s my deal. New construction, breaking ground in May. House with full walkout lower level, attached 2 car garage, garage to have full basement underneath, adjacent to and accessible from walkout lower level of house. Floor structure of garage is structural concrete slab, built with steel-supported 1-1/2″ 22 gage corrugated deck, slab thickness 4″ atop deck. Shop space basement under garage will have in-slab hydronic radiant heat. I was planning to screw 1x furring strips at 24″ centers against the steel deck (before the pour), and use the furring to attach rigid polyisocyanurate foam insulation board (Dow, others), and then face the foam board with painted 3/8 plywood. Thus the space between the top of the foam board will be the 3/4″ more or less from the furring, plus the 1-1/2″ channels in the decking. The garage will have insulated rollup doors and one insulated window and one insulated door. I think I should insulate the garage walls (2×4 framing) and truss ceiling (R-50) with fiberglass batts. Location is upstate NY where winter temps can go to 40 below. What concerns should I have about condensation in the space above the foam board at the ceiling under the slab? How thick should the foam board be?
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Temperature here also go to -40 and I love my radiant-floor heated garage. To each his own. Include receptacles in convenient locations so you can plug your car in on the coldest days.
To minimize condensation within the air gaps jsut blow the concrete, you should create a vapor barrier. I'd put 6 mil poly, taped at the seams and edges (they make a special tape) over the blueboard. And then do the ceiling in 1/2" sheetrock. A good taping/mudding job and coat of paint is an additional vapor barrier.
Another consideration is the certain discoloration and possible condensation on the ceiling (whether sheetrock or plywood) where the screws and any studs are. After about 5 years (less if there are smokers), those spots discolor from grease and dust preferentially depositing there.
I don't understand about insulating the garage walls? Is the garage heated or not? If not heated, don't insulate. If heated, yes insulate, but then why are you worried about condensation on the bottom of the slab?
If the garage is heated (to 50F?), ignore my thoughts about thoughts about discoloration and have minimal insulation under the slab. If the garage is not heated, I'd go two layers of 1-1/2" blueboards at 90 degrees to each other so you can have 2x2's at 90 to each other for supporting the ceiling. -David
*GeneI built a similar house in 1979. The walls are 2x4 with 3/4" foam sheating on the outside. I have about R-30 overhead. The garage door is a hollow core type of construction and I am guessing that it has higher losses than your insulated door. I am in Kansas City and the temp in the garage will not drop below 40 until the outside get down to 0.The one difference is that mine floor was poured using temporary shoring and is flat, exposed concrete on the bottom. Because of the hill side that I am on about 60% of the two side walls are exposed. A few years ago I finished part of the area for a office. I did some rough heat loss calculations using 40 degrees for the "outside" of the ceiling and still found that 3 times the loss through the ceiling as through the sidewalls to 0 degree air.I mounted conduit for the lights. Then I glued 3/4" foam to the ceiling cuting out around the conduit and the electrical boxes. I then 12" sq ceiling tiles and glued them to the foam. While more insulation would be better the fact that you have still, moderated air. I think that 1 to 2" of foam would work for you. Specially with the radiant heat.While the there is some concern with the condensation this is different from normal insulation problem in that the "cold" side is much warmer than normal and that not normal living space so that the moisture load will be low. I think that caulking and sealing the sheets of foam will be enough.If I was building it today I think that I would look for some kind of layer system where the plywood for the pouring would be permanate and there would be foam on top of that with somekind of anchors that would go from the plywood, through the foam and anchor into the concrete.
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Here's my deal. New construction, breaking ground in May. House with full walkout lower level, attached 2 car garage, garage to have full basement underneath, adjacent to and accessible from walkout lower level of house. Floor structure of garage is structural concrete slab, built with steel-supported 1-1/2" 22 gage corrugated deck, slab thickness 4" atop deck. Shop space basement under garage will have in-slab hydronic radiant heat. I was planning to screw 1x furring strips at 24" centers against the steel deck (before the pour), and use the furring to attach rigid polyisocyanurate foam insulation board (Dow, others), and then face the foam board with painted 3/8 plywood. Thus the space between the top of the foam board will be the 3/4" more or less from the furring, plus the 1-1/2" channels in the decking. The garage will have insulated rollup doors and one insulated window and one insulated door. I think I should insulate the garage walls (2x4 framing) and truss ceiling (R-50) with fiberglass batts. Location is upstate NY where winter temps can go to 40 below. What concerns should I have about condensation in the space above the foam board at the ceiling under the slab? How thick should the foam board be?