Garage/Attic Insulation delema
Hello,
I’m finishing an 896 square foot garage in SW Washington that has in floor radiant heat. It has a 10’ ceiling built with attic trusses and therefore a 12’ x 32’ inhabitable space running down the center above the garage which we plan to use for heated storage. I have already insulated the attic space to R-31 in the ceiling (batts and foam board) and R-21 in the sidewalls. I plan to insulate the garage and would like to use the radiant heat to keep the garage at between 55 to 65 degrees for working during the winter months. Because of the heated livable space above and the moderate temperatures planned for the radiant heating in the garage, I’m unsure if I should insulate the entire ceiling to R-30, the ceiling below the inhabitable floor space, or just the ceiling of uninhabitable attic space. In any case, my intent would be to have the vapor barrier to the winter side of the inhabitable attic space and to the winter side of the garage in the uninhabitable attic spaces. Any recommendations are appreciated.
Replies
Nothing but gut feeling to go on but if the floor is carpeted and you have two zones, I don't think you'll notice the difference if you just leave it uninsulated. Now if you go with hardwood or tile and you are sock in the house kind of guy, you'll notice the floor temps. Any heat you lose from the upper to the lower won't really be lost just less heat you'll need downstairs. The weak point IMO is the insulating between the garage and the unihabited space. Don't just run bats up there and stop at the upper wall. You need to make sure you put in a solid stop sealed with caulk in each bay at the interior point you stop or you'll get a convective loop across your upper floor/ceiling.
Voice of experience here. Builder did this on our house and my only solution (without cutting out drywall the whole length of the house) was to go to the second story and blow in cellulose to pack the entire joist bays. So now I have continuous insulation between my first and second floor. Thankfully, the second was still unfinished at the time and I did it my self so it cost me some $$$$ for the bags but little else. Would have hated to open up the ceilings to put in the stops and then have to deal with the drywall finishing in my wifes new house.
Two different thermostat setpoints
It sounds like you want the attic space warmer than the garage, and if so I would insulate the floor of the attic to R30. That's what I did in my shop, which is a two story building with most of the first floor as 55-degree shop and the entire upstairs as 67-degree living space.
Your roof at R-31 is inadequate, if I am not mistaken, and should be R-38 minimum. The fact that you may have used trusses with too-small top chords does not alleviate this requirement.
What about the vapor retarder
Thanks David,
The actual insulation consists of R-38 batts on the ceiling and the sloped walls contain vent baffles, R-30 batts, and 2" foam board (R-10) on the interior side of the batts. The vertical walls are R-21 batts. I had to build up the top cords at the sloped walls to accommodate the R-30 batts and the foam is attached over the studs.
I’d like to maintain the upper floor temperatures between 63 to 68 degrees, but only while we are living upstairs and hopefully only be for a couple of winters; thus my dilemma on how best to install the insulation.
I guess what I should be asking is did you keep the vapor retarder facing the garage over the entire ceiling or did you install blocking under the attic walls and flip the batts under the living spaces so the vapor retarder facing the floor sheathing?
I used
unfaced fiberglass in my floor. The vapor barrier that matters to me in this situation is under the concrete slab. I do not have any humidity issues in the building at all, and no reason to expect any. I suppose in theory I could have used faced batts with the facing on the warm side (facing up), but I just don't think it makes any difference. My floor above has 15# felt and nail-down wood flooring. It has never cupped or moved at all since installed 3 years ago.