Room with radiant floor over unheated garage – where to place vapor barrier?
JohnWalker
| Posted in Energy, Heating & Insulation on
I’ve got a new build with Joist Trak radiant floor panels under the subflooring of a room over an unheaded garage. Normally the vapor barrier goes on the “warm side” of the insulation (floor will be insulated). In this case that would be near the subfloor. What is normally done for vapor barrier in this case?
Thanks
John
Replies
Vapor barrier would be on the warm side, but you need to be sure to have an air barrier (drywall, sheathing, Tyvek, etc) on the bottom.
Agreed, but how to implement the VB? Staple 18" wide strips between the joists against the subfloor and seal to the joists? Otherwise I have no idea. (unless sealing the subfloor from above with some super paint and caulk is the way to go....)
Since it's a staple-up, I'd recommend foil-faced RFBI underneath the staple-up. I use polyiso, it's about R-6 per inch. If you want to friction fit them, I recommend using 1/2" thick. If your framing is good, you can cut the RFBI slightly wider than the joist bay is wide and then friction fit the foam between the joists, tucked up high just below the staple-up.
You can certainly go with thicker foam board, up to 2" for R-12. If so, then I'll cut them narrower than the joist bay is wide, tack them in place, then use canned foam to foam the edges tight to the joists.
I usually rip the foam to width on a table saw. Fast.
Add unfaced FG batts under the polyiso as needed if additional R-value is desired.
Don't forget to insulate the rim or band joist at the ends of the joist bays. You don't want cold air from the rim to have easy access to the gap between the top of the foam board and the underside of the subfloor.