I have some left over FG batts (R-13) and some 1/2″ sheets of foil wrapped isocynate insulation (R-3.6) from previous project. With these, I plan to insulate my attached, but unheated garage.
The garage has sheetrock over the framing, but no insulation. Would I be better off:
1) removing the sheetrock, putting the batts in the bays, sheathing the wall with the rigid foam, then re-rocking?
2) removing the sheetrock, cutting the rigid foam to size of each bay, putting in the batts, then re-rocking?
I guess there are benefits to each. In the first case, I’ve created a continuous thermal break between the outside wall and the inside, right? However, I know FG’s performance drops off as the temps drop.
With the second option, the FG won’t be exposed to the same cold temps because the foam will be the first line of defense, right? But, then I don’t get the benefit of a continuous barrier.
Thank.
Stace
Replies
I'd go with the first plan; put the FG in the bays, foam over that (and the studs), then rerock. That reduces the thermal short-circuiting of the wood studs and gains more R value overall. It will be less work, also. Putting segments of foam into the bays first means the FG would have to be compressed more to fit on top of it. The R/inch of the FG might be improved slightly, but more than offset by loss of thickness of the batt. Also, the thermal bridging of the studs hurts overall wall R value a lot. As to the loss of effectiveness of the FG when it's cold out, it isn't so much a matter of the average temperature within the FG but more the temperature difference across it. On a cold day, the outside of the batt will be near the outside temperature, while the inside of it will be substantially warmer. The air within the FG batt will be much more dense next to the sheathing than next to the foam or rock inside, and convection currents will set up within the batt, in effect carrying heat from inside to outside. That is why dense blown FG (BIBS) or cellulose is better than FG batts for insulation.
Why mess with demoing the sheetrock? Blow cellulose thru holes in it, better insulation for less effort. Heat rises, so roof or attic insulation is more important than the walls. What do you have up there? What about the garage door? Weatherstripping and R-value there are a big factor.
Bottom line, you have to think about the whole envelope, not just how to use up some leftover stuff.
-- J.S.
Yeah, good thoughts. We put in some insulated doors last spring (neighbor was remodeling – we got the doors for nothing) and that's when I starting paying attention to how cold it really was in there. We did Icynene in our attic and the difference is amazing. The walls are Fg batts, original to the house. Like most young home owners, we're taking it a step at a time. I'm amazed though that the previous owners didn't seem to mind the gaps that wev'e caulked, sealed, and weather stripped. I guess oil was cheaper back then.Thanks