ridged Foam for truss attic upgrade
I’m embarking on the ugly task of retrofitting/air sealing the backside of a truss attic wall. CZ 5-6b
It has a vented 9/12 roof, (eaves and ridge) with 2″x 4″ vertical members 24″ o.c.
There are currently fiberglass batts at least 5 1/2″ behind 5/8″ Sheetrock and nothing containing the back side of them.
My questions are:
1) If I use foil faced 2″polyiso ,Taped and can foamed in for containment and air sealing, is there a risk of condensation on the inside of the foam? Would this be eliminated if I used another layer of 2″ to keep the inside surface of the foam warmer? Essentially a 50/50 insulation ratio on either side of the impermeable foil? R-40 wall?
2) Does anyone think it’s mechanically, necessary to fasten the sheets to the back of the vertical wood members.
The batts are deeper than the 3 1/2″ bays. The rigidity of the 2″ thick taped foil panels spray foamed around all truss webs, and chords seems like it would be sufficient.
I’d like to leave the added insulation depth rather than changing batts or trying to mash them into the bays.
Any input or thoughts would be welcome!
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Replies
Sounds like the plan then is to basically put 2" foil faced polyiso on the back of an insulated studwall facing the attic, so the stackup from attic to interior space would be polyiso/batt-insulated studwall/ 5/8" drywall. If that's correct, I see no problems here.
I'm not clear on your "second layer" though. If you stack two layers with seams staggered, to get a 4" total layer of polyiso outside of the batts, that is safer than one layer. If you want to put a layer of polyiso on either side of the batts, then you have a double vapor barrier issue that could potentially pose problems.
Regarding your second question, I think you'd be fine with just glue or canned foam holding things in place. My preference would be to use full sheets of polyiso against the back of the wall if possible though, in which case you'd want to use adhesive and not canned foam. My own experience doing this sort of thing has found that mechanical fasteners make things easier -- but use adhesive as the primary means of attaching the foam to the wall. The mechanical fasteners hold things in position while the adhesive cures, so avoid sags, voids, and sometimes sheets of foam detaching from the wall before the adhesive has had time to set.
Bill
I really appreciate the reply.
I guess what I left out is the foam delivered is aluminum faced on BOTH sides.
My concern is water vapor reaching the inner aluminum surface facing the room and condensing. wondering if the 2" of foam on the cold side was enough.
If I had to, I could add a second layer to keep the inside face warmer.
maybe I'm not thinking about this right or am overly concerned.
I hear you on the preference for mechanical fastening. Your experience likely Trumps my imagination.
I might just experiment on the first pieces and see how firm they are after fitting around everything and taping the seems.
It seems like Polyurethane spray foam holds like the devil. The Rigid foam boards would be trapped by the roof pitch after fitting to the vent baffle.
Dave in Durango co
Polyiso often does have a foil facer on both sides. That doesn't change how you can use it or where you can use it, at least not in your application. I wouldn't worry about it.
The problem using canned foam as glue is that it tends to push things apart as it expands. I tried that once, and I won't do it again. It sounds like a good idea at first, but once you try it you'll quickly find out the error of your ways :-) You can use canned foam only if you can put it ALL THE WAY around the PERIMETER of the piece. Even then, in some cases it can cause the piece to push itself out of place as the foam expands and cures. You can mitigate that somewhat by beveling the edges so that the force of the expanding foam pushes the piece against a surface, but it can still cause issues. I prefer to use polyurethane SEALANT as the adhesive here, since it's basically just extra sticky caulk and it doesn't push things apart as it cures.
Bill