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Thanks tb! I will look into 1" rigid foam from those brands...Another question before I have all my 'basic info' and proper language in place to feel comfortable asking pros on another forum...Really how important is temperature transfer from the rafters???Im trying to decide if rigid foam and then the spray foam is enough ($$$) or is it really going to be worth it to go the extra bit and hat-channel the drywall off of the rafters, so we could cover the remaining part of the rafter with, say an inch of spray foam --after the hat-channel is on, of course, so the spray foam coats around them...~J
Hey all,If i am going to use rigid foam to create the air channel what type should I use? and could I use foil backed, with the foil side out, facing out towards the underside of the roof decking to radiate heat away? Also, would it be ok to just snug-fit and glue the rigid foam in-between the rafters? or will I have to nail in a ledge of sorts for it securely sit on? Getting there!Thanks so much!Jason
Notice that if the drywall is screwed directly to the rafters the drywall screw heads will be cold (through thermal bridging) and humidity will condense on them. When this happens you see little black spots on the wall or ceiling where the screws are. In my room this was so severe that the drywall around the screw heads would swell up.
The 1x2 strips to attach channel bottoms just make life a little easier as you are installing foam board. You can't accidentally push it in too far It doesn't have to be pretty lumber, or even continuous... an intermittant stop is adequate, particularly when spraying foam over the top which will lock it place. Around here I can buy a bundle of 8' furring strips for about $10, I think that's a dozen or maybe sixteen.Thermal bridging is a real concern, but the reality is it wasn't really considered much before the last 10 or 15 years. Since you're there anyway, we're only talking about a bit of material and time and the loss of 3/4" of headroom...almost no downside. Did you look at the Mooney Wall? Unless I already had the hat channel, I'd strip it in wood on 16" centers. My guess is there's significant out-of-plane rafter to rafter, with furring you can shim the low ones and get a flatter ceiling to boot.Have you thought any about dense packing cellulose? You could even do it yourself if you're adventurous. A "flash" coat of closed cell foam to seal things up, strip the bottoms of rafters, install insulmesh and blow it in. This might be helpful;http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm
Peter J,I see what you are saying about the furring spacers... I talked to my guy that will be helping me with the rigid foam portion and he agrees with the wood strips, so thank you. I think I am just trying to 'pare down' where I can but I see this is not the stage to do so.I did look at the Mooney Wall and am very intrigued with this approach now...If I could use 1x2's or 1x3's instead of loosing 2 inches for the "Mooney strips" that would be a plus. IF I could still get the dense-pack around the end of the rafters in that 1 inch gap between rafter and drywall. ???I did blow in cellulose under the floor myself. It was actually kinda fun! Since there was no explanation of difference in cellulose product, I assume this is the same stuff I used for the floor, just packed in real tight.So, to recap --we will have a 1" air gap, 1" rigid foam layer, then...Is there an R-value gain with 4 to 4 &1/2 inches (depending on how thick the 'flash' coat of spray is) of dense-pack cellulose over the 3 inches of R-21 spray-in that has been quoted by an installer???I am going to think hard on this approach... But the two things holding me back is the extra time I would have to put in with my hired helper (as opposed to having it all sprayed in a day by an installer) and we would then be in for three different types of insulation in the end... I get that it would probably work out to maybe $1500. or so less invested but.... Hmmm... The time and the added hours paid to my helper for us to do this ourself may just even it out in the end. But if there is a nice R-value gain with dense-pack, I may do it.One more question:We have two skylight installed... Should I seal these two bays top to bottom, since they cannot be vented? Hoping this is OK and not another issue.~J