Foam insulation for ceilings ?
antlerchaser
| Posted in Energy, Heating & Insulation on
I have a cathedral ceiling with 2×12 rafters on 24 ” centers and have been looking at my options for insulation. I have installed a ridge vent and facia vent (continuous). I talked with a contractor that sprayes foam and he said that they spray directly to the bottom of the decking and this air channel is not necesary with this application (6″ thick) I am skeptical and was wondering what you guys thought on this. Thanks, Good Guy P.S. I live in northern WI
Replies
Rut Row...
First, if you have a building official up there, ask the building official. Some inspectors demand that the roof be vented, some don't. Even if the code demands venting, you can skirt the issue with foam under the alternative methods section of the code...if yor inspector is open to alternative methods.
Second...realize that not venting the roof may void the useless warranty provided by your shingle manufacturer, and that it may reduce the life of your shingles by 2-5%.
If you go this way, ensure that the finished sprayed foam is thick enough so if any water vapor gets into the rafter bays (which it will) that the dew point occurs within the thickness of the foam. This will prevent water vapor from condensing out and ruining your ceiling.
In terms of being a valid construction method, yes, you can do it. Your installer needs to do a damn fine job so the foam doesn't shrink away from the framing. that shrinkage could allow moist warm air to contact the cool framing.
Drip....drip...
I'm with Mongo, although I couldn't have expressed it so well
Cold climate, cold roof (i.e. vented), IMO. There are other options, but they are trickier to do right and downsides to not getting the roof right are significant.
Why doesn't anyone post their location? Saint Paul? Saint Petersburg? Makes a difference.
My advise would be to install rigid foam and leave the air space and ventilation alone. 10" of rigid foam would be a very good insulator, however it is a little more expensive.
MikeB
Hi Mike, Are you suggesting that I cut the foam sheets to fit between the rafters? Not a problem , just curious. I assume that's what you are saying but knowing that the rafters are'nt perfectly spaced, I will have some small gaps on the sides , can foam time maybe ? I really believe in the air channel and went through the trouble of allowing for it so will go ahead and utilize it. Thanks for your input as well as all the others, THANK YOU !! Good Guy
You can keep the continuous air channel and spray foam over it.
Or keep the channel and blow cells into the rafter bays.
To me, either of those are preferrable to air channels and FG batts.
When I finished off my own attic (catherdral ceiling), it already had FG batts in open rafters. I added 2" of polyiso to the faces of the rafters, foamed the gaps between the sheets, furred, then rocked. Tight as a drum.
Our buddy Home Depot supplies a corrugated spacer to fit between the rafters and provide air movement over your insulation. I think its pricey but it does the job and I have used it with fiberglass as well as spray foam. One feller that I did a job for added an attic fan to pull air through but I think that was not needed---just his insurance.