insulating a cathedral roof
A newbie here, but the “framer” in my name refers to gold leaf picture frames, not buildings, so please help!
I live in central Montana and I’m helping build a new shop for myself. It is 32×40, stick frame (6″ walls), and with infloor radiant heat installed in the concrete floors. It is a shed roof style (bci joists) with a 4/12 pitch cathedral ceiling, 24″ soffits. It will be sided in cedar to match nearby buildings.
I’m getting conflicting info regarding insulating the ceiling. The lumber yard has informed me the composite shingle warranty will be void if I have 2″ of expanding foam sprayed in and then fiberglass batts to fill – installed without any vent system. Several builders have informed me that’s the only way to go, and forget about the potential heat damage to the shingles.
Any advice?
Replies
There are other shingle makers who do not deny this kind of use.
There was a study done ( probably several since) a few years ago that demonstrated the reduced life expectancy for shingles on unvented surfaces.
Basically it is because the volatile oils in the asphalt shingles cook off faster at higher temperatures. so when heat cannot escape the roof assenbly to the back side, the shingles stay hotter and do not live as long.
but the percentage was minimal, only enough to make a difference of a few months on a 25 year shingle.
For comparison, you get more redection in shingle life moving from Maine to Georgia or from the north slope of a roof to the south slope of the same roof.
So I wouldn't worry about it.
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Thanks for taking the time to reply and offering some good advice - several times!I appreciate it ...
"the lumber yard" check with the shingle manufacturer, I bet your lumber yard also sells the FG insulation.
Go with the foam.
Shingle manufacturer's used to void a hot roof, some now warranty them.
Still, why worry over losing 1-2 years off a 30-year shingle by going with foam?
Either that or over the course of 30 years you'll lose enough heat to pay for the roof two times over by not using the foam.
Yes, you purists, that's an exaggeration. But probably not.
Mongo
Either that or over the course of 30 years you'll lose enough heat to pay for the roof two times over by not using the foam.
Maybe even more in Montana.
Besides, who wants to work in a cold shop?
Joe H
Thanks for taking the time to drop a note - good advice and a good perspective on the choices!
check out the building science site, I think its just buildingscience.com, they have some good articles and you can check out this http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/97/970504.html
I'll try to find another article that I had floating around about a test house in florida where they test various products.
Also here is something from icyene. I know certainteed and Elk are fine with unvented roofs and spray foam, the others should be but they are behind the times. Do a google search for unvented roof, shingle life and you should probably get some good hits with studies, etc. The foam company you want to use, corbond, icynene, etc should email you warranty ok's they have from the shingle makers.
As piffen said, the life change on the shingles in the studies is a few months less on a 25 year shingle.
Edited 9/5/2007 11:47 am ET by DDay
Thanks for taking the time to help with my quest ... the Florida studies are interesting, but I worry they don't have a lot of application to my Northern Montana, relatively dry climate.Obviously there's more than one way to build a shop!I'll check into the Building Science site - thanks.
OK, one more time . . .
With a hot roof, no vent space, are you prone to get ice damns?
One view has it that roof insulation does get somewhat warm from the transference of heat from the interior. With a vent space, any warming of the air above the insulation is quickly moved to teh ridge and out and does not warm the roof deck, and does not melt snow which then travels to teh eave to freeze.
With a hot roof, the heat migrating through the insulation does heat the roof deck.
Any problems here?
If you have enough heat migrating through the insulation to cause ice damning, you do not have enough insulation.
Never seen an ice damn with urethene foam insulation.
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bigskyframer,
Aren't you using SIP's? If you are then simply use a product like cedar breather to provide the needed venting..
Cedar breather is simply a recycled plastic mesh material that holds shingles off the deck and allows the backside to dry so the deck and shingles don't overheat or rot..
Cedar breather is to provide a drainage plane. The very minimal venting it provides does nothing to cooling a roof surface.
And it is for cedar shingles, not for composition shingles.Try to stay on subject here old man or you'll have the poor guy tearing it all down and building a timberframe and putting shellac on the roof...;)
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Piffin,
You don't need to build a timberframe to use SIP's I'd have thought you would be aware of that..
Come on frenchy, your comments have nothing whatsoever constructive to deal with the situation as presented in this thread. You are off in your own little world drumming your own hobbies without helping him and if he used your suggestion for Cedarbreather he could likely ruin his shingles .Stick to commenting when it is something you know a little bit about.
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Well Piffin, according to the expert, a timber frame would be cheaper. . . . That is if pigs actually had wings.
Frenchy, you're losing it.
Stick framed with BCI for roof rafters.
Not a word about SIPs.
You been sniffing glue again?
Joe H
Joe H.
SIP's work without timberframing.. you can stick build a roof and put SIP's on.. Infact where I bought my panles that was the majority of panels they sold.. timberframers were rare..
I turned a "florida" room into a cathedral ceiling a few years ago. I was thinking about foam insulation when I got a job working some issues for a guy that was trying to sell his house. One of the issues was "cracks" in the main supports of his house. These actually were the joints in spliced beams, and I "fixed" them by adding extra layers, but it got me thinking.
What if some "inspector" saw that I had not vented the roof and made me vent it when it came time to sell? How could I prove that there was foam up there? How could I prove things were ok?
Yeah, with new construction, perhaps the inspector's signoff would be ok, but this is Jersey. Here, even that isn't golden.
So, I vented the ceiling and put in fiberglass insulation. I know that will pass.
"What if some "inspector" saw that I had not vented the roof and made me vent it when it came time to sell? How could I prove that there was foam up there? How could I prove things were ok?"A series of photographs taken during the process and before covering up the insulation can prove a lot.There are still a few backwards jurisdictions trying to require venting with foam, but Corbond has done studies to help get them over that. Contact them for the white papers on this.
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That works well with true building inspectors, but I was talking about the "inspections" from "engineers" that prospective buyers have done when a house goes up for sale. The companies that do these vary in quality; some are actually pretty good, but some inspectors are old-timers trying to eke out the social security.The main problem is that they don't work for the seller. The seller usually is not even home at the time. The report goes to the buyer, who is usually clueless. There's no possibility of communicating with the inspector and getting something changed on the report. You can, of course, simply refuse to do it. The sale frequently falls through at that point, or the buyer may try to get the price lowered.George Patterson
Like piffen said, corbond, incynene and others have information they can give you.You could also just let them see your heating/cooling bills before the foam and after. FG just is just not a good insulation, it adequate but does not compare to the foam.
Well, I used Dupont foam panels as the bottom surface of my venting and as blocking over the walls at the eaves. That really improved the fiberglass - there's no air infiltration through that.George Patterson
First, I will say that Joseph Lstiburek and the Building Science folks are guru's on exterior barriers and anything they write is worth not only reading but heeding. I was fortunate enough to work with them on some ugly projects in the Northeast and they know their stuff.
Re. spraying the roof, I helped my buddy build his shop 20 years ago and he sprayed the roof. (I advised against it not for the shingles but for the risk of trapped moisture and the sheathing deteriorating.
About 10 years ago he replaced about half of the sheathing and most of the shingles. The shingles were doing ok. The sheathing was not able to be walked on. It was gone.
I always make sure I vent my sheathing and have for 25 years. (but I am in the Philly area and it is hot and humid in summer, rains in spring and fall and is wet in winter)
You gotta know your area but round here, I vent everything!!
Good Luck
You don't say what kind of foam he used or how much. That usually happens with the icy foam, not urethene
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