Closed cell foam: 3" as good as 6"?


Hi,
Just received a quote for roof insulation with closed cell foam between the rafters. The quote is for 3" and I am being told that 3" is 90% as good as 6". Do I believe them?
Thanks, Aaron
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That kind of stuff makes me paranoid. Foam just seems way too flammable a material to entombing your home with.
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wrote:

Foam is treated to make it self extinguishing. It will not burn unless something next to it is burning. Code requires it to be covered with sheetrock if exposed to the living space.
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I had a roofer tell me 1" of his foam equaled R 35, the dam liar. From what I have read your foams go from aproximatly R 5 - R 7 per inch, and thats it. Polyurethane is about R7" and other types less so going by factual R values 3" is R15 to R21 depending on type of foam used, ask them whose product they use and check that companies site, but no foam is 90% different and R 21 isnt squat for insulation what I live in Zone 5. Your bidder is lying to you.
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Are you insulating between the rafters because you'll be turning the attic into habitable space?
As to your question, 6" of foam will be about twice as good an insulator as 3" of foam. If your concern is about reducing the heating and cooling costs of the entire house, then the useful of the extra 3" of foam depends on how well everything else is insulated.
Here's a simplistic example: suppose the roof represents 20% of the surface area of the house, and lets call F-1 the insulation value of 1" of foam. If the rest of the house is a perfect insulator (impossible), doubling the roof insulation will halve the energy loss. If the rest of the house is insulated to F-6, then the F-6 roof will reduce the system energy loss by 17% over the F-3 roof. If the rest of the house is insulated to F-3, then the reduction is only 10%. And if the rest of the house is F-1, the reduction is only 4%.
Cheers, Wayne
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Wayne Whitney wrote:

Don't think so. Resulting efficiency rate drops off as thickness increases. If you're at 50% with 3", you might be at 70% with 6" and 80% at 9" and 85% at 12" (numbers are made up).
You're spot on about insulating the rest of the house. It won't do much good to insulate the attic if you leave the front door open.
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wrote:

I agree with Bub on this one. It is like SPF numbers on sunscreen. Pretty soon you reach the point of diminishing returns.
Attic insulation is really a winter thing more than a summer thing anyway. At a certain point you have to decide heat rises. You cold air loss is down on the walls, windows and doors. Cooling your attic with better ventilation may return more than an extra few inches of insulation.
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snipped-for-privacy@aol.com wrote:

That depends. There are places where winter temperatures never drop below 50F but summer temps can reach 130F.
I'm in Houston and I'm blessed that I've actually seen snow. Not many of my neighbors have.
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wrote:

Snow? I have heard of this thing. People are more likely to snort it than shovel it here tho ;-)
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wrote:

Although that is very true, it's also true that more heat escapes upwards than outward. Overhead insulation tends to be the most important. Better to have r50 in the attic and r20 in the walls than the other way around.
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It depends on where you live, his 3" of foam could be R15-R21, mimimal code for my zone 5 area is R 35 and optimal is R60. Heat rises and in cold areas you do see a difference in even room comfort going to R 60. If he is in a cold winter area 3" of foam for a roof-attic is a joke in my area where it was -20f last year. Sure you get diminishing returns but I was paying near 120 a month gas where a neighbor with same size house that had crap windows and near no insulation paid near 700 one month for gas, up north here it is better to overdo efficency, so you dont overpay utilities for the rest of your life. It also increases resale value alot, or can make the sale in tough times.
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I'm not sure what kind of efficiency you are talking about. If the R-value of 3" of foam is R-15, then the R-value of 6" of foam is about R-30.
Cheers, Wayne
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Wayne Whitney wrote:

But, the energy saved by adding attic insulation is the same, regardless of the other insulation in the house.
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I guess my main point was that you may be better off directing your dollars towards insulating elsewhere in the house.
As to your comment, it is mostly true. Adding attic insulation may change the mechanism of heat loss that the house experiences. So you need to know something about the other insulation in the house to figure out the impact of your added attic insulation.
Cheers, Wayne
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Aaron Fude wrote:

Where do you live? What is your area building code? I have R60 attic, R24 ext. walls, R12 basement walls insulation. Roof is ceramic coated steel tiles. Hottest summer temp. 33 deg. C. Coldest winter -37 deg. C. Wind chill could go down to -50 deg. C but not very often. We have Chinook frequently. Very dry climate year round.
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