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Old 02-15-2012, 05:22 PM   #1
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highest R-value spray foam


I'm looking into my using spray foam to replace batts for my roof. I have 2x8's for my roof rafters and I think I'm going to lose ~1 inch for venting, so I have 6 inches left for the spray foam. In order to get to the minimum 38 R value I need a high R-value closed cell foam. I'm told some types of Polyisocyanurate spray foam have an R-value of 8 per inch, does anyone know where I can buy these? I checked the normal places but had no luck.

Also, I've heard if you get foil faced Polyisocyanurate boards that the foil gives you an extra R-vale of 2 since it reflects one of the types of heat emission, is that true? if so I could use it for my framing of the roof venting.

Thanks for the help

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Old 02-15-2012, 06:45 PM   #2
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closed cell is the highest R. you are not spraying yourself are you

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Old 02-15-2012, 06:58 PM   #3
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I'm looking into it, if it cost effective I'd spray it myself although I have enough that it may be cost effective to have a contractor do it.

So I'm under the impression that there are different types of closed cell, mainly polyeurathane which I've read is an r-value of 6.5-7 and the one I listed above at 7.5-8. I just haven't found any DIY kits for the above listed one since I'm chasing the highest r-value possible. Heck, if I could do aerogel DIY with an r-value of 10 at a reasonable cost I would
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Old 02-15-2012, 08:15 PM   #4
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where are you located? i would not perform spray foam as a DIY project. for something permanent like this which we just finished up doing a professional is what i recommend. i have a thread with pics in this section somewhere
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Old 02-15-2012, 10:13 PM   #5
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Lots of suppliers of closed cell foam are now recommending what is called "Hot Deck" installations. With this technique you no longer ventilate the attic area. Instead it becomes part of the conditioned area of the home. Studies have shown it has no effect on the life of the roof and it's certainly much simpler to do!
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Old 02-15-2012, 10:21 PM   #6
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^thats what we did.
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Old 02-15-2012, 10:48 PM   #7
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Originally Posted by federer View Post
^thats what we did.
Not so sure about that since he is talking about space for venting!
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Old 02-15-2012, 11:07 PM   #8
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i was talking about what we did, not him
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Old 02-15-2012, 11:45 PM   #9
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So in the hot deck vs vented, are there any studies on longevity concerns regarding hot deck? I didn't see any on building science. Also, if the venting is done to address the problem of ice dams will going to a hot deck be cause for concern with ice dams when it snows?
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Old 02-16-2012, 12:06 AM   #10
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building science does go into it. i have the pdf somewhere. suffice to say its ok as long as you do it correctly. make sure the roof is completely sealed from the outside. no penetrations coming in, and no penetrations going out as a normal ventilation system dictate. close off all soffits, gable vents, etc etc. and i would seal any air leaks. its also best if the ducts are in the conditioned space.
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Old 02-16-2012, 12:44 AM   #11
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The basics: http://www.buildingscience.com/docum...r-all-climates

Here is another one: http://www.buildingscience.com/docum...d-roof-systems

Code stuff: http://resourcecenter.pnl.gov/cocoon...r/article/1520

Looking in the wrong spots: http://www.buildingscience.com/docum...rchterm=attic+

Actually, they are all from my library: https://docs.google.com/viewer?a=v&q...JDTxZl32fFfiDg

The best: http://www.buildingscience.com/docum...n-roof-venting

Foursquare: http://www.buildingscience.com/docum...next-100-years


Just click on some in the "related list" at the ends.
Cape: http://www.buildingscience.com/docum...h-century-cape

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Old 02-16-2012, 01:11 AM   #12
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i know they write about it because you have posted the links before!
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Old 02-16-2012, 03:33 PM   #13
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So in the hot deck vs vented, are there any studies on longevity concerns regarding hot deck? I didn't see any on building science. Also, if the venting is done to address the problem of ice dams will going to a hot deck be cause for concern with ice dams when it snows?

Hot deck is not vented and not a DIY project with spray foam.

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