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Old 02-14-2015, 06:31 AM   #1
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Styrofoam insulation panels for attic floor?


So I found a source for 4'x8' 4" Styrofoam insulation panels for next to nothing and thought what if i doubled them up to install on the attic floor on top of the ceiling joists, since my existing fiberglass insulation is almost to the top.
At R 4 per inch that would give me R 32 on top of what I have which would be good for my area. I'm not sure that I favor the fire rating of Styrofoam but I don't see too many houses having a fire in the attic. Perhaps once in my life I've seen that, so that's not going to prevent me from doing this. The nice thing is that I would be able to still store some things up there that would not pierce the insulation, I have a problem with fiberglass, blown is is messy, this Styrofoam is dirt cheap..... what are your thoughts?
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Old 02-14-2015, 07:28 AM   #2
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You cannot leave it exposed, even in an attic.

Plenty of fires start in an attic and while rigid foam has definite application in a great many insulation scenarios, this is not one of them.

Rent and insulation blower and blow loose fill cellulose in there.

Better yet, air seal the attic floor prior to doing the additional insulation. There are a bunch of links on here and Gary in WA has several threads that have all the links cataloged.
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Old 02-16-2015, 09:14 PM   #3
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Foam board facing the attic is the BEST way a DIY'er can do (in a heating climate). Much much better than any air-permeable insulation; loose-fill cellulose, fiberglass, etc. Cover with an ignition barrier for safety/insurance claim if ever needed... think wall turned on its side; http://www.buildingscience.com/docum...e-perfect-wall

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Old 02-17-2015, 08:29 PM   #4
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I'm sure that you've already thought of this, but what is the size of your attic scuttle? The shine can quickly wear off of things that sound great in theory when presented with the logistics of the actual execution... Then again, if its a walkup or something like that this may be easier and therfore worthwhile.
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Old 02-24-2015, 11:25 AM   #5
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Quote:
Originally Posted by Gary in WA View Post
Foam board facing the attic is the BEST way a DIY'er can do (in a heating climate). Much much better than any air-permeable insulation; loose-fill cellulose, fiberglass, etc. Cover with an ignition barrier for safety/insurance claim if ever needed... think wall turned on its side; http://www.buildingscience.com/docum...e-perfect-wall

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Gary,

Can you please elaborate on the foam board facing procedure? Do you mean installing rigid foam between the ceiling rafters directly on top of the drywall instead of fiberglass batts?

I absolutely trust your opinion and expertise so I would like to know if that is what you meant.

Thanks!
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Old 02-24-2015, 12:19 PM   #6
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EDIT: Sorry, I meant putting the rigid foam in between the ceiling joists, not the rafters.
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Old 02-24-2015, 01:04 PM   #7
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Putting them between the joists will accomplish little to nothing if the attic is vented.

The envelope (i.e. air) barrier and insulation barrier must line up and occupy the same plane.
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Old 02-25-2015, 12:41 AM   #8
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"So I found a source for 4'x8' 4" Styrofoam insulation panels for next to nothing and thought what if i doubled them up to install on the attic floor on top of the ceiling joists, since my existing fiberglass insulation is almost to the top."------------- that is the way to do it. It is similar to a wall with drywall, cavity insulation, foam board, exterior air space; same as your attic- if it is vented. This stops the wood joist from thermal bridging and stops any attic convective loops from forming. It also stop air degrading your air-permeable fiberglass,cellulose or similar insulation. Your thermal (insulation) barrier is the cavity fiberglass AND the foam board on top; the air barrier is the room ceiling drywall. Add required ignition barrier against fire as a "thermal barrier" won't work because of the storage;minimum safety code; http://publicecodes.cyberregs.com/ic...9_3_par191.htm

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Last edited by Gary in WA; 02-25-2015 at 12:45 AM.
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