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shumakerscott 09-04-2008 01:23 AM

Diference between Fiberglass and Hard Foam
I'm trying to decide between either fiberglass rolls or hard foam for my roof insulation. I would like to know what 4" of hard foam is equivilent to in fiberglass. The firms here say I should use at least 6" foam board. It costs an arm and leg for that stuff. I already have 2 rolls of perferated radiant barrier, the foil stuff. I was looking at about 12" of fiberglass. My ceiling will be open vault inside with tounge and groove. On the outside I plan on a vapor barrier with the 12" fiberglass directly on top. Then the perferated radiant barrier.I will leave a 2" gap for ventilation then my roof. Does this sound right? Of course I want the best for the least amount of money, don't we all. Thanks SHU

OldNBroken 09-04-2008 08:22 PM

If you are only looking at cost, batt is considerably cheaper than polyiso (rigid insulation).
R-value of rigid insulation is approximately 5.6 per inch for polyiso and approx 3.8 for 1.5# EPS
Cost difference per R-value of either type of rigid insulation is negligible.

To obtain, for example R-30 with either product, around here, would be approximately $1.80/square foot for either type. Compare that to batt and see where you are. Polyiso would have to be 2 layers of 2 1/2", EPS would be approximately 8 inches thick.

shumakerscott 09-06-2008 01:24 AM

Thanks for the reply. As I understand it then, Fiberglass is about 30% cheaper but needs about 30% more thickness for the same R value? The price difference here is quite abit more. I don't care about thickness, I have lots of room. I'm going to go with fiberglass, 10", with a perferated radiant barrier on the outside. On the underside of the fiberglass I will use a breathable vapor barrier. Do you see any problems with this? Thanks, SHU

OldNBroken 09-06-2008 12:16 PM

Sounds like you've got it together. Make sure your batts fit snug in the bays and snug against their laps, but not tight and bunched. Your 2" gap must be clear and unobstructed from soffit to ridge. I agree with you on interior insulating, batt installed properly is quite effective and, if you have the room for it, there is no need for rigid insulation. It is mostly used underneath our roof systems when one cannot properly insulate the interior of the bldg.

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