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insulating attic storage space

I've just gained access to the attic space of my house, and in addition to adding a lot more insulation (there's only about R-13 now and I'm in NY), I want to build a finished-off, insulated storage space. My roof is gabled, the attic space is about 24' x 24', 4' top-of-joist to bottom-of-rafter down the center, and is framed with 2x6's 16" o.c. I'd like to build a couple of knee walls about 3.5' either side of center, and end walls 4'-5' shy of either gable end (so the vents can still function).
Most of the job seems pretty straight forward, but I'm unsure of how to deal with insulating the ceiling of the space. Since I want to maximize "headroom", my first thought was to put 3 layers of the 2" pink foam board between the rafters, and one continuous layer across those. At R-10 per layer, that would give me R-40, and I only lose about 3" of ceiling height. If I use R-38 fiberglass I lose about 8". Some posts I've read caution against using sprayed foam insulation against the roof sheathing because it may cause roof shingles to overheat (I have asphalt shingles). Does this also apply to foam board? Should there be an air gap between the foam and the sheathing? Should I not even use foam in this application because of flammability, or other issues? Can fiberglass be placed directly against roof sheathing without causing overheating problems?
Any advice would be greatly appreciated.

Re: insulating attic storage space

Instead of using pink rigid board, try using two layers of rigid poly-iso-cyanuarate or "high-R" board. Each piece of 2" board has an R-value of 12.8. Two layers between the rafters would yield 25.6 R-value and take up four inches of your 5 1/2" rafter depth. If you added a third layer of 1/2" board between the rafters, that would bring your R-value to 28.9 and you would still retain a 1" air gap between the insulation and underside of roof deck, for proper air flow ventilation. Then, in keeping with adding an additional layer of 2" board over the rafters oriented in the opposite direction, you can achieve an R-value of 41.7 total and only still lose that 3" of head room. Keep in mind that to maximize your R-factor, you will pay. The average cost for a 4' x 8' x 2" thick piece of "High-R" board is around $32/per sheet and 1/2" runs about $9/per sheet. I live in the upstate NY area and that is what I would average to pay per sheet. One sheet will fill three 16 o.c. rafter cavities.

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