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Insulation for attic of Cape Cod house

Posted by TheodoreTheodore (My Page) on
Tue, Nov 6, 12 at 1:00

I insulated the 'attic' of my cape cod house 2years ago. The attic joists are 2x6s, as are the rafters. The attic is only 3ft high, so I did my best to add 6' kraft-faced insulation between the joists, and then 9' unfaced insulation laid perpendicular to the joists. Of course, it was impossible to get new insulation from the attic down to the knee walls (between the joists): just too difficult no matter how I pushed/pulled. Now that Hurricane Sandy damage requires me to strip off and replace all sheetrock and insulation, I want to do a good insulation job along the sloped ceiling/wall too (i.e. from the knee wall to the attic). These upper rooms get unbearably hot in the summer, radiating heat from the sloped wall/ceiling. I can't afford spray foam. Is rigid XPS foam board between the joists better than fiberglass batts? Maybe 2 layers of 2' thick, which would leave 2' air gap between the XPS insulation and the roof deck? I'm also thinking about adding 1' XPS between the joists and the sheetrock, to act as a thermal break, but I'm concerned about popping screws.
Your advice greatly appreciated.


Here is a link that might be useful: cross section

Follow-Up Postings:

RE: Insulation for attic of Cape Cod house

You might want to consider converting your small attic to a conditioned unvented attic by closing off all roof and/or soffit vents and installing the insulation from the top down with layers of XPS or spray fibreglass. Which materials you use are dependent on your climate. (See link.) And since you are removing all drywall you would have access to the inside portion of the roof beyond the kneewalls.

Wishing you well and an eventual return to a better home than you left.

[I'm originally from Long Beach, Long Island, New York and remember seeing my Mom's photos of the aftermath of the Long Island Express that slammed into the northeast in 1938. This time, in Long Beach, the ocean and bay met sweeping in a wall of water up to three feet high.]

Here is a link that might be useful: Moisture Safe Unvented Wood Roof Systems

RE: Insulation for attic of Cape Cod house

look at the bottom of page 3
for details of how best to seal knee wall areas.

insulating kneewalls with batts
then comming back with foam sheathing
installed tightly and caulked for
air sealing will allow these kneewalls
to perform.

here in the hot humid south we use the
foilfaced foam sheathing with foil
facing into attic space.
reflects heat back into attic rather
than into wall.

best of luck.

RE: Insulation for attic of Cape Cod house

here in the hot humid south

And I always assume posters are in the healthy bracing air of the Northern climes!

RE: Insulation for attic of Cape Cod house

I really like it when people post where they
are located.
not that I wan't to stalk them..
but so that you have an idea when you
offer advice.
some sites have a place where the OP can
enter this info.
so much information is climate specific.
not all, but some.

its going to be chilly & humid here next week.

have a good weekend folks.

RE: Insulation for attic of Cape Cod house

Hi, I'm the OP. House is located in New York. I'm very comfortable with airsealing all of the typical Cape Cod leak locations. I'm most curious to know opinions on thermal break and whether what I'm suggesting will cause drywall screw pops and joint cracks.

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