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Insulating a split level attic

I have an attic above my kitchen with 3 interior walls and I am trying to figure out how to insulate it correctly.

Currently the walls facing the bedroom and the entry way have fiberglass insulation.
The floor has cellulose insulation. ~2-3 inches.
The ceiling rafters has ~4" of white fiberglass/denim insulation inside of perforated plastic bags between the ceiling rafters, this appears to be pretty consistent, but I suspect it is leaking a bunch of air.

The roof line is several inches about the existing insulation and connects to the space above living rooms vaulted ceiling.

I am thinking about putting rigid insulation boards below the ceiling to air seal the space, is this a good idea or would it cause problems?

Asked by John Reuter
Posted Aug 20, 2014 1:47 PM ET
Edited Aug 20, 2014 2:12 PM ET

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John,
It's hard to visualize the walls and ceilings that you are describing, but a few principles apply.

If you plan to use fiberglass batts, make sure that the batts are enclosed on all six sides by an air barrier. Rigid foam can be used as an air barrier. Check with your local building inspector to find out whether you need to protect the rigid foam with gypsum drywall, or else use Thermax foam (which doesn't need the drywall).

Here are links to two relevant articles:

Installing Fiberglass Right

Two Ways to Insulate Attic Kneewalls

Answered by Martin Holladay
Posted Aug 20, 2014 2:10 PM ET

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