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Community and Q&A

What is the smartest way to insulate a finished attic?

Randy Aubrecht | Posted in GBA Pro Help on

Hi. I am a newby on this site and am looking for advice on how to correctly insulate the living space in my attic.

I believe in a vented roof so I vented the rafter bays with 1″ Dow Tuff R rigid insulation spaced 2″ off the underside of the roof sheathing. It is sealed all the way down to the floor and it ends about 6 inches above the collar ties. I sealed all the edges with foam and I taped all the seams.

The rafters are 2×10 and the collar ties are 2×8. I live in northern New Jersey in IECC zone 4. The rigid insulation is R6.5.

I have space in the ceiling for R38 fiberglass batts but I am not sure what I should do with the slanted sections of the roof that will have sheet rock attached to it. Having used the rigid insulation to create my vent chutes, I am not sure how this affects what kind of vapor barrier(or retarder) I should use.

I am planning to insulate the knee walls for added R value. What am I doing right, what am I doing wrong and what am I supposed to do?

Thanks for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    1. You live in climate zone 4; according to the 2009 IRC, you need a minimum of R-38 in your ceiling.

    2. Your 2x10 rafter bays are 9.25 inches deep. You have already installed a ventilation chute and rigid foam that take up 3 inches of that depth, leaving you 6.25 inches.

    3. Your rigid foam has an R-value of R-6.5, so you have to figure out a way to add an additional R-31.5 in your roof assembly. If you want to put it all between your rafters, that means you need an insulation product that performs at R-5 (or better) per inch. The only insulation product that would do that (other than rigid foam, which is hard to install) is closed-cell spray polyurethane foam.

    4. If you supplement the insulation you install between your rafter bays with a layer of rigid foam under your rafters, you could install fiberglass batts or cellulose between your rafters.

    5. More information here: How to Build an Insulated Cathedral Ceiling.

  2. Randy Aubrecht | | #2

    Thank you Martin for your response. If I decide to use rigid foam for cost reasons, do I need to seal each piece of foam board? It is difficult to get a perfect fit between the rafters.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "If I decide to use rigid foam for cost reasons, do I need to seal each piece of foam board?"

    A. Yes.

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