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Community and Q&A

Roof rafter insulation plan

Dean Tyler | Posted in Energy Efficiency and Durability on

I have a 1989 reverse saltbox (cape cod) style house in Maine. The long, front roof has a roughed-in eve attic. The eve attic is insulated at the floor and eve walls, not at the roof. The peak attic has a ridge vent and bat insulation on the floor. We are doing renovations and we are planning to install insulation at the roof to expand the building envelope. Currently, there are vents at the drip edge and peak, but air communication between the eve and peak attic with blocked by bat insulation. The roof deck is in great shape and no signs of moisture. We did do blower door on the house and it is pretty tight. The architect has one roof insulating plan and the insulator has another. I am not sure which is best.
Plan A
1. Install rafter vents boards
2. Build a knee wall at the base of the long roof attic to increase the insulate at the roof’s bottom edge
3. Building out roof rafters 4” and install netting or felt to hold cellulose insulation
3. Fill the rafters with cellulose
4. Blow in cellulose in the peak attic floor
Plan B
1. Seal the drip edge vents
2. Install Thermax sheeting to the rafters
3. Fill roof rafters with cellulose
4. Blow in cellulose in the peak attic.
I like the idea of using the Thermax to hold the cellulose in the roof rafters, since it gives me a better R value and a thermal break. Is the Thermax a vapor barrier so the venting is not necessary? Is it OK to have Thermal, cellulose, then rafter vents? Also, the roof is asphalt and over 20 years old. When it is time to replace I might consider metal or asphalt. Is there any problem with a metal roof with either insulation plan? Lastly, we are installing two mini-split 2×2 cassettes in the peak attic floor (bedroom ceiling). Are there any issues with blowing insulation over these?

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  1. GBA Editor
    Martin Holladay | | #1

    If Plan A includes ventilation baffles extending from the eaves to the ridge vent, that's the way to go.

    Plan B violates the building code and violates recommendations from experts. You can't install cellulose insulation between rafters unless either (a) there is a ventilated air space between the top of the cellulose insulation and the underside of the roof sheathing, or (b) there is an adequately thick layer of rigid insulation on the exterior side of the roof sheathing.

    For more information, see these articles:

    How to Build an Insulated Cathedral Ceiling

    Insulating a Cape Cod House

    -- Martin Holladay

  2. Dean Tyler | | #2

    Thanks for the information. Is it feasible to have combine the plans to use rafter vents, cellulose and 2" thermax to hold the cellulose up?

  3. GBA Editor
    Martin Holladay | | #3

    Yes. That approach is listed in one of the articles I linked to, How to Build an Insulated Cathedral Ceiling.

    -- Martin Holladay

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