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Insulating bonus room of a Cape Cod

Thomas Muller | Posted in Energy Efficiency and Durability on

After much reading and searching exisisting Q and A’s on this site and finehomebuilding and i’ve decided to write in for some guidance. I am planning on finishing my bonus room above the garage in my cap cod and i have questions regarding insulating it propery.
First some info, the floor above the garage is insulated with R-19 Fiberglass batts. The area has two intersecting roof lines so not every roof joist cavity will have a soffit vent. Perhaps only half the area would contain both ridge and soffit vents. The area contains standard 2×4 walls on two exterior walls. The walls from the exterior are cedar clapboards then 1/2″ fiber board. The other walls have a knee wall transitioning to a cathedral ceiling. The cathedral ceiling is an 8ft run of 2×6 rafters that ends at the rafter ties for a small attic space. Hopefully i described this clearly.

I am trying to build this as eneryg effecient and also cost effecient as possible. Here are my options for insulating.
1) install baffles, then R-19 fiberglass, then 2inch of poly -iso board air sealed for an R- value of 35. or
2) tack 1” spacers in the ceiling joist cavity then omit the baffles and use xps foam board for a 1″ air channel air sealed, then R-13 fiberglass, then 2″ of poly-iso air sealed for an R value of roughly 35 again.

For my exterior walls i was going to use r-13 unfaced fiberglass with 1inch of poly-iso board on the interior, air sealed of course.

i was not going to insulate the knee walls as they will be in the building envelope provided by the roof and floor insulation.

My questions:

1 Can i sandwich fiberglass with foam as in option #2?
2 For rafter bays not supplied by both soffit and rafter vents, do i still provide 1″ of vent space?
3 The small attic space will be approx 6ft wide, do i continue with 2″ inch of foam over the drywall or can i insulate like a typical attic, fiberglass and loose fill cellulose. I will make the drywall airtight.
4. with all this detial, vented roof joists and unvented roof joists, kneewalls etc am i better off working lots of overtime and paying for 2″ closed cell in the walls and 3″ in the rafter bays? this option is rather costly as im sure everyone knows.
any thoughts concerns and/or advice is greatly appreciated, sorry for the long post

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Thomas,
    Q. "Can i sandwich fiberglass with foam as in option #2?"

    A. That method will probably work fine. However, you might consider using 1-inch EPS instead of 1-inch XPS to create the vent channel, since EPS is a little more vapor-permeable than XPS. (Note: see the answer to the next question before you decide how to proceed.)

    Q. "For rafter bays not supplied by both soffit and rafter vents, do I still provide 1" of vent space?"

    A. If you have rafter bays that have no access to soffit and ridge vents, then your roof is not a good candidate for a vented roof assembly. Vented roof assemblies require simple gable configurations without valleys, dormers, skylights, or other roof interruptions. You probably want an unvented roof assembly. More information here: How to Build an Insulated Cathedral Ceiling.

    Q. "The small attic space will be approx 6 ft wide. Do I continue with 2" inch of foam over the drywall or can I insulate like a typical attic, fiberglass and loose fill cellulose?"

    A. The answer to your question can be found in this article: Two ways to insulate attic kneewalls.

    Q. "With all this detail, vented roof joists and unvented roof joists, kneewalls etc., am I better off working lots of overtime and paying for 2 inches closed-cell in the walls and 3 inches in the rafter bays?"

    A. Probably. But only you can make that decision.

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