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Community and Q&A

Thermal envelope (Zone 4A)

user-1088294 | Posted in General Questions on

We are finishing off a room in the attic, above an unheated 2 car garage. The room has 5 ft knee walls and a cathedral ceiling. There are no HVAC components behind the knee wall. The floor of the room is already insulated (R30 batt) to the garage below and has OSB flooring. The room will be heated/cooled with a high seer mini-split ductless heat pump. We plan on using open-cell spray foam between the rafters in the ceiling. I believe we have 2 options. First is to spray from the ridge vent all the way to the soffits and insulate the floor behind the knee walls (that space overlies the garage as well but is currently not insulated or floored) with either spay foam or batt or cellulose. If we do that, I assume we don’t have to insulate the knee wall itself. The second option is to spray from ridge vent to just beyond the knee wall and then insulate the knee wall itself with batt that has an air block behind it (OSB). From the information I have gathered from reading this forum, I believe the advice will be to go with the first option, but I have a question. If I go with the first option, the area behind the sheetrocked knee wall, even though it becomes located inside the thermal envelope, is not heated or cooled, and will always be either hotter or cooler than the finished room. Won’t that negatively effect the temp inside the finished room and cause the heat pump to work harder than if I insulated the knee wall itself?

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  1. Expert Member
    ARMANDO COBO | | #1

    1. You need to make sure the wall to the house and floor system under the bonus room is closed and sealed from the rest of the house.
    2. You have 3 options to encapsulate an attic: a) install closed cell foam under the roof decking to meet R38 only, or b) 2” closed foam plus open cell to meet R38, or c) Install 2" rigid foam above the sheathing (it requires to reroof) and open cell under the roof sheathing to meet R38.
    3. Batt insulation is extremely permeable and installed wrong under the floor space. Once you condition the bonus room with option 1, you’ll be sucking the air and fumes from the garage inside the bonus room. You need to remove those floor batts and install 2” closed cell foam minimum, and then you can fill the cavity with blown-in cellulose or fiberglass insulation, or all close cell foam, or closed/open cell foam..
    4. If you encapsulate the whole attic, you do not need to insulate the knee wall.

  2. user-1088294 | | #2

    2) My inspector has told me that for the angled cathedral ceiling, R30 meets code; I would really prefer not to use closed cell on the roof line
    4) I just don't understand how the non-heated/cooled space behind the knee wall will not negatively effect the temp in the finished room

  3. Expert Member
    ARMANDO COBO | | #3

    - 402.2.2 Ceilings without attic spaces. Where Section 402.1.1 would require insulation levels above R-30 and the design of the roof/ceiling assembly does not allow sufficient space for the required insulation, the minimum required insulation for such roof/ceiling assemblies shall be R-30. This reduction of insulation from the requirements of Section 402.1.1 shall be limited to 500 square feet or 20 percent of the total insulated ceiling area [for the whole house, not just one room], whichever is less.
    - Some open cell foams are air impermeable at 5.5", some at 7, and some at 10". As long as you have room to have the right amount you could be ok... See 2009 IRC R806.4, and N1102.2.2 or 2009 IECC 402.2.2 (above)

  4. GBA Editor
    Martin Holladay | | #4

    If you install insulation along the sloped roof, extending the insulation to the eaves, then the area behind the kneewalls will be within the conditioned envelope of the house. Assuming you do a good job of air sealing and insulating, the temperature behind the kneewalls will be close to the temperature in the conditioned areas of your house. Believe it or not.

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