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Community and Q&A

Is this a good insulation strategy for apartment over garage (new construction)?

DavidDrake | Posted in Plans Review on

In early planning stages for a detached single-car garage/small shop with ~600 SF apartment above, as accessory dwelling unit (rental) to my house. Would like to approach Passive House standards for air sealing and overall energy use.

Footprint ~ 14′ x 42′, on a relatively narrow, deep lot. Thickened slab foundation. Stair to second story apartment inside building envelope. Climate zone 5, 6700 heating degree days, 10F winter design temperature.

Considering Remote wall system, as follows (from inside out):

First floor wall : Drywall, 2×6 stud wall w/ R-19 batts in cavity, OSB sheathing, water/air barrier (Grace Ice & Water or 6 mil poly), 4″ EPS foam, horizontal 3/16″ coroplast furring, corrugated metal siding.
Second floor wall: same as above, except 2×4 studs, R-13 batt, and 6″ EPS.

Garage ceiling: drywall, 2″ EPS, 11 7/8″ TJIs w/ blown cellulose, subfloor. Not sure about air sealing.

Roof: Cathedral 12/12 pitch (pitch matches existing house). Leaning toward open-web floor trusses used as rafters, with blown cellulose. Foam over sheathing (2″ ?), corrugated metal roofing. Not sure if this assembly is best vented or unvented. For aesthetics, considering a plywood ceiling in apartment–will this be adequate for air sealing?

Thanks in advance for input.

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  1. GBA Editor
    Martin Holladay | | #1

    [Edit: Pardon my brain cramp. The paragraph which used be be here, recently deleted, was written before I had a cup of coffee. Thanks, Dana.]

    Your plan for the cathedral ceiling needs to be modified.

    In your climate zone (Zone 5), you need at least R-20 of rigid foam to keep the sheathing warm enough if you are building an unvented roof assembly -- so 2 inches of foam won't work. For more information, see How to Build an Insulated Cathedral Ceiling.

  2. Reid Baldwin | | #2

    What temperature are you planning to keep the garage in the winter?

  3. Dana1 | | #3

    In a zone 5 climate only 25% of the wall-R needs to be outside the condensing surface for walls, and 40% for roofs, as long as the interior drywall is painted with a standard latex paint, making it a class-III vapor retarder.

    Regarding the first floor walls, an R19 batt is only R18 when compressed to 5.5" from it's tested 6.25", and there is R16.8 (assuming 1.5lb density EPS) on the exterior. That's WAY more than 25%- it's 48%, which poses NO condensation risk for walls in a zone 5 climate, and would still have a bit of margin in zone 7 or 8.

    Even if you let the garage coast at 50F average winter temp (it'll probably be warmer), unless there is a humidity source the dew point of the garage air dew point will track more closely to the outdoor dew point averages than the fully conditioned upstairs, since it's nearly impossible to make garage doors fully air tight.

    If you can find some magic R10 / inch foam for the roof and promise not to go more than R30 under the roof deck 2" of exterior foam would be adequate there. (That foam exists- ask the tooth fairy about it next time she rides by on her unicorn! :-) )

  4. DavidDrake | | #4

    Thanks--wasn't thinking about how much foam was needed over sheathing for unvented roof. With open web floor trusses used as rafters, will thermal bridging be insignificant enough to just do an all-cellulose vented roof assembly?

    Don't plan to heat garage/shop regularly. Maybe during the day some weekends. Would like it to get to temp fairly quickly when I do. Does r-30+ in the garage/shop walls seem like overkill then? Part of the reason I was thinking of 4" of foam on the first floor walls is it allows the inside and outside walls of both storys to be in same plane, and give 2x4 2d story framing full bearing on the 2x6 plate below.

  5. GBA Editor
    Martin Holladay | | #5

    Your walls will be fine. Here at GBA, many of us are in favor of an insulation approach that some people call "overkill." If you want to save money, however, you can put less insulation in your garage walls.

    Open-web floor trusses don't have as much of a thermal bridging issue as solid rafters, so you probably want to go ahead with the vented roof assembly approach. Make sure you aim for a minimum of R-49.

  6. Reid Baldwin | | #6

    I have faced similar issues regarding heating a garage/workshop. The home I am building has an attached airplane hangar that I expect to spend some time in in winter, so occasional heat would be nice. My plan is to eventually install a natural gas radiant heater from the ceiling. These make you feel warm soon after you turn them on as long as you are out in the open. They wouldn't help if you are working under a car.

    I am curious about your selection of open web floor trusses as rafters as opposed to using roof trusses. I am not critiquing your decision. This is a subject that I have not educated myself about so I would like to know the logic you used.

  7. DavidDrake | | #7

    Thanks, Martin. I've been poking around the GBA site for a few weeks--it's a pretty amazing resource. I started thinking about my project in terms of passive house/ pretty good house standards for a couple reasons: one, the university where I teach just submitted a proposal for the 2017 Solar Decathlon competition--if we're accepted, I'll be playing a lead role in helping the students with construction. So some of planning my own project is actually planning strategies for Solar Decathlon as well. Two, talking with a colleague who suggested super insulation + electric baseboard could actually be more cost effective (up front and in terms of monthly bills) than conventional insulation + mini split heat pump. I did some heating load calculations and it looks like that's the case, at least if I do most of the work, which I had planned on.

    Reid, the open web floor truss idea came from this GBA article:
    I still haven't looked into local availability of floor trusses in the depth I need, or the economics of this approach vs. solid rafters or scissor or parallel chord trusses. I was concerned blown cellulose might not work well with scissor trusses, esp. given a 12/12 pitch. I suppose parallel chord trusses wouldn't have the same issues?

    One last thing: if I'm aiming for R60 in a 12/12 roof, should depth of insulation be measured perpendicular to the roof slope, or plumb? (Maybe that's a silly question.)

  8. GBA Editor
    Martin Holladay | | #8

    Q. "If I'm aiming for R-60 in a 12/12 roof, should depth of insulation be measured perpendicular to the roof slope, or plumb?"

    A. Perpendicular to the roof slope.

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