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Community and Q&A

Envelope for garage under duplex

mcdavemt | Posted in Energy Efficiency and Durability on

Planning a PGH, all electric, duplex on a small lot in Zone 6 Whitefish, Montana. Lot size/setbacks dictate that the two-story duplex with shared wall sit on top of the two-car garages. A scan of info from this site would indicate the garages be designed within the thermal envelope, as opposed to the envelope border being the ceiling of garage/floor of first floor. Any suggestions for a less expensive way to design the envelope than including the garages?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure why you think that a garage should be within a home's thermal envelope. On the contrary: because of air quality concerns, including a garage in a home's thermal envelope is a bit dangerous. It is far preferable to have the garage outside of the home's thermal envelope. (In fact, most green builders prefer a detached garage to an attached garage because of air quality concerns.)

    So I strongly advise you to design your duplex so that the garage is outside of the home's thermal envelope. You also need to include robust details for the air barrier separating the garage from the conditioned space of the home, to prevent car exhaust fumes and fumes from stored fuel, paint, insecticides, and so on from entering the home.

    Just because a garage is outside of a home's thermal envelope doesn't mean you can't insulate the garage. If you think that the garage needs to be heated in winter, the garage can still have insulation. But give the garage its own thermal envelope in that case.

  2. JC72 | | #2

    I think you have it backwards. Garages should be, and in some locales it's code, sealed off from the living space and I've never seen this site ever suggest otherwise. Of course detached garages are optimal, but they add a lot of additional expense to the build in terms of construction and require a larger lot.

    On a side note, try your best to refrain from providing garage access to the HVAC system. It's convenient but an overall bad idea.

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