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Community and Q&A

Heat Pump Water Heater for Conditioning Garage

Cousinmike | Posted in General Questions on

I have an attached, unvented 400 sq ft garage that I’m looking to insulate mainly to keep things stored in the attic space from getting exposed to too much heat. My home is in zone 3C (Bay Area, CA). The garage is air sealed w/ Zip panels on the outside walls and roof.

My question is whether having my 65 gallon heat pump water heater in the garage would classify my garage as being a conditioned space and if so, whether I can get by with just adding some R-15 batts to the underside of the roof sheathing (2×4 trusses) and walls and call it a day or whether moisture and condensation would still be a concern?


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  1. user-2310254 | | #1


    I had a 65 gallon HPWH in the garage of another home (in CZ3A). It cooled the garage when running, but I never considered this conditioning. There was no return air path, for example. And the HPWH only ran in response to the demand for hot water.

    If it were my house, I would duct the HPWH to the outside and use dampers to control the airflow. Maybe someone else can give you advice on whether to insulate or not.

    1. brad_rh | | #2

      Steve, why would you duct it to the outside?

      1. user-2310254 | | #5

        Hi Brad,

        The ducting and dampers give you more control. For example, you can pull in air from outside when conditions are warm to hot while keeping the cooler exhaust inside the garage. During the winter, you can use the air inside the garage as your heat source for the HPWH and exhaust the cooler air to the outdoors. This helps to keep the garage from getting too cold and probably helps with efficiency.

        1. brad_rh | | #8

          You'd be pulling cold outside air in to the garage to make up for the exhausted HPWH air.

          1. user-2310254 | | #11

            Not necessarily. The dampers allow you to open and close the supply and exhaust vents.

            Ducts aren't required but give you more control over how the HPWH operates.

  2. charlie_sullivan | | #3

    It should be possible to estimate the amount of cooling you'll get from an estimate of how much hot water you use. You'll also want to consider things like what happens if you go away for a long weekend in the summer and don't use any hot water--would the resulting rise in garage temperature be OK?

    And also, are you OK with the garage being cooled down to lower temperatures than it sits at now in the winter.

    The Bay Area has widely varying microclimates, so we also might need some more local data.

    1. Cousinmike | | #6

      Thanks for the response. I'm on the Peninsula. Very mild weather here. Highs during the summers are usually in the 80's, sometimes creeps into the 90's with a rare 100 degree day here and there.

      I'm ok with cooler temps. It never gets that cold here to start with.

  3. Expert Member
    BILL WICHERS | | #4

    No, this would not be considered space conditioning. This situation is no different than putting a freezer in the garage, storing meat for your BBQ in that freezer, and they claiming that the freezer is heating, and thus "conditioning", the garage in the winter.

    A "conditioned space" is CONTROLLED, so you need some type of equipment that is intended to condition air to be installed there for the purposes of conditioning the space, and you need some type of control apparatus (typically a thermostat) to maintain a setpoint. It is this "maintaining a setpoint" that makes the space conditioned, since you are now actively controling the enviornment in the space for the purposes of maintaint a specific set of conditions in that space. Since the water heater's primary function is to heat the water in it's integral tank, not to control the temperature in the room it's in, so you can't consider the water heater to be "conditioning" anything other than the water in it's tank.


    1. Cousinmike | | #7

      Thanks Bill. So now that we established that I can't call this a conditioned garage, I have 2 follow-up questions:

      1) if I were to add r-15 batts to the roof and walls of the garage will that make an appreciable difference in how hot the attic will get in the summer?

      2) is there significant risk given my climate zone of accumulating moisture and condensation if I only add r-15 batts to this unvented space? I'd like to avoid spending the extra money to spray closed cell spray foam to this area to insulate it if at all possible.


      1. Expert Member
        BILL WICHERS | | #10

        You'd probably be better off venting the attic if you want to keep it cooler up there. Vented attics can get hot. Unvented attics can REALLY REALLY HOT! The quickest/easiest way to cool it down is probably to vent the soffits and ridge, then take some foil faced polyiso under the rafters with some rigid mineral wool over that. That would get you a radiant barrier (the foil facing on the polyiso), some insulation (R value of the polyiso + rigid mineral wool board), and fire barrier for the polyiso (rigid mineral wool board). This would install easily, but wouldn't be particularly cheap since rigid mineral wool is fairly expensive as insulating materials go.

        I don't think you're likely to have any mositure issues adding some insulation to this space, especially if you also vent it.


  4. Expert Member
    PETER G ENGLE PE | | #9

    1) Probably. It depends a lot on how much air leakage you have through the door seals and other areas.
    2) Not unless you have a significant source of interior moisture. There are very few weather conditions in your area that would result in condensation issues for more than a few hours at a time, once in a while. Also, the HPWH will dehumidify the interior air a bit as well as cooling it. It's just not controlled and it's a little bit hard to predict its value.

  5. jameshowison | | #12

    I'm having a bit of trouble visualizing this situation. You write of an attic, and of an attached garage. Is the attic just the top of the garage space? Or is the top of the garage open to the rest of the attic over the rest of the house (with insulation on the flat ceilings in the rest of the house)?

    If the top of the garage is open to the rest of the attic, then I think it won't matter much what you do. We had a situation like that and had a HPWH in the garage, water sure got hot but the temp throughout the garage didn't change at all. It was hawt, both at ground level and especially if you poked your head up near the rafters.

    If the garage space (including the area above the joists) is separated from the rest of the house, including the attic of the rest of the house, then I think things become a little more manageable.

    I would imagine that the primary source of heat is radiant from the roof sheathing; the HPWH isn't going to impact that sufficiently.

    So to maintain storage space above the ceiling joists, you'd be trying to create a cathedral ceiling over your storage space. So I'd look at the options for cathedral ceilings in your climate zone. Another option might be to do a "fur-up" of a ceiling for the attic, add drywall, then lay fiberglass or blow cellulose above that (and deep enough to cover the kneewalls on the side of the fur-up). I'm imagining something that ends up looking like a plenum truss.

    A lot of work for a relatively small amount of awkward to access storage space! Maybe a regular drywall ceiling at the joist level, insulation above that, and invest in a great set of cabinets around the walls of the garage?

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