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Hot tub room in a ‘heated garage’

user-1078381 | Posted in GBA Pro Help on

Hi GBA Pros,

We just got a call from a client who wants to build a hot tub room in their garage.

My colleague sent me this email about it.

The ceiling is 2x10s filled with fiberglass insulation and clad with 2” of rigid, foil faced, poly-iso with the master bedroom above that. One wall will be along the house wall (thermally neutral) , one wall will be on an exterior wall of the garage, and two walls will be built independently in the garage space that it heated to/kept at 55 degrees by a Modine heater. What would the best design be for the walls with respect to insulation, air barrier, and moisture migration?

I read the blog about the indoor pool. It has some insight which is helpful and one of those small indoor pools is like a hot tub.

I don’t know yet if the foil faced poly-iso is properly taped or if there is a window planned or extant in the exterior wall.

From that article I gleaned:

-Cover the hot tub when not in use.

– Ventilate: it is a small room though need to consider where the makeup air is coming from. If we ventilate with a high volume continuous duty exhaust fan where is the makeup air coming from? The garage? The house? Using a humidistat to turn the fan on and off? If we consider an HRV it is a huge cost for such a small space.

– Protect the wall against moisture. From the BSI-055 article about pools I see that using rigid foam as the insulation instead of batt insulation in the stud bays is preferred. That is where I was going as well. Then XPS on the inside of the studs, taped and sealed? Then a layer of sheetrock with a Class II liquid vapor barrier on it (paint)? Should the sheetrock be installed directly on top of the XPS or should there be strapping?

– The ceiling now has 9.25 fg in the ceiling joists (with MBR above) and 2″ foil faced poly iso. Should this the also be covered with sheetrock with the class II liquid vapor barrier?

What is your advice for this project in climate zone 6?

We appreciate any input.


Chris West

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    You probably know that this type of room is likely to have a high rate of expensive-to-fix envelope failures (rot and mold issues). Keep that in mind as you decide (a) whether you want the job, and (b) whether adding ventilation is an unnecessary expense.

    The safest type of insulation to use is closed-cell spray polyurethane foam, but well-taped rigid foam should also work. No matter what type of insulation you choose, pay close attention to airtightness.

    If you insulate with either closed-cell spray foam or rigid foam, you don't need an additional vapor barrier, so the type of paint you choose doesn't matter much.

    Choose moisture-resistant materials for interior finishes -- paperless drywall or plaster.

    You need a ventilation system. A small HRV would work, or two small fans -- one installed as a supply fan, and the other installed as an exhaust fan.

  2. user-1078381 | | #2

    Thanks a bunch Martin. That all makes sense.


  3. STEPHEN SHEEHY | | #3

    I have a hot tub in a separate room in an unheated garage. There really is very little moisture, since the cover is always on unless we're in the tub. The walls are just v-match pine. No mold.

    You'd have to be nuts to not cover a hot tub when not in use. Then you'd have a serious moisture problem, not to mention insane costs to heat the water. With the cover on, in an unheated space, we estimate we pay about $20 per month for electricity at .15kwh.

    I'd suggest the client skip the heat, skip the insulation and just put a fan through the wall to outside to ventilate when in use. A nice hot soak when it is frigid out is pretty nice. I'm in Maine, zone 6, by the way.

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