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Community and Q&A

Do heated shop buildings have the same moisture considerations as a house?

Rick Van Handel | Posted in Energy Efficiency and Durability on

Hi,

I’m in the planning stages of building a 1200 sf heated woodshop building at the northern edge of climate zone 6. I have more or less decided on 2×8″ framing on (19.2″ 0r 24″ oc spacing), taped exterior plywood sheathing, tyvek WRB, horizontal furring strips, and ribbed steel siding. On the interior of the stud wall I’m planning on dense pack cellulose with additional 1.5″ interior furring strips (“Mooney wall” to help reduce thermal bridging. The interior sheathing would be 3/8″ plywood untaped. Alternatively, I could omit the Mooney wall idea and use 1″ or 2″ of rigid foam with taped seams for an interior vapor barrier and then plywood.

Heat will be provided by a 95% efficient direct vent ceiling hung space heater and will be kept at 40 degrees unless I’m working in the shop in which case I usually turn it up to 60. Air conditioning will not be installed.

My question is, with only intermittent occupancy and intermittent use (nights/weekends), no cooking, no showering, single occupant, which insulation route would you go? I’m guessing that moisture drive is less of a concern than a house, but is this still something to recognize? Is it still a good idea to have the interior vapor barrier? If so, what would you recommend for the interior vapor barrier using the Mooney wall?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rick,
    1. Your shop will probably not generate a lot of interior moisture, unless the hobbies or work you engage in are wet.

    2. Your worries about vapor barriers and vapor diffusion problems are unfounded, and are based on old-fashioned ideas that have been discredited by scientists. Your shop does not require an interior vapor barrier. An interior layer of rigid foam is fine if that's what you want to install, but you certainly don't need any interior polyethylene. For more information, see:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    Do I Need a Vapor Retarder?

  2. Rick Van Handel | | #2

    Great. That was my thought also, but I wanted expert opinion.

    The shop is for woodworking only, so I have to keep the moisture low to prevent problems with warping. I use a solar kiln for drying wood, so it's already down to 12% m.c. before any wood enters.

    Did you have any preference between the Mooney wall and interior foam? The whole wall R-value would be slightly higher with the foam, but the cost will be higher as well. The only real advantage I could see is that you could have air barriers on both sides of the wall if I taped the foam.

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