Insulate an unconditioned structure?
I am working with a client to design a detached 3 car garage in the Kansas City metro area (climate zone 4A). The structure will have either a brick veneer (with air space) or stucco cladding over a standard 2×4 stud wall with sheathing and weather resistive barrier. The roof structure will be trusses. The attic will be vented at the eaves and ridge. The client wants to install gypsum board on the walls and ceilings as well insulation in the wall cavity and above the ceiling plane. There are no plans to condition or ventilate the space.
I have suggested that insulating an unconditioned space is probably not a good use of resources and, to me, raises durability concerns. Given the assemblies described above with interior gypsum board finish are there dangers of mold growth within the wall or ceiling assemblies? Would it be better to omit to omit the insulation? If not, is it better to use paper faced or non paper faced batts? Any other thoughts?
Any input is always appreciated.
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It's a three-car garage, but the owner might want to use some of that space for other purposes. Insulation will make the space more comfortable summer or winter. Whether the space is insulated or not, there shouldn't be a mold issue if it is properly sealed.
Another poster had a somewhat similar situation, you might find the discussion of insulation options helpful: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/98348/insulating-finished-garage-unvented-cathedral
Unless there is a source of moisture drive from the interior, the dew point of the air in the garage will track that of the outdoors. No garage is truly air-tight, and actively ventilated or not there will be some air exchange with the outdoors.
As long as there are no true vapor barriers between the sheathing and the interior, moisture drives from sun-on-wet-stucco will still dry toward the interior.
Kraft facers are not necessary here, but they aren't harmful either. When bone dry the have a vapor permeance of about 0.5 perms, but when the moisture content is high enough to support mold it rises to about 5-10 perms. Latex paint is 3-5 perms, so it doesn't slow down the drying rate very much.
Don't put polyethylene sheeting anywhere, and use an appropriate lower-permeance weather resistant barrier suitable for stucco or brick. Vent the stucco/brick to the outdoors both with weep screed/weep holes at the bottom, and at directly out at top of the cavity (or into the vented attic space if there is no other option.)
Matt,
Dana advised "Don't put polyethylene sheeting anywhere." I think he meant to add this clause: "...except directly under the concrete slab, where a layer of polyethylene is essential."
DOH!!!
Yeah, what HE said!
("...anywhere in the wall stackup...", I meant.)
I think you'll gain a little moisture robustness by adding insulation. On days when the daytime dewpoint peaks higher than the nighttime low temperature, the inside air in the garage will still have a lot of moisture when the outdoor temperature drops and drops the temperature of the enclosure materials. You could approach a scenario where you get dew forming on interior surfaces. If there's insulation, the temperature drops more slowly, and that gives the interior air's moisture content more time to catch up with the exterior air's lower nighttime moisture content before the surfaces get too cold. I'm not saying it's a problem without insulation, but I think you are a little better off with insulation.
I wish my garage had better air sealing. This would make dehumidification feasible and would reduce rust on valuable tools and vehicles.
Wall/ceiling insulation would have some minor value in terms of reducing temperature swings. Also under slab insulation - to reduce floor condensation on high dew point days.
Thanks!