Pacific Northwest: Insulating a detached garage
Climate zone 4, the Pacific Northwest: I have a detached garage that has no insulation, other than in the roof. We store things there and they are covered with mold due to the constant damp, although there are no obvious leaks. I want to insulate the garage and add heat to decrease the cold and damp and make it usable.
My thought is to put a tyvek type wrap against the interior studs, add insulation, then drywall. I can’t tell if in this climate I should add a vapor barrier as well, or if that will just keep the damp in the walls and let mold grow on the insulation.
I don’t think there will be an ideal solution given the situation, but I don’t want to make it worse! Suggestions?
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Replies
Do you still use the building for parking cars?
It could be that moisture is coming up through the slab. In that case, heating the building would likely just make the problem worse, since mold grows faster in warm humid conditions. You'd have to somehow deal with the moisture first. It could be the result of poor drainage around the building.
Any efflorescence on the slab might indicate that moisture is coming up through it. Another indication is if you move a box or something and the concrete appears darker where the item was sitting. You can also do a test by taping a roughly 2' x 2' piece of clear plastic sheet to the slab, making sure the entire perimeter is sealed, and leave it for a couple of days. If water droplets appear on the bottom side of the plastic or the concrete turns darker, then moisture is definitely coming through the slab. This test will work better in warmer weather or if you can leave a heater in the garage. But as long as your temps are staying over 40 °F, it should work ok, though you might leave the plastic in place for a few days.
The constant damp is the issue that needs to get resolved. Insulation and heating might be the solution, but just as likely might not, or might not be needed once the moisture issue is resolved.
I have a couple detached storage spaces in the PNW too (one of them insulated) and neither have any mold issues because they're both dry.
While there is doubtless moisture coming out of the slab, the changing temperature in an uninsulated building causes moisture in the warmer air to condense on cold objects which I have had cause mildew or rust
I don't park my car in the garage- I use it as a work space and storage. I don't believe the moisture is coming from the concrete slab- nothing on the floor has ever been wet. It's boxes on the shelves that feel soggy, even though they aren't wet.
So the question remains about wall insulation, use of tyvek and/or a vapor barrier for this wet region of the country.
Nancy,
Like Andy I've got several sheds here on Vancouver Island which I think represent the two successful strategies for outbuildings here: One is insulated and heated, the others un-insulated and vented. None have the moisture problems you describe. In the vented ones cardboard boxes stay dry, and nothing grows mold - even when there is some on the exterior siding.
Your garage unfortunately represents the worst of both worlds. The roof insulation means it doesn't to benefit from solar drying, and the lack of ventilation means it has no way to dry, while experiencing temperature swings that bring the air below the dew point causing things inside it to take on moisture though adsorption.
Like others I think you are getting moisture from below. Not enough to wet things left on the slab, but enough to elevate the humidity levels sufficiently to cause problems. Heating the space will help, but you may also need to either limit the moisture drive though the slab with a vapour-barrier, or dehumidify the space. Get a cheap humidity reader. See what it is now. Aim for below 60%
The walls will be fine with batts and poly. No need to allow them to dry to the inside.