Garage mold: cause and design solutions?
I’m in Portland, Oregon, climate zone 4C. Last year we bought an existing house (vintage 1943), with an attached 2-car garage. The garage is unconditioned, uninsulated, and unfinished, with the exception of the wall connecting the garage to the house, which is finished, and presumably insulated. It originally was a 1-car detached garage, but in the late ’90s, the previous, long-time owner built a kitchen addition connecting the garage to the house, and also expanded the garage to 2 stalls. The garage roof has ridge venting, as well as a few soffit vents. The garage also has two southern-facing skylights, which were apparently installed by the previous owner a fairly short time prior to the sale.
During the winter months, I didn’t spend much time in the garage, nor did I pay a whole lot of attention to it. However, a few weeks ago, I noticed what appeared to be mold growing on some of the wall studs. We had it tested, and sure enough, it is mold. We have been parking one car in the garage, and given its Portland, the car is often wet when it is parked.
My two-part question is this: what might be causing the mold, and what is a potential solution?
Now for my theory. The wet car is obviously bringing additional moisture into the garage, and there are cold wood surfaces (3 walls plus rafters) on which the moisture can condense after evaporating from the warm car. But most garages in Portland have this scenario, and I’ve never heard of anyone else with this issue. I think that the skylights, which add a significant amount of heat to the garage, may be significantly exacerbating the situation. Thoughts?
As for a solution, I think a first step may be to install a Panasonic ventilation fan (a 110-150cfm unit, possibly triggered by a condensation-based control). Perhaps I’d also need to close up the ridge vent (and possibly the soffit vents) to avoid the fan drawing air from it. But while that may help, I’m not sure that will entirely solve the problem, given that the sky lights will still, in some sense, be conditioning the space via their added heat. If that’s the case, then I suspect I may want to remove the skylights. Any insights would be much appreciated!
Might another potential part of the solution might be to finish and insulate the garage, reducing the cold surfaces on which moisture might condense? I’m less confident that this will be sufficient (and I’m also not excited about such an expense).
Thank you all for any advice that you may have.
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