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Community and Q&A

Garage mold: cause and design solutions?

D_Elliott | Posted in General Questions on

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.

Doug Elliott

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  1. GBA Editor
    Martin Holladay | | #1

    Doug Elliott,
    In the Pacific Northwest, conditions in unheated buildings, or even in shady locations outdoors, are conducive to mold growth. Mold needs temperatures in the right range, and enough moisture, and a food source (wood) to grow, and if those conditions are present, mold will grow.

    The mold on your garage studs is probably harmless, unless you have an unusual medical condition. Most people live near mold without getting sick.

    Increasing the ventilation rate in your garage may help, or it may not help, depending on the outdoor conditions on the days that you ventilate. The only certain remedies are expensive: heating the garage or running a dehumidifier will work.

  2. D_Elliott | | #2


    Thank you very much for your reply. One thing I'm confused about is why others we know in Portland don't seem to have this issue. There seems to be something unique about our garage, and other than it being attached to the house (which is somewhat uncommon here, but certainly not unique), the only thing I can really point to is the skylights.

    You're certainly right that we'd often be pulling in air with a fairly high relative humidity if we added ventilation. But my thought was that the absolute humidity of the make-up air would be lower, as I'd be exhausting the warmer, moist air from the wet car. It seems that ventilation would result in a situation no worse than that faced by unfinished, unconditioned, passively-vented attics, which is the norm here. On a related note, I can't say that I'm clear as to why such attics, given the moisture here, don't seem to have problems. One thing they don't have (given decent air sealing of the house's conditioned space) is a source of warm, moist air.

    If ventilation doesn't do the job, and we have to resort to a dehumidifier, I would think that we'd need to seal the existing venting in the garage roof. Absent that, we'd be continually fighting the humidity of the incoming air, which would be higher in relative humidity than the dehumidified garage. In any case, it's certainly a frustrating issue.

    Thanks again,

  3. GBA Editor
    Martin Holladay | | #3

    Go ahead and experiment with ventilation. It can't hurt.

    The skylight isn't making things worse -- it's making things better (by supplying a little solar gain), as long as it isn't leaking when it rains.

    One possible source of dampness is damp soil under your concrete slab. This would only be a problem if your builder forgot to install polyethylene under the concrete.

    I really don't recommend running a dehumidifier in your garage, since mold is usually harmless. But if you want to install a dehumidifier, you would have to seal up the soffit vents and ridge vents, as well as any other air leaks, before plugging in the dehumidifier.

  4. D_Elliott | | #4

    Thank you, Martin. I think we likely will see what some additional ventilation does for the problem.

  5. charlie_sullivan | | #5

    Given your climate, you may be not be able to avoid fairly high humidity, so you might want to treat the wood with something to retard mold--a primer with anti mold stuff in it, just a mold retardant like Concrobium without the primer.

    If you want to get geeky about it, you could control the ventilation fan based on the absolute humidity inside and out. You could get a wireless weather station and put sensors in the garage and outside, and turn on the fan whenever the dew point inside the garage is higher than outside.

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