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Advice Please: Dehumidifier in Detached Garage?
Old 06-03-2018, 01:47 PM   #1
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Advice Please: Dehumidifier in Detached Garage?

Our summer place has a two-car detached garage with a guest cottage above it. It is generally very humid here year-round, and this structure is well shaded, so it stays pretty cool on the garage level. The space above is kept heated to 50 degrees in the winter, not sure how much heat makes it into the garage space. This is in a place close to the ocean, so salt corrosion is an issue. On humid summer days, the garage floor (and everything in it) gets damp from condensation.

We keep outdoor furniture, garden tools, and an older vehicle in the garage over the winter. We have noticed that mildew sometimes grows on the walls and ceiling, and the wooden garage doors are starting to show signs of rot.

I'm thinking of getting a dehumidifier for the garage space and running it year-round connected to a hose that would drain to a sump pit. Any advice? BTW, I'm not sure how to figure out how much electricity it would use, but electricity is pretty expensive here, about $.21 per kWh.
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Old 06-04-2018, 07:46 PM   #2
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I'd get a humidity gauge to measure the levels inside vs outside and try an exhaust or circulator fan before going to the extra expense of a dehumidifier.
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Old 06-04-2018, 08:10 PM   #3
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Originally Posted by 45th Birthday View Post
I'm thinking of getting a dehumidifier for the garage space and running it year-round connected to a hose that would drain to a sump pit. Any advice? BTW, I'm not sure how to figure out how much electricity it would use, but electricity is pretty expensive here, about $.21 per kWh.
Unless the unit upstairs is adding moisture to the air of the garage (shower, washing machine, cooking, etc), then it is unlikely that you'd need to run the dehumidifier in the winter. Though the garage will still be cool, it will be warmer than the cold air outside, so its RH will be lower. Plus, mold, rot, mildew, etc won't progress very much at temps below 50 even if the RH is high.

Summer: If you really need to run the dehumidifier, it's gonna cost some appreciable electricity. The label on my unit says it draws 7 amps (so, about 770 watts), so if it is running 1/2 of the time, it will cost you about $2 per day (interestingly, the label says it removes 1.77 liters of water per KWH of electricity used, so if you knew the air leakage into the garage and the water content of the air, you could get a pretty close estimate of the power you'd use). And, a dehumidifier will warm up the garage, adding to the AC load of the unit above. So, if you decide to go with a dehumidifier, definitely take the time to air-seal the garage as much as possible to reduce the amount of moist air that comes in.

Another option is a small window AC unit. It could be cheaper to purchase than a dehumidifier, and it will cool the garage while getting the moisture out, so it won't add to the heat load upstairs. The down-side to this is that most of these units don't have a humidistat, so it will only run when it is hot in the garage (not when it is moist).

There are "smart" ventilators that sense the temp/humidity inside a space and the temp/humidity outside, then run the fan only when it will serve to dry out the inside area. These are much more energy efficient than an AC unit or a dehumidifier, but they work best in places that have significant swings in daily outside humidity. If you are near the coast and it is humid outside whenever it is hot, they won't be much good to you.


If the concrete floor of the garage is wet, it could be due to condensation (esp if the floor is cold and it is exposed to warmer, moist outside air). Or, it could be due to wicking of ground moisture up through the slab (esp if there's no plastic barrier over the gravel and below the slab). To tell what you've got, securely tape a sizeable (e.g. 2' x 2') sheet of foil or plastic down in the middle of the slab. Wait a few days, then on a day when the slab isn't wet elsewhere pull off the tape and look underneath the foil: if it is visibly wet compared to the rest of the slab, then the water is coming up from below. Tape it back down, and then wait for a day when the slab is wet inside the garage: Is it less wet under the plastic? Then that wetness elsewhere is due to condensation. You can apply sealers to a concrete floor which will do a fair job of reducing the water vapor from coming up, and that might be worth a try if this looks like an appreciable source of the humidity in the garage. There's a good chance it is if the garage is more than a few decades old and the soil is not very well drained (putting plastic under the slab of a garage is a relatively new building practice).
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Old 06-05-2018, 06:26 AM   #4
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Thank you for the advice, especially samclem for your detailed response.

I'm afraid that a fan won't do much good because the relative humidity is generally very high. As I type this, it is 53 degrees outside with 93% humidity. It is humid here when it is cold and when it is hot. Dense fog is a relatively common occurrence.

I'm glad to hear that running it in the winter is not necessary since the unit I'm looking at won't run below 41 degrees.

I can run the condensation test on the floor, but the house is not old (2004-05), and the garage floor has an epoxy coating. The soil here is very sandy and drains well, even though the water table is fairly high. I have noticed the dampness on the floor is on days when it is very humid and the floor is cold.
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Old 06-05-2018, 07:35 AM   #5
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I run a dehumidfier in a similar structure in the summer.

I have it on a timer so it is only on a few hours per day.

This is just enough to knock back the Carolina humidity such that it doesn't get moldy or musty. Since it doesn't run a lot, the cost is minimal.
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Old 06-05-2018, 08:34 AM   #6
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I can run the condensation test on the floor, but the house is not old (2004-05), and the garage floor has an epoxy coating. The soil here is very sandy and drains well, even though the water table is fairly high. I have noticed the dampness on the floor is on days when it is very humid and the floor is cold.
It sounds almost certain that the floor dampness is due to condensation (warmer moist air contacting the cooler concrete floor slab), not moisture wicking up. Now and the next few months will be the worst time for this type of visible moisture on the top of the slab, it will occur less often as the slab (and ground) continues to warm up and the outdoor temps eventually decrease (= a smaller difference between the slab temps and the outdoor temps). An inch or so of rigid foam insulation under the slab would have helped a lot (material cost--about $200 for a 2 car garage) but it is too late for that now.
If you get a dehumidifier and set it to run when humidity is higher than 75% and temps are also higher than 70-75 deg f, that should do a lot to help. If you can get the drained water outside, or assure the sump pit has a tight cover, that will reduce the money you pay to re-remove the same water from the air. Even with the dehumidifier, the relative humidity may be higher than the humidistat setting near your walls (when they are cooler than the room), the ceiling (when the upstairs AC is on, cooling the ceiling), in the corners, etc. Getting a fan to move the air around inside may help reduce the moisture/mold issues in these spots.
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Old 06-05-2018, 10:05 AM   #7
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There is little you can do about condensation (rust), short of a dehumidifier or heater as already mentioned.

As for the mold/mildew buildup, if you still have that after dehumidifying (or decide not to dehumidify), I recommend looking into a product called Wet & Forget. I have used it on our North side wooden deck, siding and fence. It does everything they claim and one application lasts > 1yr. They now have a sister-product specifically for indoor use. I would try it in the garage.
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Old 06-05-2018, 02:54 PM   #8
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Thanks again for the feedback. I'm going to give it a shot and unplug it during the winter. It will drain into a sump hole that is in a separate furnace room, so not too much moisture should come back in. I'm not terribly concerned about the affect on the cooling of the space above, because the building is pretty well insulated and the garage space stays pretty cool in the shade anyway. A mild bleach solution has been working well to get any mildew off the walls and ceilings, but if we could avoid it forming in the first place, that would be good.
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