Damp Basement
September 12, 2017 7:54 AM   Subscribe

I have a property that we're not in a lot, so the air inside gets stale -- and this is most noticeable in the basement, which apparently traps humidity.

The walls -- poured cement -- do not appear damp, we have not noticed any standing water, but the mostly-unfinished (there's one drywall separating wall) basement has a musty, wet smell to it, and we've noticed a little mold on exposed wood.

This is a properly we've been slowly restoring, so the furnace is currently off, which would probably fix a lot of this issue if we could turn it on just to have the fan run (this is on my to-do, to see if I can get it to just run the fan). There's no air conditioner yet.

When googling answers, they seem to be focused on finished basements, or actual water presence. We're fairly certain there aren't leaks, although it's also in our plan to make sure all the basement windows are sealed as good as possible the next time we're down there. There *had* been a roof leak, and the roof was replaced last year, so I'm thinking that the moisture that got in, without much air circulation, slowly migrated to the basement.

So, we're looking for tips to help get rid of the musty, humid air in the basement, that can be left unattended for the times between when we're on-site (couple of weeks), so that mold doesn't expand. The basement itself is probably around 800-1000 square feet. Are box fans sufficient, just to keep the air circulating? Do we need to invest in a big dehumidifier? Do you have any tips for maximizing the dehumidifying affect?
posted by AzraelBrown to Home & Garden (13 answers total) 4 users marked this as a favorite
A dehumidifier should do the trick. Alternately, if there are windows you can use, a small a/c unit which will drain to the outside of the walls.
posted by notsnot at 8:01 AM on September 12 [1 favorite]

Seconding a dehumidifier. If there's a sink in the basement, it may be possible to have the dehumidifier on top of the sink and drain into that instead of its usual catch bin, meaning you don't have to come by and empty it.
posted by Capt. Renault at 8:05 AM on September 12 [1 favorite]

Yeah, a dehumidifier with a hose to the floor drain. You can leave that running unattended without worrying about it overflowing. Get a dehumidifier that is sized to your basement. Just circulating the air with fans won't help as the air will still be damp.
posted by fimbulvetr at 8:06 AM on September 12 [1 favorite]

BTW, I have lucked out getting cheap ($20!) second-hand dehumidifiers. Look for one of the big old, ugly, metal box ones. They are indestructible, do an excellent job dehumidifying, and because they are ugly they usually go really cheap.
posted by fimbulvetr at 8:12 AM on September 12 [1 favorite]

Yeah, chances are the air in the basement is at or near 100% humidity, meaning that no amount of circulation will dry things out because the air just can't hold any more water.

You can either ventilate, heat, or dehumidify your basement. Ventilation will exchange the wet air for dryer air taken from outside. Heat will allow the air to hold more moisture. Dehumidification will remove the water from the air for disposal.

If you go the dehumidifier-into-a-drain route (which I agree is probably the best way to go) make damn sure that your drain setup is reliable since if something goes wrong you may come back to a flooded basement. Also don't run your dehumidifier 24/7 if you're planning to walk away from it for a long time. Either use a humidistat or a timer; many dehumidifiers come with one built in. They may say they're good for continuous use, but if I were going to run one unattended I'd want to make sure that it was on a gentler duty cycle than that.
posted by Anticipation Of A New Lover's Arrival, The at 8:16 AM on September 12 [1 favorite]

I've thought of trying Damp Rid which was recommended here. It's some kind of "crystals" which suck up moisture. Maybe try that while you're sourcing a dehumidifier and just see if it makes a difference. On Amazon for under $20. You just open the package and leave it.
posted by amanda at 8:21 AM on September 12 [1 favorite]

If there's a sink in the basement, it may be possible to have the dehumidifier on top of the sink and drain into that instead of its usual catch bin, meaning you don't have to come by and empty it.

This is what I do for my unattended basement. Dehumidifier on a timer. If there are particular wood areas that get moldy you can also just point a fan directly at them on low and that will (should) keep air moving around them and minimize damage.
posted by jessamyn at 8:24 AM on September 12

DampRid is just calcium chloride, which absorbs moisture and liquifies as it does so (you just pour it down the drain once it has completely liquified). It's much cheaper to buy it in bulk form. Check your local hardware store to see if they have any sacks of ice melt around, read the label to ensure it's calcium chloride, then just fill several bowls with it and re-fill as needed.

That being said, that solutions is best for a fairly low-level moisture issue. Being a basement (i.e. a hole in the ground, and holes collect water), it's pretty much a given that you will need to use a dehumidifier or, if you live in an area where it's usually not too humid, a basement air exchanger. Those are basically wall vents with a humidistat and duct leading down to the floor. There's a fan inside the duct at floor level. The 'stat turns on the fan when the humidity exceeds the set point, and the cooler, damper air (which is at floor level) is sucked up and vented outdoors, while the presumably drier, warmer upstairs air flows downstairs.

Regardless of what you use, also place at least one and preferably two fans in the basement to circulate the air. Set them on timers. The airflow will improve the performance of whatever solution you use, and especially with the dehumidifier that means less runtime and wear and tear on the device. Modern dehumidifiers unfortunately have a short lifespan compared to the old models (and a distressing number of them have started on fire), so the suggestion above to look for an old secondhand model is very sound advice.

In terms of energy used, naturally the DampRid will be the cheapest, followed by the exhaust vent. The dehumidifier will use the most electricity, but again, running fans on timers will improve its performance and reduce its runtime, and fans use a lot less electricity, so it's a win.
posted by Lunaloon at 9:05 AM on September 12 [1 favorite]

Basements are below grade, so they're cooler and damp. Dehumidifier that drains into a drain is best, but uses lots of electricity. Ventilation helps - even leaving the door at the top of the stairs open, and maybe a window cracked on an upper floor, as long as there's airflow. Fans help.
posted by theora55 at 9:28 AM on September 12

Thanks all --- dehumidifier it is.

I had bought a garage-sale "metal box" dehumidifier and put it down there last year, but it didn't seem to help so I figured it was too small; on researching more about dehumidifier size, it should have been plenty to dehumidify , so now I'm thinking I bought a broken dehumidifier. Yesterday at lunch I asked if my dad had one, he let me borrow it, and I plugged it in to test here and just the outside edge of the cooling coil immediately frosted over, so I'm 0 for 2 on working "old metal box" dehumidifiers. I may need to invest in a newer one.

Also, in looking at fans -- not just moving air around, but moving it out -- I had an epiphany, along Lunaloon's suggestion: there's a dryer vent leading outside, but no dryer; I'm going to put a duct fan on there to blow basement air to the outside and hopefully that will help too.
posted by AzraelBrown at 4:39 AM on September 13 [1 favorite]

The outside edge of the cooling coil frosted over? The entire surface or just around the edge?

A dehumidifier is basically a refrigerator or air conditioner with both coils set up to first cool the air (causing condensation of water), and then heating the air with the second coil. All of these devices are actually heaters that incidentally create a cold zone of some sort as a "side effect". In a dehumidifier the coils are both working on the same air, so in the end you should wind up with slightly warmer air coming out of the unit.

It wouldn't be that unusual for a dehumidifier to build up some frost on the cold coil, especially in a very humid basement. Older units are basically a circular fan separating the cold and warm coils, and the slower airflow around the edges of the coil would be more prone to condensation collecting and freezing. If it is totally frosting over, that's not going to be helpful, obviously, but as long as there's a good portion of it without frost, it probably isn't a big deal, and will probably stabilize and melt as the humidity improves.

Be careful about using fans to vent air to the outside from a basement, especially if you have any natural gas appliances (dryer, furnace, water heater, etc). If you create a low pressure area inside the house, without any way for the pressure to equalize, this can cause a buildup of carbon monoxide ("backdraft"). There's a whole mess of problems associated with this kind of strategy, depending on your specifics, so please read up and be careful.
posted by jgreco at 4:45 PM on September 13

No reason you can't use a fan to blow air into the basement, though. It'd be safer, but even so you should probably leave the basement door cracked so that the pressure can equalize more quickly (for better airflow). Of course, now you're basically trying to dehumidify the great outdoors (since whichever direction the fan is blowing, air is coming into the basement from outside) so maybe just try the dehumidifier by itself for a while and see how you like it.

For any future readers, Damp Rid (even bought in bulk as ice melt) is not a great solution here. A five-gallon bucket of calcium chloride can absorb less than 5 gallons of water (because some of the space is taken up by CaCl) before it is used up, whereas a typical dehumidifier of the size that someone would put in their basement can remove about twelve gallons of water from the air per day on an ongoing basis. So if you need to remove any significant amount of water and keep doing it over time, a dehumidifier is your best bet.
posted by Anticipation Of A New Lover's Arrival, The at 6:45 PM on September 13

The outside edge of the cooling coil frosted over? The entire surface or just around the edge?

The cooling coil on the back side is a flat spiral -- just the outside edge frosted; the rest of the coil was cool, but not cold, to the touch, and there was no moisture in the bin or other than the frost on the coil; my amateur diagnosis is low freon, and googling says these are generally not rechargeable.

Also, furnace and gas is off, as I mentioned in the original post. There's no door to the basement at the moment.
posted by AzraelBrown at 6:58 PM on September 13

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