dehumidifier or air conditioner in damp, unfinished basement??

Molly BrownJune 28, 2008

Hi , I posted this question on the Home decorating forum, the conversation side, and this is one of the replies I got...Could you all with more experience than me on the subject weigh in your opinion on the subject? Thank you in advance!

"Hi all,

We live in an almost 200 year old cape. The cellar gets damp in the summer, so we have a dehumidifier (which EATS electricity). The cellar only has 2 cellar windows, both on the same side, and a bulk head on the opposite side that we open on nice days. The windows have screens for the summer and glass for the winter that my husband has to screw off and on to exchange the two.

Now, my theory is that with the screens on and the dehumidifier on, when it rains or is very humid out, the dehumidifier will keep sucking all the moisture from outside to the inside. I think that we should keep the glass on in the summer and use the dehumidifier to try and keep the cellar dry. There is moisture that comes from the fieldstone foundation that has very little cement between the rocks.

My husband thinks that we should keep the screens on to allow for ventilation in a poorly ventilated space, even if it rains. He puts the screens on in spring and takes them off in late fall and these are times where it's not humid down there and the air is good. But in the summer it's humid and condensation occurs.

I don't know what is the best thing to do...What would you do?

(Old houses have their own set of issues......;-( "

Here was one of the replies and thanks to the poster of this info...

"Bear with me while I try and coherently regurgitate the thermo-dynamics lesson my engineer DH just gave me to explain what you should do, LOL!

First, he's assuming you do not have central air in your home. If by some chance your 200 year old home has been upgraded to have central air but does not have vents into the cellar, that's your solution. Get the conditioned air flowing into the cellar and shut the windows - not in that order. Moving on because you likely do not have central air ;-)

There are a couple of things to understand;

1) the warmer the air the more moisture it can hold

2) dehumidifiers put out a LOT of heat

3) because the dehumidifier is raising the temperature in your basement, the air has the ability to hold more moisture

4) because there is more moisture in the air, anything colder than the air (your stone walls, concrete or stone floor, cold water pipes) will sweat because the moisture in the warmer air condenses when it is in contact with these colder surfaces

5) the sweating surfaces add moisture to the environment which makes the dehumidifier work harder which makes the air warmer ........ and so on.

Your absolute most effective way to reduce moisture in your basement will be to go to Home Depot or whatever is cost effective for you, and buy a small, energy efficient window air conditioner and put it in one of your basement windows. It does not need to be set at the coldest setting, just make sure it can drain freely to the outside. Close the other cellar windows and block up the open area around the AC unit. You'll use less electricity to run a small window AC than the dehumidifier and you'll be using the right 'tool' for the job. A window AC takes the moisture out of the air and gets it outside, out of the space you are trying to cool and dehumidify. A dehumidifier heats up the air in the space you are trying to dehumidify, making the air able to hold more moisture, then keeps that moisture in a bucket in that same space.

I know your primary objective is to remove moisture, not to cool the air but they go hand in hand.

Funny thing is, I actually understand this. you live with an engineer long enough and eventually some of it has to make sense!!! "

So what should I do??? Thank you!

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Get a dehumidifier. Here's the problem with what was said:

Air that can hold more moisture is a good, not a bad, thing because you are talking about relative humidity -- the amount of moisture in the air relative to what it can hold at its temperature: So, by doing nothing other than warming air, you can lower its relative humidity. If you were to put a heater in your basement and warm it from, say, 65 degrees to 75 degrees, you would drop the relative humidity of the basement because, even though the air has the same absolute quantity of moisture in it, the 75 degree air could hold more than the 65 degree air, so the relative humidity is less.

But a dehumidifier is even better than a heater because it removes moisture in two ways -- it drops the air to a temperature below its dewpoint, removing actual moisture, and then warms the air, lower the relative humidity further.

An air conditioner is a problem for at least two reasons. First, because it cools as it dehumidifies, its ability to dehumidify is limited to situations where cooling is needed. If it's below 70 degrees or so outside, the a/c evaporator is liable to freeze, and at that point it'll keep running but do nothing as far as cooling or dehumidifying until it's shut off and the evaporator is allowed to thaw out. Second, because the a/c lowers the temperature of the air, it makes condensation on those walls and other surfaces more, not less, likely. Why? Because if the surface of a wall is, say 64 degrees, and the air is 65 degrees at a high humidity level, it doesn't take much heat transfer for the wall to pull the air near it down just one degree (and if the humidity approaches 100 percent, that might be enough to create condensation). On the other hand, if the air is 75 degrees with the same absolute quantity of moisture it had at 65 degrees, the wall has to drop the air 11 degrees to create the same condensation. Obviously that's much less likely. And that's the benefit you'd get from a heater alone. With the dehumidifier, the air won't have the same absolute quantity of moisture at 75 as it had at 65 -- it'll have less. Finally, the walls won't stay 64 degrees once you warm the room up. They'll gradually warm up too, further reducing the risk of condensation.

    Bookmark   June 28, 2008 at 12:12PM
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When I wrote the above response, I focused on the response you got but not your original question: Keep the screens in or use the glass year round. I think it probably depends on conditions, but I would say this -- it is most likely a waste of energy to both run the dehumidifier and have the screens in.

With all of the air changes you're getting from the screens, the dehumidifier is, as you say, just sitting down there eating electricity. (They do use a lot -- usually 500 to 800 watts, and many run about all the time -- so you're looking at 350 to 500 kilowatt-hours a month, which would add $50 or more to your electric bill in many areas of the country.) It's not, as you said, sucking in humidity from the outdoors, but the dry air it's producing is basically blowing out the windows, so shut it off when you have the screens in.

Could the screens alone be an adequate moisture control measure? Here's the plus and minus: If it's 65 degrees in the basement and 80 outside, bringing warm outside air in may produce additional condensation. But, if you kept enough outside air moving through the basement over a period of days, the basement would eventually start to warm up. And as it did, the walls, floors, etc., would give up more of their moisture to the warmer surrounding air, causing the basement to get progressively more dry.

I think what I would try if I were you is one window open with a screen and an exhaust fan in the other window, so you pull air through the basement. Give it a few days of constant running of the fan, and see what happens. The best time to run the fan would be during the warmest parts of the day. If it's cool and damp at night, or if it is raining or has recently rained, shut it off. The fan would use a small fraction of the power the dehumidifier uses, so it would be worth a try. Whether it will work or not just depends on how much moisture is present and how humid it is where you live at any given time. If that does not work, you can always close the basement (put the glass windows in) and use the dehumidifier.

    Bookmark   June 28, 2008 at 7:58PM
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Molly Brown

Thank you so much for your time, ky114. Got to find a fan to fit the small window.

    Bookmark   June 28, 2008 at 10:15PM
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Hi, I live in the north east and have a very similar problem. I have an almost 200 year old house with dirt floors, just had central air installed last year. this year it has been so damp and wet, the dirt floors were like mud. i am running a dehumidifier and the AC in the basement - although the cool air is not vented into the basement that's where the furnace is. when the company installed my furnace and a/c they sealed up an old wood door and the only window in the basement, i figured that's why it was so damp. i unsealed the window but am wondering if that was a mistake. can you tell me what worked for you? thank you.

    Bookmark   June 1, 2011 at 11:30PM
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We live in New Haven where summers tend to be on the hot side and the humidity varies weekly from moist one week to dry the next.

We have been using an inexpensive window fan to ventilate our basement now for many years and absolutely love it! The air is always fresh and the warmer moving air dries out the dampness.

We do the following: all windows are closed save for one, where we install the fan. I have cut a piece of extra thick foam core to perfectly fit in the window opening and have also made a cutout for the fan. The extra thick foam core works well because it is flexible, but plywood works well too and is more durable. Getting a reasonably tight fit between all the parts is important and it may take a few tries to get it correct.

We run this fan in exhaust mode on its slowest setting 24 / 7 from late spring to late fall, from when the heating season ends through when it begins again.

A really big advantage of the fan over an ac or dehumidifier unit is that the fan will also exhaust any radon in the air out of your house!!! The EPA ranks radon as the most serious indoor air pollution problem in houses. Not all areas have radon issues, but many do, ours included. I think the fan uses a lot less power than the other options too.

By exhausting air from our basement, warmer dry air is pulled through a first floor window which we leave open, and down into the basement. I can tell this because the air current will pull the basement door closed when it is opened just a tiny crack.

I also use a window fan in a 3rd floor window to pull hot air out of the house, using roughly the same idea as described above. The fan keeps us comfortable most of the year, but we supplement it with 2 window ac units, in 2 out of our 4 bedrooms, for portions of July, August and September, as needed. We also use small 6" diameter table fans in our bedrooms at night to keep the air moving and also to provide a little white noise.

I think this solution is as energy efficient as you can get while still maintaining a high level of comfort and also provides the healthiest air quality.

    Bookmark   September 14, 2011 at 10:34PM
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