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Efficient basement dehumidification – how to?

Our deep energy retrofit of a 112 year old masonry building in Chicago includes a basement unit (garden apartment). The old foundation walls always draw some moisture and for that reason are exposed on the inside so that they can dry out. That works fine during fall, winter and spring, when the ERV removes that excess moisture. During summer, however, that moisture starts building up and I have a really hard time to control it.

I am hesitant to use an efficient dehumidifier, as it produces not only the desired dry air but also an undesired thermal load (we currently don’t need AC in the basement unit). I am contemplating to add the Munters DryCool HD, as it promises to minimize the undesired thermal load. (http://www.munters.us/en/us/Products--Services/Dehumidification/Dehumidi...)

Does anyone have experience with the Munters DryCool HD, or should I move the ERV to the second floor and go with a completely different product for the basement unit ventilation and humidity control? Any comments or suggestions would be appreciated!

Asked by Marcus de la fleur
Posted Wed, 06/25/2014 - 10:35
Edited Wed, 06/25/2014 - 10:36

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8 Answers

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1.
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Marcus,
It makes no sense to encourage a damp basement wall to dry toward the interior. The amount of moisture on the exterior side of the foundation wall is infinite; you don't want to encourage any moisture to enter your home.

The usual solution for your type of foundation is to install a layer of closed-cell spray polyurethane foam on the interior of the wall. This provides a vapor barrier as well as an insulation layer.

For more information, see Fixing a Wet Basement.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/25/2014 - 11:51

2.
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Outdoor air infiltration is a source of summertime basement humidity that often exceeds vapor-diffusion through the foundation walls & slab.

Insulating foundation walls is often cheaper/easier with rigid foam than with closed cell spray if the masonry foundation is sufficiently flat, but it still needs to be air-sealed well.

Answered by Dana Dorsett
Posted Wed, 06/25/2014 - 11:59

3.
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Martin, Dana, thank you for your comments.

The foundation wall is insulated from the outside, eliminating (or managing) that infinite outside moisture source. The small amount of moisture I have to deal with rises from the footings (old foundations like this didn't have the damp proof layer [capillary break] between the footing and foundation wall).

The basement unit can't be called wet or damp. Again, the problem I am trying to solve is to reduce indoor summertime humidity from around 70% to around or below 50%. We are talking about June through mid September only.

Answered by Marcus de la fleur
Posted Wed, 06/25/2014 - 12:48

4.
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Marcus,
Q. "The problem I am trying to solve is to reduce indoor summertime humidity from around 70% to around or below 50%."

A. The standard solution: minimize the operation of any ventilation system so that you are ventilating as little as possible, and install a dehumidifier.

If the dehumidifier raises the indoor temperature to uncomfortable levels, the standard solution is to operate an air conditioner.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/25/2014 - 13:11

5.
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The average summertime outdoor air dew points in Chicago are in the mid-60s F, and last week it was averaging around 70F. Pulling 65F dew-point air into a 70F basement results in an 84% RH in the basement. If the basement is even cooler, the RH will be higher.

An ERV doesn't remove moisture- it can't. It only transfers moisture between the incoming and outgoing air volumes. To hit 50% RH @ 70F when the exhausting air stream is 70%RH @ 70F, the incoming ventilation air has to have a dew point well below 50F- which is rare in Chicago during the summer months.

You simply CAN'T get down to 50% RH in the basement in summer without mechanical dehumidification/air-conditioning, even the basement walls & slab were hermetically sealed.

Answered by Dana Dorsett
Posted Wed, 06/25/2014 - 15:57

6.
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Dana, you summarized the challenge in a beautiful precise way. As wonderful as it is to have the cool basement almost all summer long - the RH is always elevated because of the high dew points.

Martin, been there done that. And I mean, done exactly that (run the dehumidifier and AC alternately). And it feels rather rudimentary - like rubbing two sticks together to start a fire. What I am after are the match sticks or the lighter - something slightly more elegant or advanced. My research lead me to the Munters DryCool HD (referenced above).

I am looking for opinions/experience on/with the Munters - or even better - for suggestions and solutions that may be even more elegant, jet reasonably energy efficient.

Answered by Marcus de la fleur
Posted Sun, 06/29/2014 - 21:54

7.
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For most houses big dehumidifiers such as Munters DryCool HD would be extreme overkill- like swatting flies with a sledgehammer. Yes, they are more efficient than standalone room dehumidifiers, but not enough to matter.

An Energy Star rated ~60-75pint/day unit is probably the sweet spot in bang/buck for most basements. Even that is going to be 3x oversized for the actual loads for most houses.

https://www.energystar.gov/index.cfm?c=dehumid.pr_basics_dehumidifiers

Answered by Dana Dorsett
Posted Mon, 06/30/2014 - 16:03

8.
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Dana, that is very helpful, including the link. Maybe it is time to rethink and do some more research into the stand alone room dehumidifiers.

Have you heard or do you know of the option to duct the exterior ERV supply air into a close with a dehumidifier - where the air is dried out before it gets to the ERV? I was thinking that this way I would get dehumidified air to the ERV, and also could get rid of some of the extra thermal energy (from the dehumidifier) with the help of the ERV.

Answered by Marcus de la fleur
Posted Thu, 07/03/2014 - 21:15

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