Basement Humidity

gene_2007September 1, 2007

The quick question - Should I / Can I add a return duct to the basement in order to reduce humidity in the basement?

The details -

I have a "finished" basement. It doesn't feel like one since it was done in the early '60s with panelling and other out dated materials. Some areas of the basement are not finished (eg. exposed foundation), like the laundry room and a closet.

The basement is very humid - at least it is now in the summer. I haven't checked it in the winter. My dehumidifier is filling to capacity and shutting off at least once a day. I empty it before work and when I get home, it is full. When the dehumidifier was not running for a few days, the humidity was 80%. When it is running, the lowest I can get it is about 65%.

One thing I'm surprised about is that I can't find any condensation anywhere. I checked the concrete walls, panelling and the cold water pipes. Nothing.

I am about to remodel my basement and am afraid that excessive humidity will ruin it, although I don't seem to have any problems now (other than the humidity itself). There is no mold or odors.

The basement living area has the laundry room adjoining. The laundry room also contains the furnace and gas hot water heater. The door to this room is always open, so perhaps this is where much of the humidy is coming from? There are two washers and two dryers. Yes, the dryers exhaust to the outside. When I remodel, I plan on putting in a door and keeping it shut.

There is no A/C in the basement - don't need it. There is heat, but not from the furnace. It is actually connected to a heating system from another part of the house. The basement has hot water baseboard.

My furnace has a return run that runs across the basement and through the foundation to the outside. Some people told me this is good because it provides fresh air to the house; others say this is bad because the furnace has to heat freezing air.

Finally, my questions -

1) Keep the return duct to the outside or close it off?

2) Add a return to the basement living area?

3) Why is the humidy so high in my basement?

4) Will adding a return in the basement help with the humidy?

5) Will painting Drylock on the walls help with the humidy?

6) Are there other alternatives to help with the humidy? Should I get another dehumidifier?

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Others will most likely chime in, but I'll give my thoughts to your questions.

1 - Probably not. The "return" duct to outside is most likely combustion air. Keep it. By doing so, you are not burning already heated air when the furnace is running which then has t be replenished via air infiltration through windows and doors (only to be heated). Also, the outdoor air will have a slightly higher oxygen content to make the furnace more efficient.

2 - Yes. Because cold air settles, the basement will be cooler summer and winter. Cooler air has a higher humidity content. By recirculating it you will dehumidify as it passes the air conditioning coils and actually circulate the cooler air through the house. If the humidity is still high in the summer a dehumidifier may still be necessary.

3 - See "2"

4 - Yes, see "2"

5 - I believe it will help, but most of the humidity is most likely passing through the concrete from outdoors. Concrete is porous. Newer homes have a rubberized moisture barrier sprayed on the exterior of the foundation before backfill. Yours most likely does not. I would speak with a qualified paint supplier to see if Drylock or another paint will help.

6 - When you redo the basement, I would consider rigid insulation and possibly a plastic moisture barrier across the studs. Consider pressure treated lumber for the studs since you know there is a high moisture content in the space.

Hope this helps.

    Bookmark   September 1, 2007 at 7:47PM
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Thanks, fnmroberts. This was very helpful. Regarding #5, Drylock, I was talking about putting it on the foundation walls (inside, of course). Like you said, I think most of the humidity is coming through the foundation, so that is why I was wondering if Drylock would help.

    Bookmark   September 1, 2007 at 8:17PM
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Yes, I understand. Not being familiar with the product, I would rather suggest that you speak with a qualified retailer because of the possibility that moisture trapped within the foundation walls might cause it to pop. Better to be informed than disappointed.

Photos of our finished basement are linked from "My Page". Captions accompany many imaages. Might be some ideas for your new space.

Good luck.

    Bookmark   September 1, 2007 at 9:29PM
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I have two dehumidifiers in my basement, each separately controlled by a wall-mounted Honeywell humidistat set at 50%. They ran a lot for about the first year. Now they just run occasionally. By experimenting, I discovered that mildew (fungus) may grow at 60%, certainly will at 70% or higher relative humidity. Rusting of steel also is greater at humidity levels above 50%. If the basement is cooler than outside temperature, and that is virtually certain in the Summer, the relative humidity in the basement will be higher than outside.

    Bookmark   September 2, 2007 at 8:40AM
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