Wednesday, September 1, 2010

Crawlspace Condensation - Sweating Ducts

With record summer heat and humidity, we have been receiving an enormous amount of calls from homeowners about their sweating crawl spaces. Most homeowners are unaware of what is causing their ducts, pipes, insulation, and sometimes wood structure to sweat in their crawl space. Surprisingly, the very thing that a homeowner thinks is drying their crawlspace (ie. vents) is actually the culprit behind their crawl space condensating.

Foundation Vents are causing condensation!

In the summer months when temperatures are in the 80's and 90's, the outside air is very humid. The crawlspace is consistently 55 - 60 degrees and cooler year round because of the earth's soil. When this hot, humid air is vented into a cooler environment, the coldest surfaces in the crawlspace will begin to condensate. It's the same concept as taking a cold glass of coke outside on a hot summer day. The surface of the glass is going to start sweating. So when venting a crawlspace on a summer day, there is going to be condensation on the crawl space walls, crawlspace floor (dirt or concrete), air ducts (especially when the AC is on), and cold water pipes. These surfaces are the coldest. On very hot days, floor joists, beams, sill plates, and insulation condensate as well.

Foundation Vents are causing condensation and higher heating bills!!

When your air conditioning travels through your duct work in a vented crawlspace in the summer, the cooler air in the ducts is competing against the outside humid air being brought into the crawl space. The AC unit will have to work twice as hard to compete against this outside air. Also, this humid summer air puts a latent load on the HVAC system because the AC system has to work harder to dehumidify this damp air. For more info as to why see my post "Foundation Vents are causing higher energy bills".

Foundation Vents are causing condensation, higher heating bills, and MOLD growth!!!

Another common problem with ventilation and condensation in the summer is the propagation of mold spores. Mold needs between 65% to 70% relative humidity in the air to remain active and produce spores. Outside summer air can reach 80 to 90 percent relative humidity on any given day. This humid air by itself can lead to an environment conducive to mold growth, especially when this air is reaching the dew point on cooler surfaces. Also, many homeowners spread mulch around their foundations and vents. What they do not realize is that mulch contains high levels of mold spores and that these spores are being introduced into a perfect environment for mold growth in the crawl space.

Crawlspace Condensation SOLUTION!

Since foundation vents are causing condensation, higher energy bills, and fungus growth, they should be closed and the crawl space sealed with our AmeriBrite crawl space encapsulation system. It is not recommended to close off the vents permanently without first encapsulating
your crawlspace.

If you live in Indiana, Ohio, Kentucky, or Illinois, and are tired of dealing with crawl space
condensation and interested in encapsulating your crawlspace, give us a call 1-877-409-2837 or

Thanks for reading the rambling thoughts of a crawlspace inspector,
Larry Ralph Jr., Becky, Daniel, and Aaron


  1. Doesn't the 410a freon add to the problem when this type unit is installed on an old duct system as the old duct systems weren't designed for the lower air temperature the evaporator produces. Non radius turns in the duct system could slow the air down thus causing sweating and thin wrap would also add to the problem. Seems like it would also need to be internally masked to prevent air leakage to the wrap foil. ...JIM...

  2. I appreciate your post about crawlspace condensation - I've been examining whether or not to recommend crawlspace encapsulation to my clients or not. Your comments about condensation make a lot of sense, and corresponds to what I see in the field.

    One issue that I found in a house I worked one was that after crawl space encapsulation the owner had some cracking in the walls. My theory was that it was due to contraction of the wood as it dried - my recommendation was cosmetic repairs. Have you run into this in your jobs?

    Thanks in advance,

  3. George,

    I've seen several drywall cracks this past summer and fall, but it was because of settlement from the drought we had here in Indiana. If the crawlspace was consistently damp, then it is possible for the contraction of the wood to cause minor drywall cracks, and some cosmetic repairs should do the trick. If the cracks worsen, then it is probably something else. I've never seen it occur with encapsulation, but it is possible. I've seen it happen a few times with installing perimeter drainage in wet crawlspaces. The soil dried out around the support columns holding the center beam causing it to settle.
    If what you are assuming is true, then they probably had a very damp crawlspace. You might check the wood moisture content and RH, but you would need the WMC & RH readings prior to encapsulation to prove your theory. How is the crawlspace conditioned - Heat Ducts? Dehumidifier?

  4. I agree wholeheartedly with these points. It's amazing to me -- some contractors want more vents installed, some want to install fans to vent air IN to the crawl space. Some vent air OUT of below-grade spaces (Humidex)... no one seems to know. But a few firsthand experiences and some hands-on application, a little basic science, and it's painfully obvious that venting crawl spaces is just not working.

    Amen to this article -- keep spreading the word, comrade. ;-)

  5. When your air conditioning travels through your duct work in a vented crawlspace in the summer, the cooler air in the ducts is competing against the outside humid air being brought into the crawl space.

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