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Crawl Space Dehumidification - Leakage from Air Ducts

If a liner is installed over a concrete floor and the vents are not sealed, you will probably wind up with condensation on top of the liner or concrete in the summer, as the warm humid air touches the earth-cooled liner surfaces. Don't worry, you haven't wasted your money on a crawl space liner. This was happening on your crawl space surfaces before you installed the liner anyway. It's not the liner that's the problem; it's the air leakage. If this happens to you, obviously you need to seal the vents. If you have already sealed the vents, then you need to seal up other sources of outside air leakage into the crawl space. You may have a space between the sill plate and the top of the foundation that needs to be caulked. You may have openings around wires and pipes to the outside. You may have spaces or holes open to the garage, where unconditioned outside air is entering the crawl space. You need to seal all these openings with caulk, flashing, spray foam, wood caulked in, or whatever material necessary.

Concrete block cavities at the top of the walls, which are not covered by the sill plate, are a place where outdoor air can enter. Sealing this space can be challenging, but if you must, you can flash and caulk it, cover it with 1" x 4" treated wood and caulk it, or fill it with sprayed foam.

Remember what causes the air to get sucked into the crawl space in the first place the stack effect. The house is sucking on the crawl space, and the crawl space is sucking on the ground. You sealed up the ground, so now the crawl space can only get air at the rim joists and sill plate level.

All the air that gets sucked out of a crawl space must be replaced by new air. One approach is to close up paths that air can flow upstairs from the crawl space. The less air that gets sucked up into the house, the less air that needs to come in from the outside to replace it. Seal openings in the floor system above you around ducts, pipes, wires, and other openings.

In some cases, it may be too difficult to seal all the tiny openings to the exterior, and you continue to get condensation on top of your crawl space liner. Then dehumidification is necessary. With the addition of an effective dehumidifier, you can dry your crawl space out dramatically.

Running a dehumidifier in a vented and/or dirt crawl space is futile, since moisture is flowing in constantly, you can never catch up on it. However with a liner installed and the vents closed up, you can really dry the place out, and get all the benefits of a dry home preventing mold growth, stop ping the growth of existing mold and eventually killing the spores, killing dust mites, saving energy as dry air is easier to heat and cool, etc. Further, since airflow in the home is bottom to top, the air that rises from the lower levels will now be dry air, not wet air, and will help to dry the rest of the home significantly.

If you decide to include a dehumidifier with your crawl space solution, then it should be set up to drain automatically as it takes water out of the air. A dehumidifier with a drain bucket that you must continually empty is nearly useless. While crawl spaces typically do not contain huge volumes of air, in my experience household dehumidifiers fall short when they are needed most in the summer.

High quality high-performance dehumidification units, such as the SaniDry Basement Air System, will perform extremely well no matter how hot and humid the air is outside. Because the SaniDry unit can be ducted, it does not have to be located in the space it is drying. It can be installed in the crawl space; or in a room upstairs, such as a utility room; or in the garage. You can also duct the unit so that some of the dry exhaust air dries the first floor as well as the crawl space.

Dehumidifiers are very useful in transforming a crawl space environment. They are not always necessary to get the results you want. Sometimes you don't know if you need or want a dehumidifier until after you install the liner and seal the vents. You can always add one later.

US Department of Energy - Common Caulking Compounds

US Environmental Protection Agency - Reports on Mold and Relative Humidity

US Department of Energy - Crawl Space Moisture Control