Q. Last year, I attempted to control the humidity in my crawlspace by using a portable dehumidifier. It seemed to run continuously and increased my electricity bill tremendously. What do you recommend to solve this problem?

The first step in dealing with crawlspace moisture issues is to eliminate all sources of moisture introduction into this space. Bulk moisture (water) should be dealt with first by correcting gutter failures, site grading, and drainage systems. After the bulk moisture is taken care of, you are left with two sources of moisture introduction: the ground and the foundation vents.

If you cut off the moisture supply to the crawlspace air, the dehumidifier won’t have to run all of the time. So, the goal is to eliminate 100% of the moisture introduction.
The ground has an unlimited supply of moisture within it. For this reason, the ground should be covered with a vapor barrier. By “covered,” I don’t mean covered by 50%...80%...or even 99%. I mean 100% covered. This is the only way to eliminate moisture introduction from the ground.

The walls, and holes within these walls, are the other source of moisture introduction. Most houses have foundation walls composed of bricks or concrete, both of which are permeable to moisture. So, these walls should also be covered with a vapor barrier if the goal is to keep moisture out of your crawlspace. When it comes to the foundation vents, get ready for a paradigm shift… Seal them closed forever!

These vents connect the crawlspace air to the air outside. The original idea behind these vents was to allow crawlspace moisture to escape to the outside. But, as you can see, we’ve already eliminated all of the moisture introduction sources except the moisture that enters into the crawlspace from the outside air. Often times, the relative humidity of the outside air can be in the upper nineties. Because moisture always moves from high levels to low levels, this outside moisture wants to move into your otherwise dry crawlspace and proceed to condense onto the cooler surfaces (pipes, ductwork, the ground itself). So, to truly control the humidity in crawlspaces, these vents must be sealed closed.

This same logic applies to any hole that goes to the outside: penetrations through the band joist, air gaps between the mud plate and the foundation wall, plumbing penetrations, etc.
The next step is to provide a drying mechanism, typically a dehumidifier. We recommend installing professional grade units. These units pump the water out of the crawlspace and are generally maintenance-free. A drain should also be installed in the low spot of the crawlspace, in case of a broken plumbing pipe or overflowing dishwasher episode.

Sometimes, we see crawlspaces that have no vapor barrier but appear to have dry earth. Where did the “unlimited supply” of moisture go, you ask? Well, more than likely, it has migrated up into your home via air leaks connecting the crawlspace air to the living space. This is commonly seen in older homes that have diagonal boards for subflooring, instead of plywood or OSB. These gaps and cracks allow the crawlspace air, and its hitch-hikers (moisture, mold, dust, radon, etc.), to migrate up into the house. This drying ability may also be provided due to leaky ductwork that actively vacuums the crawlspace air into the living space. This air leakage is an air quality concern too, as you probably wouldn’t want to constantly be breathing the air found in the crawlspace.

There are many things to consider when installing this type of system. I can’t stress enough how important it is to research things carefully before deciding this system is appropriate for your home. If implemented incorrectly, it could result in harm to the occupant or making the moisture problem worse. One detail worth mentioning here is, you should never seal up a crawlspace that has an atmospherically vented combustion device (water heater or furnace), as these devices require a combustion air supply. To learn more about the science behind the non-vented crawlspace, visit www.crawlspaces.org or call a building performance contractor that has experience with these systems.

Isaac Savage is the president of Home Energy Partners, Inc., Asheville, NC. For more information, call 877-511-0117.

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