Healthy Indoors Magazine - USA Edition

HI October 2017

Healthy Indoors Magazine

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Healthy Indoors 29 space has its own conditions, though, and may or may not respond well to controls in another place. Conclusion: Although it's the most common method of treating the air in encapsulated crawl spaces, it may or may not work well. 3. Exhaust air to the outside First, note that I'm not talking about the crazy idea to put large fans in the crawl space to exchange large quantities of air between the outdoors and the crawl. (Yes, compa- nies really do promote this, and it's a quick way to rot the wood in your crawl space.) I'm talking about a small exhaust fan, as described in the IRC. Here's the language from the code: Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated So a 2000 square foot crawl space would need a 40 cfm ex- haust fan. It's pulling just a little bit of air from the crawl space. The problem is that you don't really know where the makeup air for the crawl space is coming from. Yes, the code specifies a pathway for air to move into the crawl space from the house, but air doesn't always follow those arrows in the diagrams. If the band joist isn't sealed well, for example, it may pull a lot of the air from outdoors, which isn't really what you want. Conclusion: It could work but is probably less reliable than supply air from the HVAC system. 4. Install a dehumidifier In this method, you use a standalone dehumidifier for the crawl space. (Again, we're talking about encapsulated crawl spaces only; although some people try, you can't dehumidify the whole outdoors.) It's controlled by the con- ditions in the crawl space, not the house above, so it this method will do the best job of keeping the crawl space dry. Brad Brinke of The Crawlspace Company in Virginia Beach responded to my question about this topic on our Facebook page: "We have a very good track record with the dehumidifier here in Virginia. We install a humidistat and fan system to help the dehumidifier operate efficient- ly. We tried using supply air from the home and it did not work." (See the full discussion here.) If you want dry air in your crawl space, a dehumidifier is the way to go. Yes, they do have drawbacks. You have to buy additional equipment. They require maintenance. If the drain fails, you may have a bulk water problem in the crawl space. If it's a small crawl space, a standard dehumidifier that 2. Supply air from the HVAC system If you encapsulate the crawl space, adding a little bit of air from the HVAC system is probably the most common method used. If there's already a duct system down there, it's easy and inexpensive to do. The air from the system helps to dry out the crawl space air. But it also puts the crawl space under a slight positive pressure and the house under a slight negative pressure. Is that a problem? Not according to Advanced Energy's Closed Crawl Spaces guide: In Advanced Energy field tests, the small crawl space airflow causes a negligible pressure effect that is far exceeded by the effects of duct leakage, stack pressure or wind-induced pressures in the building. How much supply air do you need to add to the crawl space? Advanced Energy recommends 1 cubic foot per minute (cfm) for each 30 square feet of crawl space floor area. The International Residential Code (IRC) recom- mends 1 cfm per 50 square feet: Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated If you go this route, it may work well in the summer if the air conditioner is sized properly. If the system runs long enough, it can provide enough dry air to the crawl space to keep the relative humidity below 70%, where you want it. In the swing seasons, however, and in homes with oversized air conditioners, this method may not work well. Another thing about this method that I'm not crazy about is that you're conditioning the crawl space based on the conditions in the finished space above. The crawl

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