These cold nights should have already been a reminder to get our homes winterized. That includes the standard things like changing furnace filters, installing weather stripping, and sealing up foundation vents.
Wait a minute… foundation vents? Should they really be closed in the winter? That depends on how your house is constructed, and who you ask. At first blush, it makes sense that you would want to close them up in the winter so that you don’t let cold air into your crawl space, which would make your floor colder and drive up heating costs. And there are several people out there who would agree with that. But it may not be that simple.
While it makes sense on the surface to close them, I have had several building experts tell me that it is a bad idea. The problem is that closed vents can trap moisture under your home, and that can lead to moisture condensation along the rim joists and mud sill. In addition to that, mold and fungus not only like moisture, they also prefer a steady temperature. With the vents closed, the lack of temperature differences in the day and nighttimes could promote mold and fungus growth.
How your crawl space is insulated might come into play. Check under your crawl space. If your foundation is insulated, you will have to make a decision whether you prefer moisture under the house (closed vents), or higher heating bills (open vents). Insulating the sub floor instead of the foundation is a good solution to solve both problems, since you can keep the vents open without losing heat.
Moisture isn’t the only reason to keep the vents open. If you live in an area with high radon concentrations, and there are many areas here in western Nevada and the Tahoe Basin where radon is a real issue, you must keep your crawlspace well ventilated all year long. It’s another good reason to insulate your sub floor instead of the foundation.
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Alice Jones said:
I didn’t even realize how an open crawl space can bring moisture into the home. I’ve had mold problems in the past, so I’m looking for solutions on how to prevent it in the future. I think I’ll get the crawl space insulated and then close it like you suggested to do.