In the latest Building Science Corporation newsletter, Joe Lstiburburek argues that closets in cold, hot-humid, and mixed humid climates need air change in order to avoid mold problems. He writes:

'A classic example of mold occurring in a cold climate closet is that of an exposed closet on an exterior wall. Its heat loss is high and the closet will be colder than the bedroom it is attached to. If the vapor pressure in the bedroom is the same as the vapor pressure in the closet, and the closet is colder, then the relative humidity in the closet will be higher than the relative humidity in the bedroom. If the closet experiences mold, it is apparent that the relative humidity in the closet is greater than 80 percent. However, the key question is this: is the relative humidity above 80 percent because the closet is too cold, or is it because there is too much moisture present (high vapor pressure) in the closet? Is this a surface temperature mold problem, or is this a vapor pressure mold problem – or a combination of the two?'

One way around this problem is to install louvered closet doors, but as Lstiburek acknowledges, not everyone loves louvers. Fortunately, he presents other strategies that include active returns at the ceilings and undercut doors to equalize the temperature and pressure between the closet and the rest of the house.

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