To control mildew growth, we must first
look at the relationship between air, water vapor, and relative
humidity (RH). Water vapor is water in its gas form, and is
present in air. The warmer the air, the more water vapor it can
hold; the colder the air, the less water vapor it can hold. RH
is a measure of how much water vapor is in the air compared to
the maximum amount of water vapor the air can hold at that
temperature. As air cools, without changing its water vapor
content, its RH goes up. As the air continues to cool, it
reaches the point where the water vapor it contains is all that
it can hold — this is 100% RH. Cooling the air any
further will result in condensation as some of the water vapor
changes to liquid.
Mildew can only grow on surfaces where the RH exceeds 70%.
Closet surfaces tend to be colder than adjacent rooms, because
of poor air circulation from the heated room to the closet, and
because, relative to their size, they often have more exterior
surface area for heat to escape. A corner or cold wall section
lacking proper insulation is particularly vulnerable. Since
these areas are colder, but have just as much moisture in the
air as adjacent rooms, they have higher RH and are prone to
mold and mildew.
To control the mildew, we have to lower the RH of the closet
below 70%. To do this, we either have to raise the closet
temperature or lower the amount of water vapor present in the
When troubleshooting a closet mold problem, measure
temperature and RH in the adjacent room with a sling
psychrometer. If the room RH is 30%, then it will be difficult
(and hard on the occupants’ respiratory systems) to
reduce RH much, so look for ways to raise the closet
temperature. Either increase heat flow into the closet (in some
cases, louvered doors may permit enough heated room air to
circulate; in other cases, you may have to put in some heat
directly), or cut heat loss from the closet (add insulation,
seal air leaks).
If the room RH is 50% or greater, look for ways to reduce
the amount of moisture in the air. First, control the moisture
at its source. Is it coming from a hot tub, lots of plants, or
a gross of gerbils? Is it coming from the six cords of firewood
drying in the basement? Next, if the problem occurs in the
heating season, increase ventilation levels to replace humid
house air with cold outdoor air holding little moisture. As a
last resort, use mechanical dehumidification.
Closet mildew problems often occur with other moisture
problems that the homeowner is not aware of, so a comprehensive
"footing to ridge" assessment may be in order.
— Marc Rosenbaum, P.E.,
of Energysmiths, in Meriden, N.H., designs and engineers
everything from hvac systems to solar, low-energy use