A.Energy and sustainable design consultant Andy
Shapiro responds: The dewpoint is not a location; it is the
temperature at which water will condense out of the air. Since
the dewpoint changes with the amount of humidity in the air, as
well as the air temperature, the dewpoint for a particular
temperature and relative humidity is best looked up in a table
or a psychrometric chart (see below).
Water from the air will condense on building components when
they are below the dewpoint of the air that’s in contact
with them. Cold water pipes in hot, humid summers condense
water and drip. Uninsulated basement floors in hot, humid
summers are often below the dewpoint of the hot, moist outside
air, so water condenses on them if the space is open to the
outside. In an air-conditioned building in a warm, moist
climate like the southeastern U.S., the drywall can be below
the dewpoint of the outside air for months on end.
Just because a building component is below the dewpoint
doesn’t mean there will be a problem. Vinyl window frames
and copper tubing aren’t bothered by a little moisture.
On the other hand, wood window components and drywall
can’t handle much moisture, especially if the wetting is
prolonged and there is no opportunity for the components to dry
Determining whether a component in a wall assembly will ever
get cold enough to permit condensation — that is, be
below the dewpoint — can be complicated. If each element
of a wall acted as a solid (which fiberglass doesn’t),
then the calculation of the temperature at any point in the
wall assembly would be fairly easy. Halfway through the
insulation value of the wall, the temperature would be halfway
between inside and out.
In reality, such static calculations can be misleading,
since wall materials can absorb some moisture without being
damaged. More accurate calculations, called dynamic
calculations, take into account many additional factors but are
so complex that they are best performed with computer
A psychrometric chart provides the
dewpoint for any given air temperature and relative humidity.
Say you have a relative humidity of 50% at 70°F. On the
horizontal scale, locate the air temperature and move up to the
curve that represents 50% relative humidity. Then move left to
the saturation curve and down to find the dewpoint —
50°F in this case.
The good news is that this type of dynamic calculation is
usually not needed — as long as builders employ good
building practices that keep inside air out of walls in cold
climates and outside air out of walls in cooling climates, and
allow building components that occasionally get damp to dry
out. One very good source for building details that avoid
moisture damage is the Builder’s Guide series from
Building Science Corp. (978/589- 5100;www.buildingscience.com).