Historic Preservation - Technical Procedures

Recognizing Excessive Condensation In Buildings
Procedure code:
Cyclical Maintenance For Historic Buildings - J. H. Chambers
General Requirements
Last Modified:
Recognizing Excessive Condensation In Buildings
Last Modified:


This standard includes guidance on understanding how condensation
develops and what can be done to recognize and reduce it in older


Reference: National Park Service Preservation Brief 39: Holding the Line:
Controlling Unwanted Moisture in Historic Buildings 



Condensation in building terms is the process by which water vapor,
a gas, changes to a liquid.  There is always water vapor in the
air, the amount depending upon the local climatic conditions.
Within a building, the amount of water vapor depends upon the
amount of vapor generated by the users, type and operation of HVAC,
air and water leaks, etc.  Air has the ability to hold water vapor
in accordance with the temperature of the air.  The higher the air
temperature the more water vapor the air can hold; the lower the
temperature, the less water vapor the air can hold.  When the air
is saturated it has reached the "dew point".  If the temperature
drops, the air can no longer hold all the water, so the excess is
changes back into liquid form.  Dew on lawns and cars in the
morning is formed because the air temperature went below the dew
point the night before.

Surface condensation occurs on any building material whose
temperature is lower than the dew point.  This can often be seen on
window glass in the winter and exposed cold water pipes in
basements in the summer.  Condensation is visible on surfaces which
are nonabsorbent.  When condensation takes place on bare wood or
other porous material, the water is absorbed so that it is not
visible on the surface.  Condensation can occur within walls and
ceiling spaces; this is known as interstitial condensation.
Condensation can be injurious to building materials, especially
wood, because the moisture level can be raised to the point that
biological attack occurs; it also damages insulation and reduces
its performance.


Old buildings often are susceptible to moisture in spaces below
ground level: basements, cellars, crawl spaces, root cellars, well
pits, etc.  A basement with condensation on water pipes shows a
relative humidity of 100%.  Where water pipes do not occur another
telltale indicator is a peculiar musty or damp smell.  The right
way to find out the moisture content of the air is with a Humidity
Gauge (accurate ones are not easy to find) or a Sling Hygrometer.
When wood has a moisture content of over 20% it is subject to
biological attack.

Reducing basement humidity can be accomplished either by
ventilation or dehumidification.  Ventilation requires that air be
exhausted from the space and that drier outside air be brought into
the space at the same time from a basement window or from upper
floors.  To be effective, air must be exhausted with a fan.  When
using a dehumidifier all openings should be closed while the
dehumidifier is in operation.  The effectiveness of each system
should be checked by a humidity indicating device.  Water collected
from a dehumidifier is distilled water, a most valuable material
for general cleaning, and should be saved.


Condensation on glass can be an annoyance and may cause trouble if
the paint film on the sash and trim permits moisture penetration.
If maintenance procedures are inadequate dirt can be washed down
upon fabrics and wallpaper causing stains.  To treat condensation
on historic glass, see 08800-01-P.

Winter condensation can occur in attic and roof spaces as moist air
in the attic condenses on the cold roof surfaces.  This sometimes
can be detected by oval or brown spots on the ceilings of the upper
story.  By the time the spots are noticed the damage could be quite
severe.  An annual inspection of unheated attic spaces should be
made during the coldest part of the year.  An unsatisfactory
condition should be reported to the Regional Historic Preservation
Officer for restorative measures.

                         END OF SECTION

Last Reviewed 2012-11-30