Recognizing Excessive Condensation in Buildings

Procedure code:
Chambers, J. H. Cyclical Maintenance for Historic Buildings. Washington, D.C.: National Park Service, Technical Preservation Services Division, 1976.
General Requirements
Last Modified:


These guidelines provide information on how condensation develops and what can be done to recognize and reduce it in older buildings.


  • Chambers, J. H. Cyclical Maintenance for Historic Buildings. Washington, D.C.: National Park Service, Technical Preservation Services Division, 1976.
  • Park, S. C. "Preservation Brief 39: Holding the Line: Controlling Unwanted Moisture in Historic Buildings". Washington, D.C.: National Park Service, Technical Preservation Services Division, October 1996.


Condensation is the process by which water vapor, a gas, changes to a liquid. There is always water vapor in the air; the amount depends 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 found 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.

Inspection for Condensation

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. Wood with a moisture content of over 20% 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.

Humidity Problems

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.

Last Reviewed: 2017-08-13