Window Condensation

Window condensation is frequently cited as the key visible indicator of excessive indoor relative humidity. To prevent condensation-related damage to window components and finishes, a number of sources list the maximum allowable indoor relative humidity (assuming 70 degrees indoor air temperature) by outdoor air temperature (assuming 15 MPH wind) and type of window. For a table summarizing approximate values, please contact the author.

These values will vary depending on window tightness, distance and nature of spacing between glazings, direction and speed of wind, actual indoor room temperature, and air movement across the inside window

surface. For example, if a window is deeply recessed or covered in a way that restricts indoor air currents from reaching the window, condensation can occur at a lower indoor relative humidity. Restrictive coverings that may increase condensation include curtains, blinds, and interior screens. Also, if a window sash is not tightly weatherstripped and/or a storm window is very tight, condensation can occur on the storm window at a lower indoor relative humidity. Finally, short-term window condensation may occur with cold weather and high wind until indoor humidity falls below these values.

A major limitation in popular window condensation calculations involves a focus upon thermal properties at the center of glass sheets or glazings. This procedure neglects the thermal conductivity of spacers separating the glazings, the thermal conductivity of window frames, convective heat transfer dependent upon airspace and temperature variations, window airtightness, comfort related to radiant heat

transfer and interior or exterior window insulation. More efficient windows use plastic, not metal, spacers between glazings as well as wood, vinyl, composite, or metal with thermal breaks for window frames.