CONDENSATION on WINDOWS, SKYLIGHTS - CONTENTS: Window condensation causes & effects on the building. How to cure or prevent condensation on windows and skylights. Condensation-resistance ratings for window glazing. Outdoor Temperatures at which condensation occurs on windows at 50% Relatiive Humidity indoors
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Window condensation problem troubleshooting & repair: this article discusses problems with excessive condensation on windows and skylights and reviews the causes and cures of window condensation, secondary effects (mold, rot, decay), and also the condensation resistance of window glazing materials.
Condensation Resistance Ratings for Windows: Window Condensation or "Sweating"
In this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants.
To rate a window’s resistance to condensation, NFRC recently
developed a method that evaluates the window’s
frame, glass, and glass edge at a standardized set of temperature
and humidity conditions.
Based on the coldest
part of the window assembly, it is assigned a rating from
1 to 100.
The higher the rating, the better the window is at
resisting condensation, but the rating doesn’t predict condensation
under specific conditions. The voluntary minimum
for a “thermally improved window” under the
AAMA/NWWDA standard is 35.
The best protection against condensation is low-E
glass with gas fill, combined with warm-edge spacers and
a nonmetallic window frame, such as wood, vinyl, fiberglass,
or one of the newer composites. Table 3-6 (below) provides
a general guide to when condensation is likely to form on
different types of glazing. Without warm-edge spacers,
condensation will occur at window edges first.
How to Avoid Condensation Problems on Skylight Windows
Because warm moist air is carried upward
by convection currents, skylights are often one of the
first places to develop condensation. This can lead to dripping
and staining of the frame, well, or furnishings below.
The best defense against skylight condensation is to choose high
R-value glazing with warm-edge spacers. Good insulation
of the skylight well also helps by keeping the surrounding
area warmer. Several manufacturers offer skylights with
integral condensation gutters, a helpful feature in cold
Condensation Resistance of Windows: Measurements
The NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the Condensation Resistance of Windows (CR) as follows:
Condensation Resistance (CR) measures how well a
product resists the formation of condensation. CR is
expressed as a number between 1 and 100. The higher the
number, the better a product is able to resist condensation.
CR is an optional rating, and manufacturers can choose not
to include it on their NFRC labels.
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