In climates with a significant heating season, windows have represented a major source of unwanted heat loss, discomfort, and condensation problems. In recent years, windows have undergone a technological revolution. It is now possible to have lower heat loss, less air leakage, and warmer window surfaces that improve comfort and minimize condensation.
Winter Thermal Comfort
An older window with a lower glass temperature feels colder because more heat is radiated from a person's body to the window. Cold glass can also create uncomfortable drafts as air next to the window is cooled and drops to the floor. This sets up an air movement pattern that feels drafty and accelerates heat loss.
High performance windows with lower U-factors will result in a higher interior window temperature in winter and thus greater comfort. Proper installation along with weather-stripping will also improve comfort by reducing cold air leakage.
Look for the following features when buying new windows:
- Double-pane 2x more efficient than single
- Gas-filled (Argon) centers improves performance 6-9%
- Emissivity – “Low-E” glass can cut thermal radiation in half.
- Details – thermal breaks, etc.
Doors – similar to windows, details: Thermal breaks, core insulation for metal doors.
Raise your hand if you don't know what a thermal break is.
If the inside frame and the outside frame are both made of aluminum the two need to be kept apart by a material that will slow the flow of heat. Plastic is usually used for this.
Look for Efficient Window Properties on the NFRC Label
The National Fenestration Rating Council (NFRC) has developed a window rating system based on whole window product performance (www.nfrc.org). The NFRC label provides the only reliable way to determine energy efficient properties and to compare products. The NFRC label appears on all fenestration products that are part of the ENERGY STAR program.
The following terms which indicate the overall performance for well-made, high performance windows:
- U-Factor - The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.
- Solar Heat Gain Coefficient (SHGC) - The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits.
- Visible Transmittance (VT) - The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The NFRC’s VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.
- Air Leakage (AL) - Heat loss and gain occur by infiltration through cracks in the window assembly. Air leakage is expressed in cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the assembly.
If you enjoyed this post, then click here to subscribe to the Energy Boomer Newsletter.