Windows
By Kenton Shepard
Heat Gain and Loss Through Windows
Windows are often the sources of greatest heat loss in homes. R-value is the measure of a material’s resistance to heat flow. While the walls of a well-insulated home would typically be R-19, a modern, high efficiency window will be around R-3.
U-factor
Thermal loss through windows is described by U-factor rather than R-value because windows consist of an assembly, which includes the window panes and the frame. U-factor takes both into consideration.
The U-factor (U-value) measures the rate of heat loss or how well a product prevents heat from escaping. It includes the thermal properties of the frame as well as the glazing. The insulating value is indicated by the R-value, which is the inverse of the U-factor. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-factor, the greater a product's resistance to heat flow and the better its insulating value.
Designing Windows for Local Requirements
New technologies allow windows to be designed to meet specific needs based on the local climate, the location (orientation) of the window in the building and the building design. Here are some of the techniques used to improve window performance…
- Gas fills- Inert gases used to fill the space between multiple window panes have better thermal resistance than air.
- Heat-absorbing tints- These tints change the color of window glazing. Tinted glass absorbs a large fraction of the solar radiation entering through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.
- Insulated (double-glazed, triple-glazed)- Adding panes to a window creates spaces which trap heat and make the window more thermally efficient.
- Low-emissivity (Low-E) coatings- Low-E coatings help control heat loss through multiple-pane windows. They can reduce energy loss by 30%-50%. They can be designed for high, moderate or low solar gain.
- Reflective coatings- Sometimes known as “one-way glass”, these coatings greatly reduce solar gain, but they also reduce “visible transmittance”, or the amount of visible light passing through the window.
- Spectrally selective coatings- These coatings reflect light of specific wavelengths. Windows with these coatings may allow all visible light to pass through the window while reflecting 40%-70% of the infrared (heat) portion of the light spectrum.
Thermally-Conductance of Frames
Window frames can conduct heat out of or into a building. Wood, vinyl and fiberglass will conduct less heat than aluminum or steel frames.
Window Energy Performance Ratings
- U-factor- This describes the rate at which a window conducts non-solar heat, such as heat in the air inside a home. Lower U-factor numbers mean higher efficiency. The U-factor is the inverse of R-value, which describes resistance to heat flow.
- Solar Heat Gain Coefficient (SHGC)- is a measure of how well a window blocks heat from sunlight. The SHGC is the amount of the heat from the sun which enters through a window. SHGC is given as a number between 0 and 1. Lower numbers transmit less heat.
- Air leakage- Significant amounts of heat can be lost from the home through air movement (air infiltration and exfiltration) around windows. Windows should be sealed with foam and caulk.
Double-pane Window Failure
Double-pane windows are windows which each have two panes of glass. The air space between the inner and outer panes acts as insulation, reducing heat loss, saving on heating costs and increasing home comfort levels. This space is often filled with a type of inert gas which increases the window’s insulative properties.
The inner and outer panes are separated by a perimeter strip filled with a desiccant which absorbs moisture vapor so that no condensation forms on the glass. The two panes and desiccant strip form a single assembly which is held within the window frame. Moisture can infiltrate the window assembly through solar pumping. Over time, desiccant strips become saturated and can no longer absorb moisture. When this happens, condensation will become visible under certain conditions.
Double-pane assemblies which have only recently failed can sometimes be repaired. Repair involves drilling two holes into the space between panes and allowing moisture trapped in the dessicant to evaporate.
Windows which have been exhibiting condensation problems for long periods and which show a white haze often cannot be repaired and must be replaced. It’s usually more cost effective to replace an entire window.
Failure of the seal is indicated only if moisture is between panes. Moisture on the inside of window glass indicates high indoor humidity.
Window Replacement Recommendations
Although new double-pane windows are more energy-efficient and provide more comfort, windows are expensive. Replacing windows in a 3,000 square foot home may cost $25,000 or more. Although comfort levels will increase with upgraded windows, payback time can be 100 years! Think carefully before recommending window replacement.
Sources for More Information
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