Upgrading windows is probably the most expensive thing you can do but can reduce annual costs up to 20%. Harley (2012) recommends sealing leaks and ducts and insulating as the most cost-effective energy strategy.
Solar radiation consists of incoming visible light and infrared heat radiation (Amann et al., 2012). Glazing of glass is useful to control the solar radiation entering or heat escaping through the window. Solar Heat Gain Coefficient (SHGC) indicates the amount of solar radiation that may enter the house. For example, SHGC=0.8 means that 80% of solar energy passes through (Amann et al., 2012).
The best windows consist of 2-3 panes with an insulating gas (e.g., argon or krypton) between the panes. A U-factor is the inverse of insulation’s R-factor; higher values prevent heat from escaping the interior of the house in colder climates. Neither SHGC or U-factor should rate above 0.3, but lower is better. Other metrics (some optional) include:
- Air leakage (AL) indicating how much air leaks through the window per square foot.
- Visible light transmittance: the amount of visible light that shines through.
- Condensation resistance: Numbers range between 0-100, where high indicates high resistance to condensation.
Wood, vinyl and fiberglass frames conduct less heat than cheaper aluminum, which performs poorly in cold climates.
If you do not want to replace windows: Here are other ideas if you want to insulate your windows better for cheap (Harley, 2012)(Amann et al., 2012):
- Storm windows: to buttress single pane windows and reduce heat loss by 50%, add storm windows.
- Plastic wrap: Renters can use plastic shrink wrap to warm windows and prevent air leakage. A layer of plastic wrap is taped to the walls around the window and heated with a blow dryer to make plastic taut across the window for visibility and attractiveness.
- Drapes: Shades and drapes need to be tight-fitting to prevent cold drafts from entering on exposed sides.
- Insulating cellular shades: These are rated using R-values but must be opened and closed manually to see out windows.
- Rope caulk: This temporary sealing (a preformed soft clay string) is used to seal windows before winter and stored in foil between seasons.
To keep home cool: External shading is more effective than internal shading. Consider adding awnings, porches, overhangs and even trellises and arbors, to prevent solar heat gain (Harley, 2012). Trees and bushes can also prevent sun from entering, but in a dry climate, may also be susceptible to fires. The best windows to protect include east and west windows where the sun spends mornings and afternoons heating. South windows in summer are less susceptible, since the sun is overhead (Amann et al., 2012). Northern windows are not susceptible to solar radiation in North America.
References
- Harley, Bruce (2012) Insulate and Weatherize for Energy Efficiency at Home. The Taunton Press, Newtown CT.
- Amann, Jennifer Thorne, Wilson, Alex, and Ackerly, Katie (2012) Consumer Guide to Home Energy Savings, 10th edition. New Society Publishers, Gabriola Island, BC Canada.