Windows are thermal holes. An average home loses approximately 30 percent of its energy through them.
So it goes without saying energy-efficient windows save money. The additional amount spent for a solid, energy-efficient window will pay for itself in two to four years. Long-lasting windows cost less in the long run, while making your home more comfortable and cozy.
The heat loss of windows is measured using U-values or U-factors. U-values are the mathematical inverse of R-values: for example, an R-value of 2 equals a U-value of 1/2, or 0.5. Unlike R-values, lower U-value indicates higher insulating value.
Vinyl windows lose and gain heat by:
Transmission of energy (heat and cold) through a solid material via direct contact. The lower the conductivity, the lower the heat loss and vice versa. Double-glazed windows filled with argon gas and warm edge spacers greatly reduce conduction.
In a cold climate, heated indoor air brushes against the interior surfaces of window glass, which causes air to cool, become dense, and drop downward. As the stream of air drops, warm air rushes in to take its place at the glass surface.
This cyclical movement of air forms a convective loop and is self-perpetuating. Double-glazed windows filled with argon gas and warm edge spacers raise inboard glass temperatures, thereby slowing down the convection cycle and improving comfort in your home.
This is the movement of heat from a warmer body to a cooler body.
Clear glass absorbs heat and redirects it outdoors. Radiant-heat loss through windows can be significantly reduced by opting for low-e glass that reflects specific wavelengths of energy. In a similar fashion, low-e coatings keep the summer heat out.
Air leakage through windows is responsible for most of a window’s heat loss. This is commonly referred to as air and water infiltration.
Well-designed windows have durable weather-stripping and high-quality closing devices that effectively prevent air and water leakage. Hinged windows such as casements and awnings clamp more tightly against weather stripping than double-hung windows.
How well the individual pieces of the window unit fit together also affects air leakage. Glass-to-frame, frame-to-frame, and sash-to-frame connections must be tight & consistent.