If you are going to save energy with your windows, first you need an understanding of
how and why certain windows work, while others don't work as well.
Energy is lost through your windows in three ways:
- Infiltration: These are the cracks and crevices that allow
cold
air in and warm air out in the winter months and the reverse in the
summer months.
- Convection: This is the process of heat transfer
that involves the
motion of fluid air moving towards the window glass and through the
panes. Heat will flow from a higher temperature to a lower temperature.
- Radiation:
This is the transfer of heat from one separate surface
to another.
Today's modern energy efficient windows will reduce infiltration,
convection, and radiation through a number of technological
advancements. Double pane and even triple pane windows create greater
insulating barriers. Adding features such as Low E glass to reduce the
sun's heat from entering the living area by penetrating the glass in the
summer, but allowing its heat to penetrate in the winter is a perfect
example of reducing energy costs.
Argon or Krypton gas injected into the dead air space that exists
between the panes of glass further insulates the window. Another key
ingredient for insulating is the manner in which the glass is fused
together. In the past, the glass had metal connections from one to
another (spacers). Now the spacer is insulated with thermoplastics which
eliminates seal failure and doesn't allow the glass to conduct cold.
Ultimately the goal of utilizing windows with lower U values, i.e.; .16
will decrease energy consumption, increases savings, and keep you more
comfortable. Three key measures are used to report window energy
performance. U-value (or "U-factor") is the measure of the amount of
heat (in Btus) that moves through a square foot of window in an hour for
every degree Fahrenheit difference in temperature across the window.
The lower the U-value rating, the better the overall insulating value of
the window. Solar heat gain coefficient (SHGC) is the measure of the
amount of solar energy that passes through the window; typical values
range from 0.4 to 0.9, and the higher the SHGC the greater the solar
energy that passes through the window system. Windows with high SHGC
(above 0.7) are designed for colder climates, while windows with low
coefficients are designed for hotter climates. Air infiltration or air
leakage is given in terms of cubic feet of air per minute per foot of
window edge. The best windows have air leakage rating between 0.01 and
0.06 cfm/ft.
