Tips for Buying Energy-Efficient Windows

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When you go shopping for windows, it's easy to get confused by all the ratings and figures. Until recently, window manufacturers could rate their products almost any way they wished, which made comparisons a nightmare. Thankfully, they realized this and, in 1989, formed the non-profit National Fenestration Rating Council (NFRC) to standardize testing and rating for windows, doors and skylights.

The NFRC provides up to four ratings for every window, though only one of those (the U-value) is required.

U-value

A window's heat loss is measured by the U-value, so it's the opposite of the R-value associated with insulation and heat retention. Not surprisingly, the U-value is simply the inverse of the R-value.

U-value = 1 ÷ R-value

The most efficient windows have a very low U-value. You should target models which score below 0.3 for a passive solar home. Note that some manufacturers provide two U-values: one for the overall window and one for the middle point of the glazing, which can indicate how efficient the frame is.

Solar heat gain

This one is really important in passive solar installations, for obvious reasons. The NFRC reports values called "SHGC", the solar heat gain coefficient . The SHGC measures the relationship between the total amount of solar energy hitting a window and how much of that energy is transmitted through the glass as heat.

South-facing windows in passive solar homes need high SHGC ratings, with low U-values to retain the heat and low air infiltration (see below) for optimal efficiency. Given that SHGC ranges from zero (no heat transferred through the window) to one (100% of the solar energy transferred), you need to target a range as follows:

  • For hot climates, under 0.4 SHGC
  • For intermediate climates, between 0.4 and 0.55 SHGC
  • For cold climates, as far above 0.55 as possible

 

Air infiltration

As we've said many times, air leaks are the bane of good passive solar home design. The NFRC measures air infiltration for window frames to show how much leakage there is. They measure in cubic feet of air loss per minute per square foot of surface area (cfm/ft²), so you want the lowest possible value.

Sliding windows tend to allow more air to escape than casement or awning windows, because the hinged options usually fit tighter and have better weather stripping. However, like all things, much depends on the quality of the product.

For passive solar homes, you should aim for air filtration rates below 0.3 cfm/ft².

Visual transmittance

This is a very technical term for how much light a window lets in. It is measured as a percentage of the amount of light a window lets through compared to an empty hole of the same size and shape. Plain glass can rate up to 90% VT, whereas tinted glass can reach as low as 15%.

As a rule of thumb, most people will think a window is light if it's at 60% VT or higher and dark if it's at 50% or less.

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