Energy Features of Glazing
The choice of fenestration involves the art of compromise. The designer must balance competing issues of view to outdoors, daylighting potential, solar heat gain, thermal heat losses and even aesthetics.
Insulation Performance of Glazing
As shown in the diagram to the right, the following units have a successively increasing thermal performance (U values in brackets are imperial measurements):
A SINGLE GLAZING - no longer recommended for most climates. U-value 6.24(1.1).
B CLEAR DOUBLE-GLAZING - this is a sealed factory unit with airspace - known as insulating glass because of its good insulating characteristics. U-value 2.78(0.49)
C CLEAR DOUBLE-GLAZING - LOW-E - a transparent metallic (low emissivity) coating reflects heat back to occupied space. U-value 1.93(0.34)
D CLEAR DOUBLE-GLAZING - LOW-E - GAS FILLED - uses Argon or Krypton gazes - less conductive than air within the cavity. U-value 1.59(0.28)
E CLEAR TRIPLE-GLAZING - in many cases equivalent to D. If gazes and metallic coating are used, then performance is increased further. U-value 1.59(0.28)
F MULTIPLE COMPARTMENT - SUSPENDED FILMS - uses two panes of glass with two, low-E coated polyester films - very high performance.. U-value 0.74(0.13)
DISADVANTAGES - with the exception of some proprietary F systems, the above-noted solutions, using clear glazing, do very little to limit solar heat gain.
Insulation Performance of Frames
Emphasis has shifted from glazing performance to frame performance as the thermal weakness of the window assembly. Improvements include thermally broken frames, wood or metal clad wood frames, or less conductive vinyl and the newer pultruded fibreglass.
Solar Heat Gain Performance of Glazing
Reducing heat gain is an entirely different problem than reducing heat loss. Some traditional methods as indicated in the diagram to the right are:
G TINTED OUTER PANE - energy absorbing materials are dispersed throughout the pane, giving it a tint of varying colour and intensity with the ability to reduce energy penetration.
H REFLECTIVE COATINGS - added to the No. 2 surface reflect rather than absorb solar radiation, thus reducing overall solar heat gain.
DISADVANTAGES - both methods G and H, although effective for reducing solar heat gain, may limit daylighting potential. Spaces are typically "dark"; views to the outdoors are may be subdued; and the quality and colour of naturally available daylight may be altered.
Excerpted with permission from Design Smart published by BC Hydro.