Fully controllable conditions, repeatability, and the ability to reach and maintain steady-state conditions are necessary to determine the SHGC and U-factor
of fenestration systems.
Walls and attics are rated with R-value, and windows are rated with U-factor
An example is the presentation of the provisions for a commercial building wall in terms of an overall U-factor
for the opaque portions of the wall or for the entire wall.
Several metrics were proposed for the optical, lighting and thermal performance of TDDs, including: Light Transmittance, Normalized Luminous Intensity, Average and Peak Luminance Indices, Light Diffusion Index, Daylighting Area Coverage, Spacing Ratio, SHGC and U-factor
R-values, fenestration U-factors
, SHGCs, and economizers are now invoked as reference points when demonstrating conformance of the building envelope, insulation, and mechanical and lighting devices to approved drawings.
This means, for instance, that a substance with an R-Value of 12 would have a U-Factor
For example, a very low U-factor
envelope system may not require a perimeter heating system, and very tight, highly insulated homes have been built in Northern Europe that do not require central heating systems.
Pyrolytic Low E coatings have the best combination of solar heat gain and U-factor
(measurement of thermal transference) to provide optimum annual energy savings in north-central and northern climate zones.
The new standards also alter the U-factor
and Solar Heat Gain Coefficient for the regions (see table below).
The measurement for low heat-loss in glass is called a U-factor
; the lower the U-factor
the better the insulating value.
: Lower U-factors
(inverse of thermal resistance) will more effectively reduce heat loss in climates that require more heating.