Roof Glazing Type

U-factor: indicates how well a window keeps heat inside a building. The rate of heat conductivity is indicated by the U-value (inverse of the R-value) of a window assembly. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

The definition of Good U-value depends on the climate, the building type and the amount of glass. For example, in a warm climate, the U-value doesn't matter much. However, in cold climates a low U-value is beneficial—0.25-0.40 would be preferable. It is very important for skylights in cold climates to have low U-value to deal with condensation issues for residential constructions.

Note: GBS and Revit use actual material physical properties and layering in constructions to calculate U-Values. The U-Values listed in the materials properties are actually calculated by DOE2 from material density, specific heat, etc.

Note: Currently it is not possible to specify the abstract property of U-Value or R-Value directly, you have to select or define a material or construction.

SHGC: Solar Heat Gain Coefficient measures a window's ability to block warming caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.

The definition of Good SHGC depends on the climate, the building type and the amount of glass. For example in hot climates near the equator a low SHGC is Good— 0.20-0.35. In a cold climate or a situation where you want to take advantage of the sun for passive heating, High is Good—0.5-0.7. A low SHGC is VERY important for windows/skylights in hot climates.

Visible light transmittance (Tvis or VLT): it measures how much light gets through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. Most VLT values are between 0.3 and 0.8. The higher the VLT, the more light is transmitted. A high VLT is desirable to maximize daylight.

The definition of Good VLT depends on the climate, the building type and the amount of glass. For example, glare can become a problem if the VLT is too high and the glass area is large. However if the VLT is too low and the glass area is too small, you have spent money on glass and didn't get the benefit of natural daylight light. Selective coatings allow high VLT and low SHGC—you don't need mirrored glass to get low heat gain. New glass can have 65% visible light transmittance, 30% solar heat gain, and 0.30 Uvalue.