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GLASS AND GLAZING ARCHITECTURAL PRODUCTS

Info for Architects

Here is some helpful information for you on various properties and the physical nature of glass and the elements that can effect them.

Optical Properties

Forms of Heat

U-Value - Btu/hr/ft2/F

U-value is a measure of heat transfer due to outdoor/indoor temperature difference. Used almost exclusively to describe the heat loss of material.

Q = U A (T high – T low)
Heat = (U value) (area) (Temp. Difference)

R-Value

R = 1/U The higher the R-value, the less heat transfer

Shading Coefficient

Shading coefficient (SC) is a ratio of the Solar Factor of the glass in question to the Solar Factor of 1/8" clear glass (0.87).

Therefore, SC = Solar Factor/0.87

The lower the number, the less heat into the building.

Relative Heat Gain

A number that represents the amount of heat entering a building due to both solar radiation and conductive/convective heat gain. Calculated for a specific environmental condition.

RHG = Solar Transmission + Cond/Conv. Heat Gain

85 –90%,10 – 15%

RHG, SC, and SHGC are all ways of answering the same question – How much heat gets into the building?

Emissivity

Lump of Coal ..................1.00
Uncoated Glass ...............0.84
Reflective Glass ...............0.40
Pyrolitic Glass ..................0.15
Soft Coat Low-e .............0.09
Heat Mirror – 66 .............0.04
Tin Foil ............................0.00

Emissivity: a measure of how readily a surface absorbs and emits infrared energy (the lower the emissivity, the less infrared the glazing absorbs. Emissivity is the heat-emitting (or radiating) propensity of a surface. The emissivity number compares a given surface with that of a perfect radiator, called a blackbody (emissivity of 1). Ordinary window glass has an emissivity of 0.84, meaning it absorbs and emits 84% of the radiant heat.


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