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Advertising supplement provided by Pittsburgh Corning

Light and heat transmission. While light transmission in glass block is nearly equal to that of flat glass, its thermal values are much higher.

For example, the following figures apply to light transmission:

• Flat sheet glass — 90 percent of visible light that strikes is transmitted
• Solid glass block — 80 percent is transmitted
• Standard and thin block — 75 percent
• Tightly ribbed patterns — 50 percent
• Fibrous glass inserts and thick-faced block — 50 percent to 55 percent

As the thickness of the glass block increases, so does heat transmission values (U values):

• Sheet glass — U-value = 1.04
• Solid glass block — U-value = 0.87
• Thin block — U-value = 0.57
• Standard block — U-value = 0.51

Thermal resistance, or R-value, is the inverse of U-value. Regarding R-value, glass block can more than double the resistance of single-glazed, 1/8-inch-thick plate glass. The solid glass block R-value is 1.15, while the 1/8-inch-thick plate glass value is 0.96. With hollow glass block and its inherent partial vacuum, a jump in R-value occurs: The R-value of thin glass block is 1.75 and the R-value of standard glass block is 1.96.

The differences between the shading coefficient of glass block and flat sheet glass is also significant. Thin glass block can reduce the shading coefficient of 1.00 for 1/8-thick flat sheet by 35 percent to 0.65 (and down to 0.45 for tightly ribbed patterns). Contributing to this, experts say, is the louvering effect of glass block’s horizontal mortar joints, which helps reduce light transmission from the higher sun. The size and orientation of the block can, therefore, affect the amount of shading that occurs.

Sound transmission. Sound transmission class (STC) is a rating that provides an estimate of the sound-absorbing performance of the partition for general building design purposes. The rating is designed to correlate with subjective impressions of the sound insulation provided against the sound of speech, radio, television, music and similar sources of noise in offices and dwellings. STC value represents an average of sound transmission losses between the 125 Hz and 4,000 Hz frequencies.

The sound-reducing characteristics of glass block are most notable in the hollow block with its partial vacuum (35 STC to 40 STC). With thick-faced and solid block, the STC is 48 and 53, respectively. This compares with about 29 STC for flat sheet glass (1/8-inch thick), 45 STC for single wythe 4-inch brick wall and 50 STC for 6-inch solid c.m.u. wall system.