Blog 9

Understanding U-Values

Utilizing the potential of various coatings and glass types has other benefits. Compared to the total surface area of the building, the heat loss through relatively small areas of façade glazing is more than offset by the contribution of solar gains and the reduced use of artificial lighting. Anybody involved with architectural design and specification should be aware of U-values as a measure of thermal transmittance (the movement of heat energy) through building materials from cold to warm.

LOW-E COATINGS IMPACT ON GLASS PERFORMANCE

In addition to treatments for solar control and reflectivity, there are also low emissivity (low-e) coatings that impact on the loss of cool from inside the building by reflecting long wave radiation back in.A material’s emissivity determines the amount of thermal radiation emitted from its surface. Low-e surfaces emit less thermal radiation, and glazing units benefit from this through the application of a microscopic coating of tin, silver or zinc to certain faces of the glass panes in the unit.In contrast to the short wave radiation from the sun that heats the building interior, the heat energy transferring back through the building envelope, from cold inside to warm outside, is long wave radiation. Glass with the low-e coating reflects long wave radiation, effectively keeping more chill inside the building.

WHAT ARE ‘HARD’ AND ‘SOFT’ COATINGS?There are two types of coating: hard and soft. Hard coat is applied while the glass is still molten, whereas soft coat is applied later in the process. Hard coat is more durable, as its name suggests. Soft coat remains delicate, is only applied to the sides of panes facing into a sealed airspace and has a lower emissivity than hard coat. Selecting glass that provides high light transmittance, effective solar control and excellent ther-mal performance is the holy grail. The term selectivity is used to describe this balanced combi-nation of performance characteristics, where a higher selectivity is seen as the optimum combi-nation.