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Glass Technology for Cold Climates

Window Performance in Heating-Dominated (Cold) Climates
Creating Energy Efficient Window Performance in Heating-Dominated (Cold) Climates

Window Performance in Heating-Dominated (Cold) Climates
Homes in heating-dominated climates lose interior heat (energy) to the outside colder-temperature environment. Heat loss continues as long as the exterior temperature is lower than the interior temperature, regardless of whether it is day or nighttime. (Some climates provide significant amounts of passive solar energy, which can help heat the home in the winter.) Many colder climates have weeks or even months during which outside temperatures are significantly colder than most preferred interior temperature levels.

Heat loss (or heat gain) through windows occurs by three methods:

Radiant heat transfer
Convection heat transfer
Conductive heat transfer

A brief explanation of each follows.

Radiant Heat Transfer
Radiant heat transfer is heat flow via absorption and then re-radiation. (An easy example is when you put your arm into the direct summer sun and it starts to warm up.)

Convection Heat Transfer
Convection heat transfer is heat flow via air movement. (An easy example is when you turn on a hair dryer and the hot air is projected from the end of the hair dryer.)

Conduction Heat Transfer
Conduction is heat transfer through materials. (An easy example of conduction is when the handle of a pot on the stove gets warm. The heat is being conducted from the bottom of the pot all the way to the handle.)

An energy efficient window designed for heating-dominated climates minimizes heat loss by controlling all three sources of heat (energy) transfer.

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Creating Energy Efficient Window Performance in Heating-Dominated (Cold) Climates

The window industry uses the term u-value to measure heat flow. (A lower u-value means better thermal performance.)
There are a number of glass-related options that window manufacturers can use to design energy-efficient windows in cold climates. Most of these techniques improve the thermal performance of the glass. A brief explanation of each follows:

Insulating Glass
Insulating glass is comprised of two (or more) pieces of glass separated by a spacer material and sealed together to create an insulating glass unit (IG unit). IG units reduce convection and conduction heat loss.
Low-E Glass (PPG Sungate® or Solarban® Low-E Glass)
Low-E glass has an almost invisible, microscopically thin coating that reflects long-wave infrared energy (or heat). When interior heat energy tries to escape to the colder outside, the Low-E coating reflects the heat back to the inside. The Low-E coating reduces radiant heat loss through the glass.
"Heavy" Inert Gas in the IG Unit
Many IG units are sealed with atmospheric air which is a combination of oxygen, hydrogen, nitrogen, and other trace gases. IG unit performance can be enhanced by completely filling the air space between the glass panes with a "heavy" gas, such as argon or krypton. These heavy gases reduce heat loss across the air space by minimizing convection heat loss.
"Warm-Edge" Spacer System (PPG Intercept® Spacer System)
The spacer material separating the two glass panes can be a source of conductive heat loss in cold climates. The thermal performance of IG units can be enhanced by using a spacer material that has a lower energy conductance rate and/or has less physical mass which equates to a lower energy transfer path.