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Heat Transmission
through Glass
When searching for solutions in reducing heat exchange through glass, one must first understand how the heat is transferred. In buildings, heat flows by three physical processes: conduction, convection and radiation. Reducing or omitting these factors will ultimately determine the effectiveness of the shading system. Conduction is the flow of heat in solids. Dense materials, like glass, are generally good conductors. They transfer heat from a warm spot to a cold one. Touching the handle of a hot cast iron frying pan demonstrates conductive heat transfer instantly. Trapped pockets of dead air will increase the “thermal resistance value” (R factor) of the glass. The higher the R factor, the greater it will be at reducing conduction. Down vests and insulated underwear work on this principle.
Convection is the transfer of heat by movement in liquids and gasses. As the transfer gas (air) is heated up by convection, it “carries away” the available heat. A fan placed behind a wood stove will convect warm air to other parts of a room. Effective window insulation will keep the heat from inside the buildings from warming up the glass. Because glass panes can become extremely cold, they can help create wasteful convection currents which move warm air to colder spots within a room. Shading systems with “edge seals” (or blinds tightly installed against the window frame) will reduce convection. An air tight shading system, with edge seals, will greatly reduce condensation problems. The third method of heat transfer is radiation. Radiant heat transfer can occur in the absence of matter. All objects within a building radiate energy (our bodies are radiating about 300 BTU/hour). Uncovered windows act as cold “heat sinks”, which rob warm bodies of heat. Shiny surfaces, metallic and white colours reflect radiant heat. |
