-
Windows allow heat to flow between the home and the outdoors in many ways.
Heat flows in three main forms:
- Convection (hot air rising or cold air sinking),
- Conduction (heat movement between hot and cold objects that are in contact),
- and Radiation (infrared light emitted by hot objects).
Heat flow is resisted in the following ways:
- Convection: by creating an air-tight barrier between hot and cold air,
- Conduction: by creating layers of trapped air between the hot and cold objects,
- and Radiation: by placing reflective material between the hot and cold objects.
A single pane of glass is one of the worst possible insulating materials: it has no trapped air pockets and it is transparent, providing no resistance to radiant heat flow.
-
-
Modern windows add a considerable amount of insulation to your home.
Older, single pane windows drive up the air conditioning and heating costs for a home due to their poor insulating properties. The more windows a home has, the greater this penalty on the electric bill will be.
Modern windows have added features that greatly improve upon the insulating properties of the window.
- Double-pane: Modern windows have two panes of glass separated by a layer of gas, creating a trapped air pocket to resist conductive heat flow.
- Spectrally-selective coatings: Modern windows have a coating that allows visible light to pass through but reflects non-visible radiation (such as infrared light) back outside. This reduces the radiant heat load from sunlight.
- Low-emissivity coatings: Modern windows also have a coating that reduces the radiant heat emissions of the window to the outdoors. This reduces the radiant heat loss from the window when the weather is cold outside.
