The Window Source R-Value, U-Value, and the 4 Ways Your Windows Transfer Heat New Hampshire, Massachusetts, and Maine
We want to help you keep your home’s interior winter heat or summer cool inside. The best way to do this is to arm yourself with knowledge about what protects your home for heat transfer/loss.
Heat transfer is expressed with U-values. Here’s a couple of quick and useful definitions:
- R-values: The R-value is a measure of resistance to heat flow through a given thickness of material. So the higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties.
- U-values: The U-value is a measure of how much heat is lost through a given thickness of a particular material, but includes the three major ways in which heat loss occurs – conduction, convection and radiation. This is the measure that is always within Building Regulations.
For more of an explanation on R- and U-values, visit http://www.thegreenage.co.uk/article/thermal-conductivity-r-values-and-u-values-simplified/.
Windows lose and gain heat four ways: conduction, convection, radiation, and air leakage.
Conduction is the movement of heat through a solid material. If you hold a metal rod over a fire, you’ll run the risk of burning your hand as the rod conducts heat. Heat can pass through a window this way. A less conductive material helps to impede heat flow. This is why we offer multiple-glazed windows that trap low-conductance gas (like argon) between multiple panes of glass. This limits the conduction through the largest volume of the window. We also offer thermally resistant edge spacers and additional components help to reduce conduction.
Convection is another way that heat moves through windows. In a cold climate, heated indoor air rubs against the interior surface of window glass. The air cools, becomes more dense and drops toward the floor. As the stream of air drops, warm air rushes in to take its place at the glass surface. The cycle, a convective loop, is self-perpetuating. You recognize this movement as a cold draft and turn up the heat. Each 1°F increase in thermostat setting increases energy use 2%. Multiple panes of glass separated by low-conductance gas fillings and warm edge spacers, combined with thermally resistant frames, raise inboard glass temperatures, slow convection, and improve comfort.
Radiant transfer is the movement of heat as long-wave heat energy from a warmer body to a cooler body. Radiant transfer is the warm feeling on your face when you stand near a wood stove. Conversely, your face feels cool when it radiates its heat to a cold sheet of window glass. But radiant-heat loss is more than a perception. Clear glass absorbs heat and re-radiates it outdoors. Radiant-heat loss through windows can be greatly reduced by placing low-E coatings on glass that reflect specific wavelengths of energy. In the same way, low-E coatings keep the summer heat out.
Air leakage siphons about half of an average home’s heating and cooling energy to the outdoors. Air leakage through windows is responsible for much of this loss. Well-designed windows have durable weatherstripping and high-quality closing devices that effectively block air leakage. Hinged windows such as casements and awnings clamp more tightly against weatherstripping than do double-hung windows. But the difference is slight; well-made double hung windows can greatly limit air leakage. Craftsmanship and installation will also affect air leakage. The installed components must have tight connections. Windows with certified air-leakage rates of less than 0.30 cfm/ft2 are considered energy efficient. The lower the value, the better.
Hopefully this information is useful! More questions? Feel free to contact us. Our sales professionals are happy to help you. Here at The Window Source, we’ll bring you the best solutions for your replacement window needs. We serve all of Massachusetts, New Hampshire, and Maine.