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The energy efficiency of various windows can vary dramatically. For this reason, they are given two different types of values that measure heat loss: R-values and U-values. Windows are most efficient if they have a relatively high R-value and low U-value, so these terms can be confusing. Here is a brief explanation:
R-value:Thermal resistance—the ability of a material to resist heat flow—is measured by an R-value. The higher the R-value, the more the material resists the movement of heat. Although some newer houses are built with insulation that meets optimal standards, most older homes are underinsulated unless they’ve been retrofitted with some type of insulation.
Recommended minimum R-values for homes vary by climate and may be affected by how a house is built and the type of heating system used. Here are a few rules of thumb:
* For mild climates, R-11 in walls and floors: R-19 in ceilings below ventilated attics
* For moderate climates, R-19 in walls and floors: R-30 in ceilings below ventilated attics
* For cold climates, R-19 in walls and floors: R-38 tol R-49 in ceilings below ventilated attics
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