Energy Performance of Entry Doors: R-Value vs. U-Factor

You hear the terms U-Factor and R-Value a lot but do you really know what it means? Both are measures of energy performance but are reached in very different ways.  Most of us know that the higher the R-Value the better the insulation and therefore the better the energy savings for our homes.  We also understand that if the R-Value is high then the U-Value or U-Factor is low.  In order to understand how these ratings can help us make wish decisions about investing in energy savings we must clearly understand what the ratings mean.

R-Value is a measure of resistance to heat flow.  Generally, R-Value is given for a certain thickness of specific material, such as fiberglass wall insulation.  The R-Value of a structure can be estimated by simply adding up the R-Value of each material that makes up the layers in the wall (exterior sheathing, insulation, interior drywall, etc.).

While R-Value measures resistance to heat transfer, U-Factor (or U-Value) measures the rate of heat transfer.  The lower the U-Factor, the better the product’s ability to resist heat conduction.  U-Factor is a series of calculations created by engineers and scientists to measure the rate at which heat flows through 1 square foot of material.  But U-Factor is not a material property value.  It combines the conductance values of all the materials that make up the product.  Windows and doors are known as fenestration products.  Fenestration means “an opening”, as in the wall of a house.  U-Factor accounts for how energy enters and leaves the product, considering both conduction and radiation.  R-Value accounts only for the resistance to heat flow by conduction.

The NFRC (National Fenestration Rating Council) recognizes only U-Factors for energy ratings.  This recognition is also true for the American Society of Heating, Refrigerating, and Air-Conditioning Engineers and the International Energy Conservation Code.  Jim Benney with the NFRC says, as a public service organization, the NFRC has an inherent responsibility to communicate to consumers, government bodies, and others the most appropriate and credible information about fenestration product performance.  U-Factor is directly related to energy savings because it directly predicts reduced heat transfer.  In contrast, the relationship of R-Value to energy savings is more complicated and highly variable.

The decisions that we make about the products we put in our home are decisions we must live with for many years so we want to make wish choices.  We hope that this information will help you with your decisions about your next window and doors products.

Information for this article was gleamed from articles by Jim Benney, National Fenestration Rating Council’s chief executive officer and Ken Brenden, technical standards manager for the American Architectural Manufacturers Association.