U-Factor More Important Than R-Factor For Windows and Doors

The window and door industry have their products measured in the form of U-Factor rather than R-Factor. U-Factor is the standard adopted by the National Fenestration Rating Council for windows and doors because it measures the rate of heat transfer (or loss) by the product being rated. It is a more important factor than the R-Factor because R-Factor measures conductivity. Windows and door are obviously not stationary so the measurement of heat loss gives us a better indication of how energy efficient a window or door is.

Radiant Heat Transfer

Radiant heat transfer is heat flow via absorption and then re-radiation. (An easy example is when you put your arm into the direct summer sun and it starts to warm up.)

The NFRC gives each window a U-factor rating. The U-factor is the inverse of the more familiar R-factor used in attic insulation. So a U-factor of .5 equals an R-factor of 2. The lower the U-factor the better.

When comparing windows, check for the NFRC label. Look for the U-factor. The first number after the words "U-factor" is the rating that's appropriate for residential purposes. It will be marked "AA" or "Residential." The U-factor marked "BB" or "Non-Residential" is for commercial window applications. Use the U-factor rating to make meaningful comparisons. Be wary of a window vendor who won't provide this number.

How far should you go? Obviously, you need to get price comparisons to make a decision. Keep in mind that a good portion of your cost is installation, so it makes sense to leverage those costs by installing a better window.

Convection Heat Transfer

Convection is the transmission of heat caused by movement of molecules from cool regions to warmer regions of lower density. The word "convection" is derived from the latin term convehere (to bring together).

Convection heat transfer is heat flow via air movement. (An easy example is when you turn on a hair dryer and the hot air is projected from the end of the hair dryer.)

Conduction Heat Transfer

Conduction is heat transfer through materials. (An easy example of conduction is when the handle of a pot on the stove gets warm. The heat is being conducted from the bottom of the pot all the way to the handle.)

An energy efficient window designed for heating-dominated climates minimizes heat loss by controlling all three sources of heat (energy) transfer.

Creating Energy Efficient Windows For Cold Climates Like Minnesota

The window industry uses the term u-value to measure heat flow. (A lower u-value means better thermal performance.)
There are a number of glass-related options that window manufacturers can use to design energy-efficient windows in cold climates. Most of these techniques improve the thermal performance of the glass. A brief explanation of each follows:

• Insulating Glass
  Insulating glass is comprised of two (or more) pieces of glass separated by a spacer material and sealed together to create an insulating glass unit (IG unit). IG units reduce convection and conduction heat loss.
• Low-E Glass
  Low-E glass has an almost invisible, microscopically thin coating that reflects long-wave infrared energy (or heat). When interior heat energy tries to escape to the colder outside, the Low-E coating reflects the heat back to the inside. The Low-E coating reduces radiant heat loss through the glass.
• "Warm-Edge" Spacer System
  The spacer material separating the two glass panes can be a source of conductive heat loss in cold climates. The thermal performance of IG units can be enhanced by using the SST spacer material that has a lower energy conductance rate and/or has less physical mass which equates to a lower energy transfer path.