It is important to understand technical performance standards before investing in replacement windows. Many homeowners are familiar with R-values; however, the values used to measure the energy efficiency of windows are not as familiar to most homeowners. Like R-values, technical performance standards for replacement windows are independently established and certified. These standards are the same for manufacturers, i.e. each company is not free to establish their own performance standards. These standards provide a good basis for homeowners to compare window performance from one brand to another brand.

Required Performance Standards

There are three key replacement window performance standards that are important for homeowners to understand and compare:

  1. U-Factor
  2. Solar Heat Gain Coefficient - SHGC
  3. Visible Transmittance - VT

The U-Factor measures how well a window prevents heat loss from the home. It is a measure of how well the window insulates. The lower the U-Factor, the better the window insulates. U-Factor ratings are generally between 0.25 and 1.25. Mid South Windows recommends a U-Factor of 0.30 or lower for Knoxville and East Tennessee.

The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from the sun. Blocking solar heat gain is especially important during the summer in East Tennessee. SHGC is expressed as a number between 0 and 1. The lower the number, the better the window is at blocking unwanted solar heat gain. Mid South recommends a SHGC of 0.30 or lower for our region.

Visible Transmittance (VT) measures how much light comes through a window. VT is expressed as a number between 0 and 1. The higher the VT rating, the greater the amount of natural daylight that the window lets into the home. Desirable VT rating will vary significantly based on the home’s location and individual preferences. It is often advantageous to vary VT ratings based on whether the windows are East or West facing. Homeowners should carefully consider the VT rating before investing in replacement windows.

Optional Performance Standards

There are two optional performance ratings:

  1. Air Leakage - AL
  2. Condensation Resistance - CR

Air Leakage (AL) measures how much outside air enters a home through the window. AL is expressed as a number between 0.1 and 0.3cfm (cubic feet per minute). The lower the AL, the better a window is at keeping outside air from entering the home. AL is an optional rating; therefore, manufacturers can elect to not include AL on their labels.

Condensation Resistance (CR) measures a window’s ability to resist the formation of condensation. CR is expressed as a number between 1 and 100. The higher the CR rating, the better the window is able to resist condensation. CR is an optional rating; therefore, manufacturers can elect to not include CR on their labels.