A Guide to Window Energy Efficiency

Having a finely tuned and well-oiled heating and cooling system isn’t much good if all that conditioned air leaks right out the window. That’s why investing in high-quality windows that are built to be energy efficient is a vital step toward total home efficiency. However, windows are a lot more than just a flat piece of glass stuck in the wall. To understand which windows are the best fit for your home you need to understand a few key bits of information.

Manufacturer’s Label

Every window will have a sticker with the manufacturer's information on it. Here’s what to look for when reading this label and what it all means.

  • Frame materials- Is it made from wood, aluminum, vinyl, steel or fiberglass?
  • Number of panes: The options are single, double or triple. The more panes, the more efficiency.
  • Type of gas filling the gap in multi-pane windows- It’s normally argon or krypton.
  • Low-E- A glass treatment that reduces harmful UV rays that can fade your furniture and carpets.

NFRC Label

A very helpful piece of information is the label from the National Fenestration Rating Council (NFRC). This label will come in the form of four or five numbers in big bold print, which will rate the following factors.

  • U-Factor: a measurement of how effectively windows keep heat in your home. Lower numbers mean more energy efficiency during the heating season.
  • Solar Heat Gain Coefficient: a measurement of how effectively windows block heat from outside. Lower numbers mean more energy efficiency during the cooling season.
  • Visible Transmittance: measures the amount of light a window lets in. Higher numbers mean more light, which can lead to savings on artificial lighting.
  • Air Leakage: indicates how much air the window allows in. Lower numbers equal a tighter air seal.
  • Condensation Rating: not every NFRC label will have this rating, which is a measurement of how well a window resists condensation. The higher the number, the higher the resistance.

Energy Star Sticker

One of the fastest ways to spot a good window is to look for the Energy Star sticker. This indicates the window has met strict energy efficiency guidelines set by the Environmental Protection Agency (EPA).