Window Basics

Window 101

Depending on what type of windows you currently have, the dimensions of the existing windows, the type of fuel you use for heating and cooling, as well as how energy conscious you are will affect the savings accrued by replacing single pane windows with energy efficient windows. The following are some things to consider when selecting and purchasing new, energy-efficient windows:

Energy Performance Ratings. Windows have energy performance ratings that measure how much heat they will gain or lose as well as how much light they will transmit into your home.

The National Fenestration Rating Council (NFRC) is a non-profit organization that determines the energy performance ratings of windows, doors and skylights.  When purchasing a window, look for the NFRC label on the product.  This label contains the manufacturer’s name, a description of the product, and the product’s energy performance ratings. You can see an example label below:

ENERGY STAR looks at two performance ratings, the U-factor and the solar heat gain coefficient, when determining whether or not a window meets ENERGY STAR performance standards.

U-factor.   U-factor is the rate of heat transfer through the window (from inside to outside when it is cold, and from outside to inside when it is hot) per unit area and per unit temperature difference.  With a lower U-factor, less heat energy leaves your home in the winter and less heat energy enters your home in the summer.  A lower U-factor is always better.  In the northern climate zone, a window must have a U-factor of 0.35 or lower in order to receive ENERGY STAR qualification.

Solar Heat Gain Coefficient (SHGC).   SHGC measures how well a window blocks heat caused by sunlight.   It is the ratio of solar heat gain entering the space through the window assembly to the incident solar radiation.  Solar heat gain includes directly transmitted solar heat and absorbed solar radiation that is then released into the space.  The lower the SHGC the less solar energy transmitted to the space.  The SHGC is a number between 0 and 1.  Windows with lower SHGC values transmit less solar heat into the house.

Visible Transmittance (VT).  The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The NFRC's VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.  Select windows with a higher VT to maximize daylight and view.

Air Leakage.  Heat loss and gain occur by infiltration through cracks in the window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. At this time, the AL is optional.  Select windows with an AL of 0.30 or less (units are cfm/sq ft).

Condensation Resistance Factor (CRF).  The CRF is a numerical index generally in the range of 30-80 for conventionally glazed fenestration products.  The larger the CRF number the greater the resistance to condensation.  When water vapor in the air comes into contact with a surface whose temperature is lower than the dew point of the air the water vapor will condense.  The amount of water vapor in the air is called the relative humidity.  Controlling the relative humidity in your home is the most effective way to avoid objectionable condensation.

ENERGY STAR Qualified Windows. The Energy Star label means that a product is ENERGY STAR qualified in one or more climate zones. It is important to note, however, that manufacturers must apply in order to receive ENERGY STAR qualification. Some products without ENERGY STAR labels may in fact meet ENERGY STAR performance standards. For more information click here.