Replacing your old windows is an important step in creating a more energy efficient environment in your home. The ultimate goal in choosing to upgrade your existing windows is to keep the cold inside during the summer months and keeping the heat inside in the winter months. Of course, one of the added benefits to having your old windows replaced is the reduction your utility bills year round. To compare your existing windows with a more energy efficient window there are several things to keep in mind:
- NFRC Certification
- VLT-Visible light transmittance VLT
- U Factor
- SHGC-Solar Heat Gain Coefficient
- UV Block
- AL-Air leakage
- CR-Condensation Resistance
The NFRC helps to eliminate the question of energy efficiency. There are many window companies and many types of window so knowing which windows are the most energy efficient can be difficult. The National Fenestration Rating council has made the task much simpler by providing industry standards to measure performance of windows in certain areas.
What is Visible Light Transmittance (VLT)?
Visible Light Transmittance (VLT) is defined as the amount of visible light that passes through glass. Visible transmittance can be affected by different factors such as type of glass, the number of panes, and glass coatings such as low-E, reflective coating or tint.
The higher the percentage of VLT the more light is passing through the glazing material. Visible Light Transmittance of glass can run around 90% for simple clear glass and a standard double-pane IGU will run around 78%. The more panes, coatings and specialized insulated glass the less the VLT.
*VLT general range is 0-1
What is the U-Factor?
U-Factor is a measurable number indicating the heat transfer rate or, more simply, how well a window is insulated. The lower the window’s U-Factor the better that the window keeps the cool inside in the summer and the heat in the winter.
*U-Factor general range is 0.15-1.25
The Solar Heat Gain Coefficient is a way to measure how well a window blocks heat flowing into your home. A lower number is indicative of a product that better blocks solar radiation or heat from the sun.
*SHGC general range is from 0-1
What is UV Block?
Damaging ultra-violet rays coming through your windows can cause your furniture, curtains, paint and carpet to fade. UV block tells how well the window blocks these rays. The higher the number the better the blocking power of the window.
*Values vary per window
What is Air Leakage (AL)?
AL measure the rate air escapes through seams in the window. The lower the AL the less the air will pass through those seams. Common standards usually requires an AL of 0.3 cf·m/ft².
*Values usually range from .1-.3
*Values vary due to window construction
What is Condensation Resistance (CR)?
CR is a measure that determines the degree at which a window resists condensation on the interior surface. Condensation Resistance values on range from 0 to 100. The higher the number value given, the less condensation the window allows.
*CR Values Vary Due to Window Construction
For more information go to http://www.energystar.gov/index.cfm?c=windows_doors.pr_ind_tested