Explaining Energy Performance Ratings For Double Glazed Windows

Homeowners thinking of replacing their old windows should learn as much as they can about energy ratings and the Energy Star program.

New replacement windows can help homeowners save on their utility bills and protect and conserve the environment at the same time.

The National Fenestration Rating Council (NFRC) runs a voluntary testing, certification, and labeling energy performance program for doors, windows, and skylights. The NFRC label is found on Energy Star products; the Energy Star program only tests for U-factor and solar heat gain coefficient, while the NFRC tests other important energy factors.

Both programs provide sufficient information for consumers to make informed decisions on replacement windows.

Double Glazed Windows and Doors.

DOuble GLazed UPVC Window profile viewThese are rated by measuring their heat gain or loss. Heat gain or loss can happen due to radiation of heat into the house (mostly from the sun) and out of the house from people, furniture, and other room-temperature objects.

It can also occur because of direct conduction of heat through the glass, frame, or door. Heat loss or gain also occurs because of air leakage through and around windows and frames.

The U-factor is the rate at which a door, window, or skylight conducts non-solar heat flow. For replacement windows, the U-factor usually refers to just the glass or glazing. The lower the U-factor, the more energy efficient the product and the more energy savings a homeowner will see.

The solar heat gain coefficient (SHGC) is the measurable amount of solar radiation emitted through a door, window, or skylight. This can refer to heat absorbed or transmitted directly into a home, and then subsequently released as heat.

A window with a lower SHGC will transmit less solar heat, provide greater shading ability, and reduce energy costs. A window with a low SHGC is better at blocking solar heat gain and keeping rooms cool during the summer.

A window with a higher SHGC is better at collecting greater amounts of solar heat and keeping a room warm during the winter. Location, the situation of the house, the amount of natural shade (trees), and the climate will determine the best SHGC for a home’s replacement windows.
Air leakage is self-explanatory and refers to the amount of air flow around a window or door.

It is expressed in cubic feet per minute per square foot of frame area (cfm/ft2). It is a relatively complicated sounding formula that basically lets the homeowner know that a lower air leakage value provides a tighter fit and more energy savings.

Other Factors for Double glazed windows & doors

Two other factors that are considered in replacement windows, doors, and skylights are visual transmittance (VT) and Light-to-solar gain. VT is concerned with the visible spectrum of sunlight transmitted through the glazing on a door, window, or skylight. The higher the VT, the more it transmits visible light. Light-to-solar gain is the ratio between the SHGC and the VT. A higher number means that more light can be transmitted without excessively increasing the amount of heat.

Before purchasing replacement windows or doors, it is essential to have a basic understanding of the ratings, the numbers, and the potential savings that energy efficient windows provide.

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Doing a little research before buying will ensure an informed, satisfying, and energy efficient purchase.

Double Glazed Windows and Doors Energy Efficiency (US)