window rating

Pay attention to the windows energy rating labels. Energy Rating labels enable consumers to compare the efficiency of windows.

Energy Rating Label Elements

The windows energy rating labels involve two main elements:

- the U-factor (Solar Heat Loss Coefficient) and the...
- SHGC (Solar Heat Gain Coefficient).

They may also include the Air Leakage, the Visible Light Transmittance, the Resistance Condensation and the Annual Heating and Cooling coefficients.

The Window U-factor: measuring the Heat Loss

The windows U-factor measures heat loss, that is, the resistance of the window glass to the heat flow, and it is the most important information about windows in cold climates. It is expressed in units of Btu/hr-ft2-°F or W/m2-°C.

The lower the U-factor the better. The best windows have a U-factor of 0,3 or less. To attain such measures, the window should include features like double or triple-plane glass, low-e coating on the panes, argon gas filling, insulated frames and warm edge bridges.

The Window SHGC factor: measuring the Heat Gain

The SHGC coefficient measures the solar radiation admitted through a window, that is, the heat gain. The SHGC is expressed as a number between 0 and 1. A lower Solar Heat Gain Coefficient means less solar heat and more shading.

If you spend too much money in air-conditioning, pay special attention to this measure. Windows with lower SHGC values will reduce summer cooling...

To achieve that goal, windows use modern types of spectral selective low e-glass (the Low-E2, the Sungate 2, etc.).

The VT coefficient

Window rating may also include the VT (Visible Transmittance) coefficient. High VT coefficients are desirable to get more sunlight, which happens on the north side of a home (in the North hemisphere) and south side (in the South hemisphere), but not on the east and west sides, where sun is a cause of glare (in these sides, we should prefer windows with a lower VT).

An example of a window rating label (NFRC - National Fenestration Rating Council).

Windows Energy Star Label

The NFRC label includes 5 energy performance measures, including the...

- Condensation Resistance coefficient (a value between 0 and 100, measuring the resistance of the window to condensation; the higher the coefficient the better) and the...

- Air Leakage coefficient (a value between 0 and 1, measuring how much air passes through a square foot of window/door/skylight area (cfm/sq ft) by infiltration through cracks; the lower the AL the better).

Rating labels and Climate Zones

The U-factor, the SHGC and other window energy coefficients aren't the same in all climate zones. A very low SHGC is an excellent indicator in warm or even in mild climates, but it isn’t very relevant in a very cold climate, where the solar radiation admitted through a window, door or skylight isn’t a bad thing.

For more information, see: Windows & Climate Zones

USA Windows Rating

USA Energy Star labels (based on the National Fenestration Rating Council rates) are the major window rating reference in USA… Energy Star considers four climate zones for windows and skylights in the United States, each one with specific U-factors and SHGC values.

See: USA Climate zones & Windows, Doors and Skylights

CANADA Window Rating

Canadian Energy Star considers four climate zones for windows and skylights, each with different U-factors and SHGC values.

See: Canada Climate Zones and Canadian Energy Office

Europe & Windows Energy Rating

The BFRC (British Fenestration Rating Council) contains bands from A to G, where A is the most efficient rating and G the least. Only the most efficient windows carry the Energy Saving Recommended logo.

For more information, see: Energy Saving Trust UK, British Fenestration Rating Council

AUSTRALIA Window Rating

Australia has the Window Energy Rating Scheme (WERS). Windows are rated from 0 to 5 stars for both cooling (summer) and heating (winter).

For more information, see:

Australia Windows & Doors Labeling
Australian GreenHouse Office

See also: