This article was originally published in the January/February 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online January/February 1997


New Window Ratings to Indicate Seasonal Performance

What's the best way to inform window buyers about a product's energy characteristics? As with appliance energy labels, there is considerable debate over how to give people an accurate and useful representation of the product without overwhelming them with confusing numbers (see New Appliance Labels Emphasize Energy Use, HE Jan/Feb '96, p. 7).

The National Fenestration Rating Council (NFRC), which designed standard test procedures for U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), and air leakage, has been developing two composite ratings--one for winter performance and the other for summer performance. Researchers recently ran thousands of computer simulations and found that the composite ratings can adequately represent the effects of U-factor, SHGC, and air leakage under a wide range of conditions.

The fenestration heating ratio (FHR) and fenestration cooling ratio (FCR) show what percentage of annual house heating and cooling energy the window will save, compared to a base case single-glazed, aluminum-framed window. An FHR of 30, for example, indicates that a house fully equipped with this sort of window will require 30% less heating energy than if equipped with the base case window. The FCR indicates cooling cost savings in the same way.

John Carmody of the Minnesota Building Research Center and Brian Crooks of Cardinal IG used a prototypical house to run computer simulations for 11 different types of window. Then they changed all the basic characteristics of the house, one by one, to reflect the range of housing conditions and reran the simulations.

They found that while altering the house's characteristics changed the calculated energy use, it often did not change the relative impact of the different window types. That is, a window that saved 20% of heating energy in one test still saved 20% in a house with different characteristics. The researchers then tested the window types in 30 different climates and found again that the savings percentages remained close. The FHRs and FCRs, averaged over the 30 climates, are shown in Table 1, along with U-factor, SHGC, VT, and air leakage.

This is not to say that climate is unimportant in choosing a window. A window may have an FCR of 20 in both Maine and Florida, but 20% of cooling costs is clearly much greater in Florida than in Maine. The FHR would be far more important to the window buyer in Maine. NFRC will also provide FHRs and FCRs for typical older window types, so remodelers can compare their existing windows with new ones.

The researchers did find that the composite ratings won't be as accurate for certain situations. These include houses with a window area greater than 20% of floor area; houses with substantial window shading; older, poorly insulated houses with high air infiltration; passive solar houses; and houses where only some windows are being replaced.

Canada has already been using a composite window Energy Rating (ER). Like the FHR, the ER is a single number that indicates how window performance affects heating-energy use. The rating is used voluntarily by members of the Canadian Window and Door Manufacturers Association. The ER similarly combines solar heat gain, U-factor, and air leakage, and compares windows to a base case scenario. The base case house has equal window areas facing north, south, east, and west in a climate based on that of several Canadian cities. ER values can be positive or negative. A positive value means that a window will add more heat to a house than it loses during the heating season; a negative rating indicates the opposite; and a window with an ER of zero neither gains nor loses heat.

In the United States, FHRs and FCRs won't be available right away, but NFRC expects manufacturers to start labeling annual performance in 1997. Window labels for the other energy characteristics have increased by leaps and bounds since the certification program started. In 1995, 22,000 products were certified, compared to just 3,800 products in 1994. One hundred and twenty window manufacturers now have U-factors, SHGC, and VT certified, and at least six states and a number of counties accept NFRC certification as evidence of compliance for energy codes. It can still be difficult to find the labels on windows in a store, because some windows are tested but not labeled. However, information for all certified windows is listed in the NFRC Directory of Window Ratings.

--Jeanne Byrne
Further Reading
Carmody, John, and Brian Crooks, Selecting Windows Based on Annual Energy Performance. In 1996 ACEEE Summer Study on Energy Efficiency in Buildings. Berkeley, CA: American Council for an Energy-Efficient Economy, 1996.

NFRC Directory of Window Ratings, National Fenestration Rating Council, 1300 Spring St., Suite 120, Silver Spring, MD 20910. Tel:(301)589-6372; Fax: (301)588-0854. The directory costs $15.

Table 1. Simulations of Window Energy Performance Note: All values (even SHGC and VT) include the effect of the frame and sash, not just the glass.
Window Type U-Factor SHGC VT  Air Leakage  FHR (30 Climate  FCR (30 Climate
  (Overall) (Overall) (Overall) (CFM/ft2) Average) Average)
Single glass (clear), 
aluminum frame (no thermal break)   1.3   0.79   0.9   0.98     0   0
Single glass (bronze), 
aluminum frame (no thermal break)   1.3   0.69   0.68   0.98   -2   8
Double glass, 
aluminum frame (with thermal break)   0.64   0.65   0.81   0.56   19   12
Double glass, 
wood or vinyl frame   0.49   0.58   0.81   0.56   24   18
Double glass, low-e (0.2), 
argon, wood, or vinyl frame   0.33   0.55   0.74   0.15   32   19
Double glass, low-e (0.08), 
argon, wood, or vinyl frame   0.3   0.44   0.78   0.15   32   27
Double glass, selective low-e (0.1), 
argon, wood, or vinyl frame   0.31   0.26   0.44   0.15   27   40
Triple glass, low-e (2 surfaces), krypton, 
insulated vinyl frame   0.15   0.37   0.68   0.08   38   33
Source: John Carmody and Brian Crooks, Selecting Windows Based on Annual Energy Performance, in 1996 ACEEE Summer Study on Energy Efficiency in Buildings (Berkeley, CA: American Council for an Energy-Efficient Economy, 1996).


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