New energy ratings for windows and doors

If\OMPARING energy perfor\-rmance among windows, doors and skylights has typically been like comparing apples and orangutans; there are simply too many factors measured by different manufacturers in different ways.

But the day is not too distant when fenestration products will have a single rating encompassing thermal transmission, solar heat gain, optical properties, air infiltration and condensation resistance.

Although windows and doors now have certification labels only for Uvalues (thermal transmission), "in a few years, we'll have a single annual energy usage number, one number on all products like the MPG on cars," says National Wood Window and Door Association's Rick Markway.

The energy performance characteristics of fenestration products are unique when compared to other building materials. All may be a source of solar heat gain and light. Overall energy performance can be affected by the variety of materials used to construct the frame, sash and glazing components. As well, durability is important, since the longer energy saving attributes last, the greater the cost effectiveness of the product.

Until a few years ago, door and window manufacturers displayed energy performance in various ways. Some described their glass performance by showing R-values comparable to those of many insulation materials. Yet although the glass may have had a high performance, the rating did not characterize the performance of the entire window system. Similarly, others touted the energy efficiency of their frames or other compo- nents, which also did not reflect the performance of the entire system.

Other manufacturers did provide energy ratings for the whole product, but used different methods to arrive at their ratings. Some used various calculation procedures, while others provided a variety of thermal test reports. The end result was more confusion.

As a result,.the National Fenestration Rating Council, a non-profit public/private group of manufacturers, builders, designers, specifiers, code officials, utilities, regulators and consumers, was formed to establish a national energy performance rating system for fenestration products based on "whole product performance." So while a particular skylight may'have high performance glazing, its overall performance may be reduced by a poorly performing frame. Similarly, a highly energy efficient frame may be wasted on ineffective glazing and sealing.

The first energy factor NFRC addressed was thermal transmission

(U-value). U-value is defined as the number of BTUs that will pass through each square foot of area per degree of temperature difference from one side of a window to the other. The typical range of U-values is from 0.15 to 1.15. The smaller the Uvalue, the less heat transfer between the inside and outside due to a tem-

Story at a Glance

More energy performance factors to be rated on window and door labels next step: overall energy rating number.

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perature difference. U-value is especially crucial during the heating season when the temperature difference is greatest between inside and outside. This year, certification labels will also show solar heat gain and visible light transmission. NFRC has adopted the technical procedure for air leakage and is currently working on the certification procedure to add air leakage to the label. In November, it adopted a "Procedure for Determining the Annual Heating & Cooling Energy Ratings of Fenestration in Residential Dwellings," which will be used to incorporate all elements into a single long-term energy rating.

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NFRC will continue to refine its current procedures as well as add other rating factors such as condensation resistance and longterm energy performance. Members are also working on rating procedures for specialty products such as curtainwalls and fenestration attachments.

Brickmold (brick moulding) Standard trim piece to cover the gap between the window frame and masonry or siding.

Butt joint Joint formed by ends, edges or faces coming together.

Cames Lead strips that divide, hold and surround small pieces of glass in leaded windows.

Cap Decorative molded projection cover- jamb ing the lintel of a window.