July 18, 2017
A page from Sears & Roebuck’s 1929 catalog showed the amount of money homeowners could save on energy costs with the purchase of a newly manufactured storm sash from the retailer’s Newark, N.J. factory. “With storm sash, $60.00 for coal,” but without, “$90.00,” the ad announces. And, it’s true, drafty windows made of single pane glass were known to frost up with the cold, leading to additional energy expended to keep a home warm. Early 19th century doors and windows, while improved, still had a lot of room for growth before they could truly be called “energy efficient.”
Fast forward 75 years and improvements in material science have led to advancements in the design and manufacturing of windows and doors, which is called fenestration. For example, durable, low-maintenance framing materials, such as vinyl, aluminum and composite clad, reduce heat transfer and help insulate better. Additionally, the inclusion of multiple panes of glass—two panes of glass, with air or gas-filled space in the middle—insulate much better than a single pane of glass.
“New technologies are coming out all the time,” explains David Stammen, principal engineer for UL’s Building Envelope Performance group in Northbrook, IL. “The latest is vacuum glazing where instead of filling the area in between the two panes of glass with argon or krypton gas, the air between the two panes is extracted to create a vacuum.”
To test fenestration products for market compliance; UL provides product performance validation services for windows, curtain walls and all commercial and residential fenestration products. The three core tests, per Stammen, assess a product’s ability to withstand the harmful effects of air, water and wind. This includes testing a product’s air leakage, to see how much outside air leaks into the building, susceptibility to water penetration and its structural performance, to evaluate a product’s wind load.
For thermal efficiency, UL’s lab simulates a product’s insulating ability by calculating its solar heat gain coefficient (SHGC) and its U-factor value.
The SHGC is the proportion of solar energy transmitted or absorbed through a fenestration product. The higher the SHGC, the higher the product’s heat gain. “If your home is in Canada, you might want a high heat gain, but in Florida, the air conditioner (AC) is going to run all the time,” says Stammen. And in today’s energy-conscious environment, that is a problem.
The U-factor, which is the dominant thermal measurement, tests a product’s resistance to heat flow. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating properties. Low U-factors are becoming the norm due to energy-educated homeowners and building owners through the ever-present Low-E (low emissivity) coatings on the glass. Emissivity is a measure of how efficiently a surface emits thermal energy.
All states in the U.S. benefit from low U-factors due to Low-E glass technology because these coatings have a built-in tinting factor which reduces the solar heat gain. Therefore, Low-E glass not only reflects the heat back into the home/building in the winter season for Minnesota but can also reflect heat away from the home/building in the summer months in Arizona.
A product’s thermal properties are first evaluated by analyzing the material used and the design specifications. As fenestration products are first created using computer-assisted design (CAD) software, every design choice, from frame to pane, is on the computer. “Dennis Anderson, UL’s Fenestration Expert, will use the information to simulate the thermal transmittance and solar heat gain of a product,” states Stammen. “Once the simulation is complete, the lab needs to validate the results per the Codes.”
Validation tests are performed in the lab’s thermal chamber, a box that is approximately 6700 cubic feet. The room is divided in half with two different climates on either side. One-half of the room houses an extensive air conditioner system that can drop the temperature down to -20 degrees Fahrenheit. The other side is comprised of heaters and small fans to simulate the warm air of an interior room, typically 70 degrees Fahrenheit.
The product is placed in the center of the chamber, between the two different climates. Once the simulation and validation tests have been completed, the manufacturer knows how the product will perform under the best and worst of climatic conditions.
As with all types of performance testing, testing procedures and lab protocols undergo stringent oversight to ensure the testing is performed per industry standards. For building products, the industry follows American Society for Testing and Materials International (ASTM), American Architectural Manufacturers Association (AAMA), the National Fenestration Rating Council (NFRC) and the International Energy Conservation Code (IECC).
"When the building inspector comes in, and he's looking at your house as it's being built, he knows that the windows and doors have to perform. And, how does he assure himself? He looks for a label that shows performance," Stammen concludes.