Efficiency Metrics for Windows, Walls and Roofs

By Loren Snyder  
OTHER PARTS OF THIS ARTICLEPt. 1: Efficiency and Building EnvelopePt. 2: ASHRAE 90.1 Add Building EnvelopePt. 3: This Page

Having an understanding of what the ratings related to individual building envelope systems mean and how they are calculated will help facility executives better grasp the energy performance of the envelope as a whole.

Windows and Skylights — Windows and skylights are critical to boosting worker productivity and lessening lighting loads. But they can be the Achilles’ heel of a building envelope when not given the consideration they need. When poorly specified, they emit too much radiant heat, allow too much air leakage or limit the amount of light that’s available. For facility executives considering a retrofit or new construction, here is a primer on some of the metrics that gauge efficiency.

U-factor measures heat loss from a window. The rate of loss is given as the U-factor of a fenestration assembly. The lower the number, the better the performance of the assembly. Generally, those numbers range from 0.20 and 1.20. Most experts recommend that facility executives seek out window assemblies with a U-factor lower than 0.35 in colder climates.

Solar Heat Gain Coefficient (SHGC) measures how well a product limits radiant heat gain that is caused by sunlight. This radiation is transmitted directly to the occupied space and lessens the heating load or increases the cooling load. The SHGC is expressed as a number between 0 and 1, and that fraction denotes how much radiation makes it into occupied space. As an example, a window with an SHGC of .35 would admit 35 percent of the radiant heat that hits the window and reflect 65 percent.

Visible Transmittance is a metric that measures how much light — not heat — comes through a product, which directly affects lighting loads in a facility. In other words, this metric determines how clear the glass is. Fenestration assemblies that reduce the visible transmittance — via coatings, films or gases between the panes — can make the window more energy efficient, but limit the light that enters the space. According to the National Fenestration Rating Council (NFRC), the visible transmittance is expressed as a number between 0 and 1. The higher the number, the more light is transmitted.

Condensation Resistance (CR) is measured on the NFRC label, and it indicates the ability of a product to resist the formation of condensation on the interior surface of the product. The higher the CR rating, the better that product is at resisting condensation formation. Formation of condensation can be caused by a number of factors, so this metric cannot necessarily predict condensation formation, but serves to compare the potential for condensation among various products. CR is expressed as a number between 0 and 100, and the higher the number, the better the window assembly is at resisting condensation.

Walls and Roofing Systems — The NFRC’s label indicates window assembly air leakage by a rating that’s expressed as the equivalent cubic feet of air passing through a square foot of window area — but it’s also vitally important to determine heat loss through opaque walls and roofing systems.

“ The ultimate test for this is building commissioning,” says Charles Cottrell, vice president for technical services at the North American Insulation Manufacturers Association.

Any roofing system is comprised, generally speaking, of two parts that affect energy efficiency: the roof material and the insulation in the sub-roof assembly. In taking stock of the efficiency of a facility’s roof, executives need to keep several metrics in mind.

Reflectivity (also known as albedo) defines a roof’s ability to reflect visible, infrared and ultraviolet wavelengths. Reflectivity values are measured on a scale of 0 to 1. A reflectivity value of 0.0 indicates that the surface absorbs all solar radiation, and a 1.0 reflectivity value represents total reflectivity. ENERGY STAR certification is available on products with a reflectivity of 0.65 or higher for low-slope roof applications and 0.25 for steep-slope roofs.

Emissivity refers to a material’s ability to release absorbed heat, according the U.S. Environmental Protection Agency’s ENERGY STAR program. Rating metrics for emissivity use a number between 0 and 1 (or between 0 and 100 percent) to express emittance.

R-value is defined, according to ORNL’s Building Envelope’s program, as thermal flow resistance of insulation. The higher the R-value, the greater the insulating effectiveness. The R-value of insulation depends on the type of material, its thickness, and its density. When calculating the R-value of a multi-layered installation, the R-values of the individual layers are summed to arrive at the final R-value of a wall system.

Remember that wet or compressed insulation will have lower R-value ratings than when originally specified. Thermal bridging can also take place — causing net loss of heat (or cooling) — when insulation is placed only between joists, rafters and studs. Metal framed structures are susceptible to greater heating or cooling loss when thermal bridging occurs than wooden or masonry buildings.

“Adding more insulation can have diminishing returns,” says Desjarlais. “One of the things we’ve been looking at is thermal inertia and thermal mass.”

His point is that, in the quest for improved energy efficiency, it is important not to go too far when specifying ever greater insulation values. Sometimes, old-fashioned concepts — such as masonry structures and ballasted roofing — are new again.

Research ORNL is conducting on thermal massing mainly serves to allow facility executives to mass heat during off-peak hours so that the absorbed heat is radiated during peak hours.

Ultimately, though, whether experimenting with the latest efficiency methods and metrics or employing the tried-and-true, several constants remain: Do the research, employ the help of professionals when needed and don’t cut corners.

“I’ll always caution owners looking to design a good building who consider trading one technology for another,” says Cottrell. “You know, they’ll consider putting on a cool roof, but then go from R-38 insulation to R-20.” Cottrell says you should do them both now, because these are long-term decisions that aren’t that easy to replace if you change your mind.

Building Envelope Efficiency Tools

Loren Snyder, a contributing editor for Building Operating Management, is a writer who specializes in facility issues. He was formerly managing editor of Building Operating Management.

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