Volume 13, Issue 8 - October 2012

AAMA Analysis

Designing for Cooler Climates
Achieving Peak Winter Performance is No Sweat
by Ken Brenden

As the sweltering summer months give way to the cooler months of fall, those working on new construction or remodeling projects would do well to plan for when winter rears its ugly head.

Notable Improvements
Double glazing, long the gold standard for energy efficiency, is tweaked constantly to improve performance. Wider gaps between the lites are being recommended, with 5/8-inch considered by many to be the optimum separation. Today’s most advanced double-pane, insulating glass (IG) units typically reduce U-value by 34 percent, while an advanced triple-pane “super window” can be 60 to 70 percent lower (39 to 54 percent lower than the advanced double-glazed product).

Today’s advanced “triple-silver” spectrally-selective low-E coatings combine the best qualities of low-E: tinted and reflective glass. Multiple layers of silver in the coating, in addition to reflecting the long-wave heat radiation, also selectively transmit visible light waves and reflect infrared heat waves. This effectively filters out 40-70 percent of heat normally transmitted through insulating glazing, while allowing nearly the full amount of light to be transmitted.

Inert gas infills usually include argon, or the more expensive yet more highly insulating krypton or xenon. Argon may enable beneficial U-value improvement in a double-glazed product for as little as one percent of the total raw material cost.

Warm-edge spacers that reduce heat flow around the edges of IG units, composed of insulating silicone foam, PVC foam, extruded vinyl, pultruded fiberglass or a special polymer formulation, increase the inside edge temperature by 10 degrees or more under the same conditions. Moving from the least efficient metal spacer to the most efficient spacer in a double-glazed product can yield a significant U-value improvement.

High-performance framing systems offer potential U-value savings. Options include metal with low-conductivity thermal barrier materials such as pour-and-debridged technology or polyamide strip, or inherently insulating polymeric material such as PVC, ABS, fiberglass, wood or wood-cellulosic composites.

Codes Add Demands
In addition to decreasing energy costs, advanced features take a leading role in meeting the increasingly more stringent requirements for fenestration, embedded in everything from the 2012 IECC (which calls for U-factors as low as 0.32) to Energy Star Version 6.0 upgrades (which seek to impose U-factors pegged at 0.25) and the DOE’s R-5 (U = ~0.20) window program. These programs continue to evolve.

Energy Star Version 6.0 is of particular importance. It is still in development stages and all stakeholders—including manufacturers—should weigh in on the impact and feasibility of the proposed requirements. The development timeline has been extended, now aiming for final publication in March 2013 after more rounds of drafts and comment periods, with an effective date of January 2014. To be added to the stakeholder list, contact windows@energystar.gov.

To meet all these challenges—particularly in extreme climates—watch for products with the next wave of technological innovations. Chief among these are enhanced low-E glass using next-generation silver and other metallic compounds, dynamic glazing that can be darkened or lightened (i.e., change their reflective and emissive properties) literally with the flip of a switch and vacuum insulated glazing (VIG), which some analysts say has the potential for true commercialization in North America by 2014 with thermal performance exceeding R8.

As the cost of energy escalates, the price premium imposed by these advances becomes easier to justify in terms of payback period, a calculation that should be frequently revisited as economic trade-offs change.

Adapting the appropriate mix of the increasingly available performance enhancement features will allow builders and remodelers to stop sweating over their customers’ or code-mandated efficiency requirements and, well, chill out this winter.

Ken Brenden serves as technical services manager for the American Architectural Manufacturers Association in Schaumburg, Ill.







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