Volume 10, Issue 7 - September 2009

eye on energy

Toward .30 or Lower
How to Get There: Consider the Entire System
by Ric Jackson

Did you ever think “window U-values” would become part of the general public’s vernacular? Well, it’s happening. A new Internal Revenue Service tax credit and revised Energy Star® criteria have made U-values a top concern for consumers looking to reduce energy bills and save the environment. As such, achieving total window U-values of 0.30 or lower has become paramount for window producers. The question is, at what cost?

Weighing Product Options
Producers have two choices to respond to demand for more thermally efficient windows: 1. design a window to meet the minimum U-value requirements for Energy Star and the tax credit; or 2. design the best possible performing window. There’s a consumer base for each choice. The trick is to find the best combination of components that will meet your objective at the lowest possible cost and with the least disruption to manufacturing operations. I call this getting the biggest bang for your green buck. Various component choices can impact total window U-values and your bottom line. Here are four considerations:

Spacers. Warm-edge spacers offer the potential to improve total window U-values in a highly cost-effective manner. Table 1 shows U-values by spacer type for a variety of windows with different framing, gas fill, glass emissivity and solar heat gain coefficient (SHGC) options. In this example, producers can realize up to a 0.05 U-value improvement by moving from the least efficient to most efficient spacer.

Further, combining low U-values with efficient manufacturing is a win-win for producers when selecting spacer systems. Producers must consider the applied costs of each system to determine if any manufacturing efficiencies will contribute to an economical cost per U-value improvement. One may realize a 5 percent to 10 percent difference—positive or negative—in manufacturing costs associated with labor, scrap and energy when switching between spacer systems. The added cost of a higher-performing spacer may, therefore, be offset by reduced manufacturing costs, making the spacer choice essentially cost neutral for the added U-value improvement.

"The trick is to find the best combination of components to reach your objective at the lowest
possible cost and with the least disruption to manufacturing operations.
I call this getting the biggest bang for your green buck."

Argon. Another option with a low cost per U-value reduction is argon gas filling. Argon may provide a 0.04 U-value improvement for as little as 1 percent of the total raw material cost of a window. The challenge for manufacturers is to find a filling method with the cycle times and labor requirements that suit their production needs. Further, producers need to ensure that windows are filled reliably, repeatedly and efficiently.

Triples. Triple-paned windows represent an option for reducing total window U-values by as much as 0.08 for a triple with two lites of low-E glass. However, triples are expensive to implement due to the added costs of heavier duty hardware, reinforced framing and additional glass and spacer materials. Transportation costs increase due to the added weight. Further, producers may need to redesign their framing with a larger glazing pocket to accommodate a triple insulating glass unit. Luckily, there are a number of options for fabricating triples, and some systems lend themselves to more efficient manufacturing than others.

Industry leaders are anticipating even lower U-value requirements in the future, which means now is the time to start thinking about producing triples should they become mandatory in the future.

Foam-Filled Framing. A more dramatic change to consider is shifting to foam-filled framing, which offers potential U-value savings up to 0.04. Foam-filled frames carry a significant premium in terms of total window design cost. However, producing a foam-filled frame window may be cheaper than building a triple hollow-framed window due to the retooling costs associated with the larger triple frame.

Decide What Options are Best for Your Company
To obtain the lowest possible window U-value, producers may end up implementing all of the above options. Doing so offers potential U-value savings up to 0.21 (spacer = 0.05; argon = 0.04; triples = 0.08; foam-filled framing = 0.04) per window. Costs vary for each option and by choices within each option. Manufacturers should analyze each choice to determine which combination is best for their business.

At the end of the day, the most cost-competitive windows with the best U-values offer the biggest bang for your green buck.

Table 1. Window U-Values by Spacer Type

Window Properties Corresponding U-Value by Spacer*
Corresponding U-Value by Spacer*
Gas Fill
Glass Emissivity
(Klin = 1.8)
Steel Channel (Klin = 0.67)
EPDM Foam (Klin = 0.16)
Nonmetal Hollow
(Klin = 0.08)
Hollow PVC, Double 3/4-Inch OAT
Clear Float
Hollow PVC, Double 3/4-Inch OAT
Hollow PVC, Double 3/4-Inch OAT
Argon 90%
Hollow PVC, Triple 1-Inch OAT
Argon 90%
Foam Filled PVC, Triple 1-Inch OAT
Krypton 90%

OAT: Overall thickness of IGU 1Xe: One lite with low-E
2Xe: Two lites with low-E Klin: U total x Height of Spacer (U in W/m2-K; Height in m)
Source: Simulations were performed by Enermodal Engineering Ltd. using Windows 5.2 and Therm 5.2 as per NFRC 100.

Ric Jackson is the director of marketing and business development for Truseal Technologies Inc. He can be reached at rjackson@truseal.com. Mr. Jackson’s opinions are solely his own and not necessarily those of this magazine.

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