Volume 11, Issue 4 - May 2010

Eye On Energy

Doing a Double Take
Achieving R-5 Values with Doubles
by Ric Jackson

R-5 ratings are becoming the new gold standard for high-performance windows. New construction and repair and remodeling market standards are converging near R-5 – or U-values of 0.22 for operable windows and 0.20 for non-operable windows. Manufacturers are relying on triple-pane windows to achieve these values. However, R-5 double-pane windows are possible through a combination of low-E coatings and thermally-efficient components.

For both triples and doubles, manufacturers need to consider an optimal component combination to reach the desired performance level, including:
• Spacers: High-performance R-5 windows essentially require a warm-edge spacer system, with non-metal, third-generation spacers offering the lowest thermal conductivity.
• Gas Filling: Argon, krypton and xenon help reduce U-values.
• Frames: Framing systems with foam filling or insulated air-cell cores improve thermal efficiency.
• Glass: Low-E coatings can provide the extra U-value reductions needed to reach R-5.

Manufacturers have been able to meet the 0.22 or better U-value target for triples for many years. They can produce triples with U-values as low as 0.15 (R-7) using low-conductivity spacers (klin1 values between 0.15 and 0.07); gas filling; foam-filled frames; and two lites of a very low-E glass (range of 0.02 emissivity). Each component adds cost, but choosing the optimal combination can reduce the total manufactured cost.

For example, a triple with the most thermally-efficient spacer and frame options can meet the 0.22 U-value target without using krypton, as long as a minimum air space of 3 millimeters exists between internal muntin bars and the glass. This requirement may necessitate an offset triple design in which one glazing cavity is wider than the other. Neglecting the 3-mm grid proximity rule, or utilizing a less efficient spacer or frame, may force manufacturers to use krypton to realize R-5 values, which significantly increases costs.

R-5 values are not possible in traditional doubles. With a single lite of low-E glass, the most thermally-efficient frames and spacers, and krypton or xenon gas, manufacturers can only expect values up to R-4 (0.23 U-value). Even then, the rare gases add up to $50 to the manufacturing cost of a typical double-hung window.

Here is where low-E coatings enter the equation. Manufacturers may apply coatings to any of a double’s four glass surfaces, which include (in order from the exterior to interior of a home):
• Surface 1: outside surface of exterior lite;
• Surface 2: inside air space surface of exterior lite;
• Surface 3: inside air space surface of interior lite; or • Surface 4: outside surface of interior lite;

In a double featuring the lowest emissivity coating available (0.018) on Surface 2, krypton gas filling and the best spacer and frame systems, the best possible center-of-glass (COG) U-value is 0.23. Using the same low-E coating on Surfaces 2 and 3 lowers the window’s solar heat gain coefficient, but has no additional impact on the COG U-value. In either case, the window’s lowest total U-value is 0.23 (R-4).

Remarkably, manufacturers can reduce the COG U-value to 0.20 using argon gas, the same low-E coating on Surface 2 and a pyrolytic low-E coating on Surface 4. Add the best spacer and frame systems, and the result is an R-5 (0.22 U-value) window with an argon-filled double insulating glass unit. When applying this same concept to argon-filled triples—with low-E coatings on Surfaces 2 and 4 and a pyrolytic coating on Surface 6— COG U-values drop to 0.10, which yields total window R-values as high as R-7 (0.15 U-value). Krypton provides further COG U-value reductions, along with added cost.

COG U-values depend on the gas filling and the dimension of the air space (or “gap”) as shown in Figure 1. For example, the optimum gap to realize an R-5 argon-filled double is between 0.42 inches and 0.58 inches.

[1] Klin = U total x height of spacer (U in W/m2-K; Height in m)


Ric Jackson is the director of marketing and business development for Truseal Technologies Inc. Mr. Jackson’s opinions are solely his own and do not necessarily reflect those of this magazine.


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