Ask Yourself:
R U Ready for R5 Windows?
by Ric Jackson

Pressure is mounting for producers to improve the energy efficiency of their windows. As industry standards continue to evolve, producers need to consider investing today to meet tomorrow’s requirements, as well as those expected years from now.

New Standards
As evidence, look at the U.S. Senate Energy Committee’s current push for building efficiency improvements. The committee directs the U.S. Department of Energy (DOE) to set “targets for residential and commercial national model building energy codes [to improve by] 30 percent in 2010 and 50 percent after 2016.”

Another relatively new driver for improved window energy efficiency is the DOE’s Building Technologies Program (BTP), which is pushing for the development of net-zero-energy buildings. One of the program’s near-term goals is to help the marketplace develop affordable R-5 windows (defined as those with U-values around 0.20) through an initiative known as the R-5 Windows Volume Purchase Program.

Energy Star® 2010 windows are only required to have an R-value of 3 (or a 0.30 U-value). Increasing the R-value from 3 to 5 reduces average heat loss by 40 percent. However, the principal barrier to widespread market commercialization of R-5 windows has been cost. The DOE is working to achieve a price premium of no more than $4 per square feet compared to typical Energy Star 2010 windows. The DOE’s mandate to reach R-5 is a good indicator of the thermal performance targets that will be set for Energy Star Phase II requirements, which are expected to be in place in 2016. (For a detailed article on “The Road to R5,” see the September 2009 issue of DWM, page 30.)

What to Do
The convergence of proposed thermal performance requirements means window producers in both the new construction and remodeling markets likely will need to consider triples in the near future. Therefore, the challenge will be to build triples in the most efficient manner possible in combination with the most efficient spacer, glass and framing options for a high-performance window rated as R-5.

Spacers – Spacers offer insulating properties to help prevent the transfer of heat, improve edge of glass temperatures and resist condensation.

Glass – The right type of glass will go a long way in achieving an R-5 window. The variety of low-E coatings on the market provides opportunities to select the right glass package in combination with other components to maximize thermal efficiency while keeping an eye on cost.

Framing Systems – Energy-efficient framing systems, such as those with an integral insulated air-cell core design, also help provide the improved thermal efficiency needed to achieve an R-5 window. Manufacturers should consider recyclable framing materials to help their sustainability messages and the ability to gain LEED points.

Considerations should also be given to inserts like grids and gas filling. Most windows today have glazing cavities of 1 inch or less. Using this same measurement in a triple window with dual- or single-strength glass means each glazing cavity will be very small. That may be an issue when adding grids as producers risk running up against the 3-millimeter grid proximity rule, which requires a minimum air space of 3 mm between the grid and glass. Producers considering grid placement in triples may therefore need to make one of the airspaces wider than the other. The result is an offset triple in which the grid will not detract from the overall R- or U-value.

These considerations will help R-5 windows become a reality, but gas filling may still be needed to push the window over the R-5 threshold. Use of an exotic gas like krypton is an option, but it is very expensive compared to argon. Remember, the goal of the DOE’s BTP is to ensure an affordable, thermally efficient window. An R-5 window can be achieved affordably with argon in combination with efficient spacer, glass and frame choices.

Having R-5 windows as the standard for energy-efficient windows can’t conceivably be reached for several years. However, creating an R-5 rated window—or even an R-6 window—is more than feasible today and would give producers a differentiation in the marketplace now.

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 do not necessarily reflect those of this magazine.

DWM
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