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.
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.)
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-
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 firstname.lastname@example.org.
Mr. Jackson’s opinions are solely his own and do not necessarily reflect
those of this magazine.
© Copyright 2009 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.