Doing a Double Take
Achieving R-5 Values
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
• 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.
 Klin = U total x height of spacer (U in W/m2-K; Height in
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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No reproduction of any type without expressed written permission.