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Appendix 6
Selecting the Right
Glazing Material
*
The most important things to consider in choosing a glazing
material are appearance, durability, performance, and cost.
Since the glazing is visible, whether it is clear or cloudy, shiny
or dull, or flat or bowed, it dramatically affects the appearance
of the system. Durability is critical since the glazing provides
the outer barrier to water, cold air, ultraviolet radiation, and
weather. High transmittance of light and low transmittance of
heat affect the efficiency of the system. The glazing should be
inexpensive and easy to handle. The table summarizes
important properties of various glazings.
Glass
Glass is usually a more expensive choice but a very popular
one. Although common glass is less expensive, tempered
glass is stronger and safer. "Water-white" glass (fully
tempered) has a very low iron-oxide content (0.01 percent)
and thus the highest transmittance 0.91). Tempered float glass
is less expensive but has a high iron oxide content and a
transmittance of (0.91. However, use it only vertically and
when safety is a small factor.
Glass is rigid, it looks good, it's durable, and it resists
weathering and chemical and light deterioration. Unfor-
tunately, it is heavy and difficult to handle. It also breaks .
• Based on an article
by
Peter Temple and Joe Kohler entitled "Glazing Choices" in
Solar Age
magazine, April1979, Harrisville, NH 03450.
160
Selecting the Right Glazing Material
161
Glass prices vary significantly depending on how much
you buy and where you buy it. Tempered low-iron glass
("water-white") usually has the same retail price as float glass,
roughly $2 to $2.50 per square foot. But you can buy water-
white glass for as little as $1.00 per square foot if you shop
hard enough.
Fiberglass-reinforced Polyester
Fiberglass-reinforced polyester (FRP) glazing materials
appear cloudy, but their solar transmittance (0.84-0.90) is
only slightly less than low-iron glass. Kalwall's Sun-lite™
and Vistron's Filon™ are two commercially available FRP
glazings.
FRPs are available in 4- and 5-foot-wide rolls in thick-
nesses of 0.025, 0.040 and 0.060 inches. It is a popular
material since it is easy to cut, drill, and install. Some
people do not like its appearance it does not lie flat and
often looks wrinkled. Kalwall has double-glazed panels
onto which the FRP is stretched taut over an aluminum
frame. The panels are less wrinkled but are not entirely
smooth.
FRPs degrade somewhat at high temperatures. Kalwall
notes that their Sun-lite loses 1 %, 3%, and 11 % of its
transmittance when exposed to temperatures of 150°F, 200°F,
and 300°F, respectively, for 300 hours. Most passive
applications do not reach 200°. Tilted convective loop
collectors are the main exception.
Filon is an acrylic-fortified polyester, reinforced with
fiberglass. A thin layer of Tedlar™ polyvinylfluoride
provides protection from ultraviolet degradation
and weathering. Filon is available in flat or corrugated sheets.
The corrugations reduce the wavy appearance problem. Filon,
like Sun-lite, may require venting in higher temperature
applications to protect it from thermal degradation.
Films
Plastic films are very transparent and are relatively
inexpensive. Two of the best materials are Dupont's
Teflon™ and Tedlar™. Teflon stands up well in high
162
Appendix 6
temperatures except that it expands and sags. It is difficult to
handle, bowing between supports and sticking to surfaces
like Saran Wrap™.
Tedlar is also difficult to handle and install. Dupont
recommends that it be used only at low temperatures. Direct
exposure to ultraviolet radiation causes embitterment, and this
effect is tremendously accelerated at higher temperatures.
Used at low temperatures, Tedlar has an expected lifetime of 4
to 5 years until embitterment. If there are any hotspots (e.g.,
near a hot metal support), these places will embrittle earlier. A
new version of Tedlar, 400xRB160SE, has recently been
developed, and it is expected that this product will be less
susceptible to UV degradation.
A new product by 3M Company, Flexiguard™ 7410, may
avoid one of the problems of most films. The manufacturer
claims that it does not sag at high temperatures. It remains
rigid, but not brittle, at temperatures from -30°F to 3000°F.
A common disadvantage of thin plastic films is their
transparency to long-wave radiation (heat). The resulting
higher heat loss reduces efficiency. Glass has a transmittance
of heat of less than 1 %, but the transmittance for films ranges
from 17% for 5 mil polyester to 30% for 4 mil Teflon REP
and 57% for 1 mil Tedlar. Long-wave transmittance data for
Flexiguard 7410 is not
presently available.
Rigid Plastics
Rigid plastic glazings are strong, easy to handle, and
generally attractive. Most of them are either acrylics or
polycarbonates. Acrylics are slightly more transparent than
tempered water-white glass and resist ultraviolet light and
weathering. They are usually clear and are as attractive as
glass if they are not scratched. They tend to soften and bow
at higher temperatures, but this is not a concern for most
passive applications.
Polycarbonates are stronger than acrylics, but they have a
lower transmittance and suffer from ultraviolet degradation.
Like acrylics, polycarbonates have a high coefficient of
thermal expansion and bow inward when the passive system
gets too hot.
Selecting the Right Glazing Material
163
Insulating Panels
Some glazing materials are manufactured as "insulating"
panels, which form a rigid sandwich: an air space between
two glazing layers. Their higher initial cost may be offset by
the lower installation cost compared with two individually
installed layers.
• Courtesy
Solar Age
magazine, Harrisville, NH.
164
Appendix 6
Tuffak-Twinwall™ is a sandwich of polycarbonate
material. Although it is relatively inexpensive, it has the same
serious disadvantages of any polycarbonate: ultraviolet
degradation, low transmittance, and a large coefficient of
expansion. Likewise, the Cyro-Acrylic SDP™ panels have
the disadvantages associated generally with acrylics: a low
melting point and a large coefficient of thermal expansion.
ASG sells double solar glass panels using either their
Solatex™ or Sunadex™ glazing. These panels are designed
specifically for solar applications. The two layers of glass are
hermetically sealed.
The www.BuildItSolar.com website provides hundreds of
free plans for solar and renewable energy projects

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