Reference Publication: Parker, D., Fairey, P., McCluney, R., Gueymard, C., Stedman, T., McIlvaine, J., "Rebuilding For Efficiency: Improving the Energy Use of Reconstructed Residences in South Florida", Prepared for U.S. Department of Energy, Florida Energy Office, and Florida Power & Light Company, FSEC-CR-562-92, December 1992. Disclaimer: The views and opinions expressed in this article are solely those of the authors and are not intended to represent the views and opinions of the Florida Solar Energy Center. |
REBUILDING
FOR EFFICIENCY:
Improving the Energy Use of Reconstructed
Residences in South Florida
Appendix
D
Window Selection Guidelines for South Florida Residences
Florida
Solar Energy Center
Research and Development
Division
28 October 1992
MEMORANDUM
To: Philip
Fairey, Danny Parker
From: Ross McClutd Christian Gueymard
Subj.: Window selection guidelines for South Florida
residences
The
purpose of this memo is to offer you additional information resulting from
our studies of glazing selection options for South Florida residences being
re-built following the destruction in South Dade County caused by hurricane
Andrew.
We would like to be able to make a simple recommendation, such as providing
narrow ranges of values for the overall window U-factor, the overall solar
heat gain coefficient SHGC (or F), the visible transmittance T, and the infiltration
coefficient. However, the situation is complicated by several considerations
that make the best choice of a window depend upon a number of factors.
Accordingly, we will list the various factors contributing to the purchase
decision, provide several combinations of these factors, and try and make
a recommendation in each category.
Window
Options
There are seven basic window design types that we are considering for
the Miami area. The pros and cons for each will be listed below, following
an introductory discussion. The types are:
SP |
Single pane tinted or reflective coated glass in an uninsulated aluminum frame. |
SPWF |
Single pane clear glass with a plastic window film applied to the inner surface, in an uninsulated aluminum frame. |
SPSSLOE |
Single pane spectrally selective tinted glass with pyrolitic low-e coating on the surface facing the building interior. |
SPLAM |
Single pane laminated tinted, reflective, or low-e coated glass in an uninsulated aluminum rame. |
SPLAMSS |
Single pane laminated tinted glass with spectrally selective coating between the panes in an uninsulated aluminum frame. |
DPLOE |
Double pane spectrally selective tinted and/or reflective outer lite with a low-e coating on surface 2 or 3, in an un-insulated aluminum frame. |
DPINS |
Double pane spectrally selective tinted and/or reflective outer lite with a low-e coating on surface 2 or 3, in a moderately or well-insulated vinyl or wood frame. |
A list of commercially available glazings and glazing systems falling into these categories is provided in the Attachment along with their visible transmittance T and Shading Coefficient SC values as well as the corresponding modified light-to-solar-gain ratio, LSG’.
In
order to understand how we compared the generic window types listed above,
it will be helpful to discuss some of the technical issues involved first.
As indicated in our 22 September memo to you and the follow-on memo from
Chris dated 5 October, our RESFEN runs have shown that there
is little difference between double and single pane or insulated or uninsulated
frame windows, in 0.0 terms of the annual average monthly utility bill.
On the other
hand, solar heat gain prevention is a very effective energy efficiency
strategy for this climate, from an annual energy use perspective. We have
found a linear relationship between solar heat gain coefficient (or shading
coefficient) and annual energy costs. The lower the values of these parameters,
the lower the annual energy costs. This leads us to recommend low solar
heat gain coefficient values.
Even though the insulated window options do not exhibit good annual
energy savings, they show improved peak heating and cooling load performance.
Even though this is of little monetary value to the homeowner it is important
to the utility companies. The utilities, and the society at large (as represented
by the Florida Public Service Commission), are interested in keeping peak
electrical demand low, to better utilize the power plants used to make electricity.
This results in more efficient utility operation and ultimately lower electricity
prices for consumers. Thus, utility companies and the PSC that regulates
them should be interested in the insulated window options listed above.
The
problem is that without special incentives, the home owner presently receives
no direct benefit for reducing peak electricity use. If you are a home
owner, therefore, you would look for the cheapest window you could find,
with the lowest possible shading coefficient but still with good durability.
This generally means an uninsulated aluminum frame window with a low- transmittance
tinting and/or coating on the single pane of glass contained in it.
The problem
with this simple strategy is that as the shading coefficient is reduced,
the visible transmittance is generally reduced as well. Achieving a very
low shading coefficient could result in a window that admits very little
daylight and greatly reduces the ability to see outside even on a bright
sunny day. It would provide excellent annual energy performance, however
(as long as the windows were not so dark as to force occupants to turn
on electric lights during daylight hours). In order to avoid what we call
the “dark windows” syndrome that results from selecting overly
low shading coefficients with conventional glazing tints or coatings, a
new class of product has been developed.
These are known as spectrally selective glazing systems. These glazings
seek to lower the shading coefficient without greatly reducing the visible
transmittance. They achieve their spectral selectivity through a combination
of tinting or dyes in the glass itself and coatings applied to the outside
surface of the glass that are specially designed to reflect or absorb the
near infrared portion of the solar spectrum while admitting much of the visible
portion of that spectrum. The spectral transmittance of an ideal glazing
of this type is shown in Figure 1.
The ratio of Visible transmittance T to shading coefficient, called
the modified light-to-solar- gain ratio, LSG’, is a measure of the
ability of a low shading coefficient glazing system to admit visible light.
The shading coefficient is plotted versus T for a number of different glazing
systems in Figure 2. The better the LSG’ the farther downward and to
the right on this plot.
It is interesting
that the double pane options are the ones with the best LSG’ values
on this plot. The reason for this is that the best spectral selectivity
is generally achieved with modern sophisticated multi-layer coatings on
glass, coatings that are insufficiently durable to survive long on the
exposed surface of a single pane of glass. As a result, these high-tech
coatings are most often found on an inner surface of the sealed multiple-pane
insulated glazing units (IGUs) that are commonly found in cold northern
climates butwhich are not generally appropriate for hot southern climates.
For example, an IGU for hot climates can be made up of two clear lites
of glass with a special, spectrally selective coating on a plastic sheet
suspended between them. Although this super window is intended for northern
climates and produces a very low U-factor, it also has a LSG’ ranging
from 1.19 to 1.36, depending upon the film used and whether the outer lite
is green tinted or not.
An even better IGU for hot climates is available that is made up of
a green tinted outer lite having a sputtered low-e coating and a clear inner
lite with an insulating air space in between, to produce a visible transmittance
of .56 and a shading coefficient of .33 for a LSG’ of 1.70. The reflectance
of this system is low and therefore not objectionable visually. It works
by spectrally selective absorption in the outer lite and protection from
the heat from this lite is provided by the low-c coating to repress radiative
transfer across the air space and the insulating air space to reduce conductive
transfer of the heat absorbed by the outer lite.
A consequence of this is that if a Dade County resident wants the maximum
combination of low shading coefficient with high visible transmittance, using
window systems presently on the market, an IGU is probably the best way to
go, even though the extra insulating (low U-factor) value of such a window
is wasted, in terms of the monthly energy bill. Perhaps the PSC and Florida
Power and Light can be convinced to offer some financial incentive to pay
for the extra cost of a two-pane window in return for the improved peak load
performance that will result.
There is another alternative that we have been exploring. That is to
see if a reasonably high value of LSG’ can be achieved with a relatively
conventional single pane window. We found a special window film available
from one manufacturer which offers a LSG’ value of 1.15 when applied
to single-pane clear glass. The visible transmittance of this product on
clear glass is 0.50 and its shading coefficient is .42. Although a lower
shading coefficient is desired, this is a relatively cost-effective option,
if it can be proven to be durable enough to suit all the parties involved.
A good thing about this film is that it offers an excellent retrofit option.
Looking for greater durability than is available in an applied plastic
window film, we turned to the laminated glass manufacturers. We asked if
they could provide a single-pane glazing which would take advantage of the
lamination to protect an inner layer from exposure to the air and the wear
and tear of repeated window washing and other abrasive uses. We found that
such a system is not generally available from most laminated glass suppliers.
However, one product of this type has been developed for the California market
that has a spectrally selective green tinted outer lite and a spectrally
selective reflective inner layer that has a shading coefficient of 0.40 and
a visible transmittance of 0.67 for an LSG’ of 1.68, a value that rivals
the best of the spectrally selective IGU options.
Another laminated
option is to combine a spectrally selective tinted glazing such as evergreen
with a pyrolitic coated low-e clear lite. (Pyrolitic coatings have an emittance
that is on the order of .2 or so, somewhat higher than the values of 0.05
to 0.08 achievable with sputtered coatings on protected surfaces.) Such
a combination has a visible transmittance of .54 with a shading
coefficient of .39, for a LSG’ of 1.38, a very respectable number.
The low-e coating is in this case not intended to keep heat inside the
building during cold winter months but to prevent radiative heat gain at
long wavelengths from the hot single pane heated by absorption of solar
radiation.
The problem with the laminated glass approach, however, from the standpoint
of our goal of providing a high-performance single-pane product, is that
laminates cost about as much as the two-pane insulated glass options. Thus,
from a cost-effectiveness standpoint, it would probably make better sense
to just go with the widely available two-pane options and the high LSG’ values
they offer.
When talking with the laminated glass suppliers, however, we found
that glass-to-glass as well as glass-to-plastic laminates are available that
offer excellent strength, security, and shatter resistant properties over
conventional non-laminated glass products. Thus, homeowners concerned about
security and protection from future storm damage, may be happy to pay the
extra cost of the laminates, if they can achieve the high LSG’ performance
they desire.
The complexities and costs of these options led us back to see what
is available in tinted and coated single pane glazings. The ideal glazing
whose transmittance is shown in Fig. 2 works best from an energy standpoint
if it performs its near JR rejection by reflection rather than absorption.
In this case, the unwanted infrared radiation is reflected back outside and
not absorbed by the glass to heat up the interior. Coatings that achieve
this characteristic are not generally available in a durable form for coating
on an exposed surface of a single pane of glass. Even if they were, they
would probably offer high reflectivity in the visible portion of the spectrum
as well, a characteristic that many homeowners find very objectionable.
There are a number of tintings, such as iron oxide, that, when placed
in clear glass provide a rather spectrally selective transmittance, blocking
some of the visible and a lot of the near JR by absorption--not by reflection.
These glazings are much more attractive looking, but they present the problem
of getting hot and emitting much of this heat to the interior when strong
direct sun is incident upon them. In double pane glazing systems, this problem
can be partially overcome by putting a low-emittance (low-e) coating on surface
number 2 or number 3 (numbering from outside inward). Such coatings are very
reflective and hence low-emitting in the long-wavelength infrared portion
of the spectrum that is emitted inwardly by the hot outer glazing. The combination
of the reflective coating on surface 2 or 3 and the insulating air space
help to prevent much of the heat absorbed by the outer glazing from entering
the building as heat gain. Such glazing systems can produce high LSG’ ratios.
The question is one of how to achieve this same effect in a single-pane case.
Without the second pane and the insulating air space, it is not possible
to prevent some of the absorbed heat from conducting and convecting to the
interior. However, if a durable low-e coating can be placed on the inner
surface of a spectrally selective absorptance glazing, then at least the
radiative component of the inward-flowing heat can be reduced. A large U.
S. manufacturer does offer this option in a single pane glazing unit. The
glass itself is given a light green tint and the inner surface receives a
pyrolitic low-e coating that can withstand the abrasive action of normal
cleaning operations. It has a visible transmittance of .66 and a shading
coefficient of .58 for a LSG’ of 1.14. This is pretty good for a simple
single pane glazing, but a still lower shading coefficient is desired. The
laminated evergreen and low-e clear option, with T = .54 and SC = .39 with
LSG’ = 1.38 appears to be an excellent choice for this market and it
offers the extra advantages of strength and protection from breakage and
shattering.
With the above background information presented, we can now list and
describe various factors affecting the purchase decision.
Factors affecting a purchase decision
1. Degree
of shading. The presence or absence and degree of shading of
the windows in the residence is an important factor affecting the choice
of glazing properties. This will affect both the desired shading coeffcient
value as well as the visible transmittance. A bright unshaded exterior
environment, especially one facing a large body of water to the west,
will need not only a low shading coefficient but also a moderately low
visible transmittance for glare mitigation. A dark, well-shaded exterior,
such as a site surrounded with dense tall trees, will need a high visible
transmittance and can accommodate a moderately high shading coefficient.
The same is true for a window beneath an awning or other effective exterior
shading device. The presence of a highly reflective interior shading
device can also affect the decision. In this latter case, the energy
performance of the system is strongly dependent upon how the device is
operated and how reliably. Guideline: High shading calls
for high values of the visible transmittance, say over 65%, and
a moderate value of LSG’, say from .9 to 1.2.
2. Importance of mechanical protection. If it is important
to the homeowner to have safe windows that are unlikely to cause serious
injury when broken, then one class of laminated or safety glass is recommended.
If it is important to increase the strength of the glazing, so that it
will maintain integrity during storms, even when the glass is broken, then
another type of laminated glass or glass with an applied window film is
called for. Guideline: For mechanical protection with
good energy performance, laminated glass (or glass laminated to plastic)
with good mechanical strength, having a green tinted outer lite and a pyrolitic
coating on the inner surface is recommended.
3. Importance of resistance to outdoor noise and sound. If
it is important to the homeowner to reduce the transmission of outdoor
sound to the interior, then a double pane (IGU), perhaps one with laminated
glass as one of the lites, is recommended. Guideline: For
noise mitigation with good energy performance, a double pane IGU is recommended,
with a green spectrally selective outer lite and a low-c coating on either
surface 2 or surface 3.
4. When cost is very important. When having a low initial
cost is a very important factor, then one of the less expensive options
should be employed, at some loss of energy performance.
Guideline: For low cost, single pane, green tinted, pyrolitic
low-e coated glass in an inexpensive aluminum frame should be chosen.
5. Retrofit or new construction. If the window is intact,
then an applied window film seems to make good sense. If it is broken
and a single pane window, then the replacement glass should be upgraded
to one of the spectrally selective ones, with a low shading coefficient
and high visible transmittance. If the whole window has to be replaced,
then there are several options for the glazing, as indicated in items
1 through 4 above. Guideline: For existing windows, a
spectrally selective retrofit window film is recommended, one having a
LSG’ above 1.0 and a shading coefficient below .45. For replacement
glazings in existing single pane windows, a green tinted glazing with
a pyrolitic low-e coating on the inner surface should be used, with an
ISO’ above 1.0. For replacement windows, the highest LSG’ glazing
system should be chosen, within the constraints of the other choices mentioned
in items 1 through 3 above.
Now that we have discussed the various glazing design options and factors
affecting the purchase decision at some length, we can offer the positive
and negative characteristics of each of the above window categories for the
South Florida climate:
| Window | Pros | Cons | |
| SP | Clear glass is good for dark locations, darker, tinted glass is ok, and inexpensive for bright locations | Little solar gain protection from the clear options block more light than heat. | |
| SPWF | Moderately good heat gain rejection with acceptable light transmission and protection from glass breakage. Low cost. Retrofit applicability. | Short lifetime, susceptilbility to abrasion and scratches. | |
| SPSSLOE | Excellent heat gain rejection with good visible light transmittance in a relatively low cost single pane configuration. | Not widely available, possibly high cost. | |
| SPLAM | Excellent heat gain rejection with good visible light transmittance in a single pane option. Excellent protection from storm damage, from injury from flying glass, and from intruders. | Moderately high cost. | |
| SPLAMSS | Outstanding heat gain rejection, for a single pane window with high light transmittance. Protection from storm damage and injury from flying glass. | High cost. | |
| DPLOE | Excellent heat gain rejection with good light transmittance. Good human comfort performance by virtue of improved sound insulation, moderate inner glazing temperatures, and good infiltration prevention. | More expensive than single pane options and thicker and heavier. | |
| DPINS | Super window with excellent insulating ability, good light transmittance, high human comfort and aesthetic appreciation, and very good solar gain rejection. | High cost not justified by energy performance. | |
It is difficult to say that any one of these window options is the best for all situations and conditions. So we are not sure how best to make a recommendation about glazing selection to FPL or the PSC or the FEO. Please let us know what we can do to make our recommendations more useful and helpful.
Some Commercially Available Glazings
| Single Pane | ||||
| Company | Glazing | Tv |
SC |
LSG' |
| Typical | clear
low-e green low-e |
.84 .71 |
.74 .56 |
1.14 1.27 |
| PPG | Solex
tinted med. green uncoated Azurlite Aqua |
.75 .72 |
.69 .62 |
1.09 1.16 |
| LOF | Eclipse
3/16" evergreen Eclipse 1/4" refl. bluegreen Evergreen 1/4" |
.70 .33 .66 |
.63 .44 .58 |
1.11 0.75 1.14 |
| Single Pane with Applied Film | ||||
| Company | Glazing | Tv |
SC |
LSG' |
| 3M | IN50BR
on clear IN50BR on tinted LE50AMARL, shatter resist |
.50 .26 .50 |
.42 .35 .43 |
1.19 0.74 1.16 |
|
Single
Pane Laminated
|
||||
| Company | Glazing | Tv |
SC |
LSG' |
| LOF | Laminated Evergr. 1/4" + LoE | .54 |
.39 |
1.38 |
| Monsanto | Laminated
on clear 1/8 glass SF82/64/53 blue SF83/64/51 black SF37/57/50 green |
.64 .64 .57 |
.53 .51 .50 |
1.21 1.25 1.14 |
| Southwall 1/4" | laminated
with heat mirror inner layer laminated clear laminated water white laminated sea foam clear laminated sea foam clr low e laminated sierra green |
.75 .73 .73 .65 .67 |
.60 .50 .51 .43 .40 |
1.25 1.46 1.43 1.51 1.68 |
| Double Pane | ||||
| Company | Glazing | Tv |
SC |
LSG' |
| Typical | IGU
green tint IGU green low-e IGU green with poly film |
.68 .64 .45 |
.60 .47 .29 |
1.13 1.36 1.55 |
| Guardian | IGU NU-52 1/8" green | .473 |
.415 |
1.14 |
| LOF | IGU Energy Adv. Evergreen | .54 |
.39 |
1.38 |
| Cardinal | IGU
Loe²-171 on #2 3 mm IGU Loe²-171 on #2 4 mm IGU Loe²-171 on #2 5 mm |
.71 .71 .70 |
.53 .52 .52 |
1.34 1.37 1.35 |
| Viracon | IGU
VE7-85 azurlite IGU VE7-55 azurlite IGU VE7-40 azurlite IGU VE8-85 evergreen IGU VE8-55 evergreen IGU VE8-40 evergreen |
.61 .39 .29 .56 .36 .27 |
.38 .27 .22 .33 .24 .20 |
1.61 1.44 1.32 1.70 1.50 1.35 |
| Double Pane with Poly Film Between | ||||
| Company | Glazing | Tv |
SC |
LSG' |
| Southwall | HM 77 Clear on 1/8" | .64 |
.54 |
1.19 |
| Southwall | HM 66 Clear on 1/8" | .56 |
.45 |
1.24 |
| Southwall | HM 77 Green on 1/4" | .51 |
.38 |
1.34 |
| Southwall | HM 66 Green on 1/4" | .45 |
.33 |
1.36 |
| Southwall | HM 55 Green on 1/4" | .38 |
.30 |
1.27 |
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