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This article continues our discussion of window energy efficiency with an explanation of window choices matched to climate, use of window shading to control solar gain, the solar heat gain coefficient of windows, visible transmittance ratings for windows, window air leaks, and fading due to UV light from windows or skylights.
In this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants. We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
Due to its significantly lower
U-value, low-E glass outperforms standard double glazing
in all climates. However, which type of low-E glazing is
optimal for a building depends on several factors, including
the heating load, cooling load, and orientation of the
glass. In general, the windows with the lowest U-values
will yield the greatest savings in cold climates, while windows
with the lowest solar heat gain will yield the greatest
savings in hot climates.
Some window manufacturers market different glazing
types in different parts of the country and may be able to
provide different glazing types by special order. General
recommendations from the EPA’s Energy Star program are
shown in Table 3-3 below.
Window Glazing Recommendations for Cold climates. In climates dominated by heating
loads, select a window with a low U-value (high
R-value). Windows with high solar gain will slightly
favor winter performance and windows with low solar
gain will slightly favor summer performance, but
annual energy costs are determined primarily by the
U-value. Two exceptions to this are passive solar
homes, which should use high-solar-gain glass, and
homes with a lot of west-facing glass, which should
use low-solar-gain spectrally-selective (Spectrally Selective Window Glazing) glass.
Window Glazing Recommendations for Mixed climates. Spectrally selective glass or low-E glass
(SUNGAIN, FILMS, LOW-E GLASS) with moderate solar gains are good choices in mixed
climates. The greater the cooling load, the more important
a low solar-heat-gain factor will be. However, the
differences in annual fuel bills between using high,
low, or moderate solar-heat gain glass in these climates
will usually be small. Other issues like overall U-value,
UV light transmission, cost, and durability might be the
more important factors in choosing a window.
Window Glazing Recommendations for Hot climates. In climates dominated by cooling
loads, choose a glazing type with low solar-heat gain.
Spectrally selective coatings provide an ideal combination
of high R-values, high visible-light transmission,
and low solar-heat gain. UV radiation, which causes
fading, is also cut significantly.
glass is a big improvement over tinted glass, which
blocks solar gains, but also obscures views and creates
glare and reflections when viewed from outside.
Our photo of a warm-climate window (above, left) is in Rabat, Morocco, a climate that ranges from warm to hot (-DF).
Solar-Heat-Gain Coefficient Rating for Windows: SHGC
A window with an SHGC of .70 captures about 70% of the
available solar energy falling on the window. Clear double
glazing has an SHGC of about .75 versus .60 to .70 for
standard low-E and about .40 for spectrally selective low-
E. Which type of glazing is optimal for a given project depends
on the climate, summer and winter fuels costs, and
how glass is used in the house design.
[Click to enlarge any image]
Low SHGC. Low-solar-gain glass blocks unwanted
solar gain and provides significant savings in both
peak and annual cooling loads in hot climates. For
example, switching from clear double glass to low-
SHGC glazing can reduce air-conditioning bills by
15 to 20% in a typical home in Phoenix or Miami (see Figure 3-10 above). Spectrally selective glass , introduced in
the 1990s, combines very low solar gains with high
visible light transmittance and high R-values (up to
R-4 with gas fill). While this yields the greatest savings
in hot climates, it is also a good choice in any
climate with significant cooling loads or large
amounts of un shaded west-facing glass. (see Spectrally Selective Window Glazing).
Moderate SHGC. In northern cities like Boston or
Chicago or mixed climates with more-or-less equal
heating and cooling costs, moderate-gain glass is a reasonable
choice, balancing moderate solar gains in winter
with moderate blocking of solar gains in summer.
High SHGC. High solar-heat-gain glass is a good
choice in cold climate homes with enough south glass
to take advantage of passive solar gain, called “sun
tempering.” Savings on winter heating bills will be
partially offset by the increased cooling load in
summer, however. To avoid overheating, south-facing
glass should range from 4 to 7% of the total floor area
(avoid sloped glass, which tends to overheat in summer
and fall).With south glass in excess of 7% of floor
area, thermal mass may be needed to prevent daytime
overheating and to store heat for nighttime use. A designer
with expertise in passive solar can help determine
the right amounts of glass and thermal mass.
Visible Transmittance Ratings for Windows
People install windows primarily for daylighting and
views, so the higher the percentage of visible light
transmitted (VT), the better. Clear double-glazing has a
VT of about 80% (see Table 3-4, below). With hard-coat
low-E, that figure drops to 75%, and down to about 70%
with the new spectrally selective coatings (Spectrally Selective Low-E Windows).
All low-E coatings
reduce visible light transmittance to some extent and
some may appear slightly tinted or more reflective under
certain light conditions. The new spectrally selective glazings
are fairly color-neutral, but they may appear slightly
darker compared to clear glass.
[Click to enlarge any image]
In general, most people do not notice tinting until the
VT of the glazing falls below about 60%. The visible light
transmittance ratings listed on NFRC window labels can
be confusing since they include the sash and frame, not
just the glass.
The VT for the glass only should be available
from the window or glazing manufacturer upon request.
Beyond the numbers, it is always a good idea to examine
a sample of the glass before purchasing. View the
glass from both outdoors and indoors under different light
conditions to check for tint and glare.
In older homes, leaky windows contributed significantly
to heating loads (less to cooling loads), and the drafts made
occupants feel cold despite the thermostat setting. While
windows built today are, in general, much tighter, the
effect of air leakage can still be significant on cold, windy
days, particularly on windows with direct wind exposure.
Most windows today are built with a leakage rate of
.30 cfm/sq ft of glass area or less, the minimum allowed
under the AAMA/NWWDA standard. The best windows
have leakage rates near .10 cfm/sq ft.
Windows with compression seals, such as casements
and awnings, tend to be tighter than windows with sliding
seals, such as double-hungs and sliders. Also slide-by
weather-stripping is more prone to wear out over time and
more likely to be breached by high winds that cause the
window to flex.
With any weather-stripping system, look
for long-lasting materials such as EPDM and silicone and
heavy-duty construction that can withstand years of use
and exposure to water, freezing and burning temperatures,
and UV radiation.
Fading of Interior Carpets & Furnishings due to UV Light
Most interior materials, including fabrics, carpeting, paint,
and artworks, fade from exposure to sunlight. Although
the most potent effect is from ultraviolet (UV) radiation,
research has shown that the shorter wavelengths of the visible
light spectrum also cause fading.
To account for the
relative effects of both UV and visible light on typical materials,
researchers have developed an approach called
“damage-weighted transmittance” (T-dw), which was recently
standardized by the International Standards Association
Lower ratings are available with triple glazing or tinted
glass, primarily used in commercial construction. Low
numbers for UV transmittance and T-dw indicate less fading
potential, but some fading will still occur. The best
approach with valuable rugs, artworks, and other light sensitive
furnishings is to place them in areas with minimal
exposure to windows or to use shades or draperies that
substantially cut light transmission. Also see SOLAR SHADES & SUNSCREENS.
Condensation Resistance Ratings for Windows: Window Condensation or "Sweating"
To rate a window’s resistance to condensation, NFRC recently
developed a method that evaluates the window’s
frame, glass, and glass edge at a standardized set of temperature
and humidity conditions.
Based on the coldest
part of the window assembly, it is assigned a rating from
1 to 100.
The higher the rating, the better the window is at
resisting condensation, but the rating doesn’t predict condensation
under specific conditions. The voluntary minimum
for a “thermally improved window” under the
AAMA/NWWDA standard is 35.
The best protection against condensation is low-E
glass with gas fill, combined with warm-edge spacers and
a nonmetallic window frame, such as wood, vinyl, fiberglass,
or one of the newer composites. Table 3-6 (below) provides
a general guide to when condensation is likely to form on
different types of glazing. Without warm-edge spacers,
condensation will occur at window edges first.
Which way a window faces has a big impact on its
contribution to comfort, heating and cooling loads, and
West-facing glass is the most problematic, because in
summer the afternoon sun shines directly on the glass,
causing glare and overheating the house already
warmed by increasing afternoon temperatures. Provide
shade with plantings or light-colored shades (exterior
shades are most effective). Overhangs do not help
much due to the low angle of the sun. Also see SOLAR SHADES & SUNSCREENS.
South-facing glass (within 30 degrees of true south)
gets direct sun exposure in the winter when it is
desirable, at least in climates with significant winter
heating loads. In summer, the sun is high on the south
side, moderating the solar gain. Also, it is easier to
block the high summer sun with appropriately sized
overhangs or awnings. South-facing glass should not
exceed 7% of the building floor area, unless thermal
mass is used.
East-facing glass provides desirable morning light
and modest solar gains on cold winter mornings. Too
much un shaded east glass, however, can cause overheating
North-facing glass provides diffused light that is free
of glare and solar gains, which is ideal for daylighting
and is sought after by artists for its consistent color
In most cases, one type of glazing can work on all
sides of the house. In houses with large amounts of west
glass, however, it makes sense to use tinted or spectrally
selective glass at least on the west face to reject the summer
sun. This will dramatically improve comfort and reduce
both peak and annual cooling loads.
If the house is
also designed to take advantage of passive solar heating,
high-heat-gain windows are preferable on the south face.
Mixing glazing types can get tricky, however, and should
be handled by an experienced solar designer. One caution,
also, is that the slightly different tints of the two glazing
types might be objectionable to some clients.
Industry Associations for Windows & Doors
American Architectural Manufacturers Association
Efficient Windows Collaborative
National Fenestration Rating Council (NFRC)
Sustainable by Design
Shareware calculators for sun angles, solar heat gain,
Window and Door Manufacturers Association
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