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Windows are one of the most distinctive features of many home designs, affecting both
the exterior appearance and interior ambience of a house. However, windows can also be
"energy robbers," allowing substantial amounts of heat to escape from the house during
the cold winter months. In fact, heat loss through windows can account for over 30% of
total heat loss from a house. This is because glass offers little resistance to the
movement of heat (thermal resistance). A typical window loses up to 10 times more heat
than the wall around it.
On the other hand, properly constructed, sized and oriented windows can actually
provide more heat to the home than they lose. High performance windows can also make
houses feel more comfortable by reducing drafts and temperature flow to cold interior
window panes. They can also add to indoor comfort during periods of higher humidity
(which provides more warmth at the same temperature) without allowing condensation to
form on the glass.
Window selection and placement are key considerations from an energy viewpoint, as
well as for aesthetic reasons. This is particulary true in R-2000 homes, which are
designed to achieve a specified energy performance target. The type of windows selected,
the energy rating (ER) and the placement in the home all play an important role in
determining whether a house can meet this target.
The windows being installed in homes today are more energy efficient than those used
in the 1960s, 1970s and even the 1980s, thanks to significant advances in window design,
technology and materials. While this is good news for consumers, the abundance of window
types, styles, materials, price ranges and manufacturers' claims about energy performance
can be confusing and intimidating.
When selecting windows, consideration must be given to the energy performance, impact
and durability of all the principal components of the unit - the glazing, frame material
and spacers. Other factors to consider are how the window operates, where it will be placed
in the home, and its overall energy rating.
The term "glazing" refers to the panels of glass in a window. On of the earliest techniques
for improving the thermal resistance of a window was to add extra layers of glazing. Today,
most windows are at least double-glazed (two layers of glass), and triple-glazed units are not
uncommon. In both cases the layers of glazing are separated by an air space which actually has
much more thermal resistance than the glazing itself. Standard double-glazing has an insulation
value of about R-2000, so it loses about 10 times as much heat as a typical insulated wall of R-2000.
A window's thermal resistance can be further enhanced by the application of special low emissivity
(low-E) coating to the glazing. This usually invisible coating reflects heat back into the home during
the winter months, and blocks some radiant heat from entering the home during the summer.
The use of a low-E coating on a double-glazed window can effectively boost its performance to the
equivalent of a triple-glazed unit. Some manufacturers even offer the coating on a thin film or two
pieces of plastic placed between two sheets of glass, resulting in a lightweight, essentially triple
or quadruple-glazed unit.
Two types of low-E coatings are available: soft and hard coat. This refers to the manufacturer's
method of applying the coating to the glazing. "Soft" coats are typically more efficient at reflecting
heat, while "hard" coats are generally more durable. Tinted glass and some low-OE coatings may reduce
solar transmission by up to 50%.
Heat loss through the glazing cavity can be further reduced by filling the air spaces between the
panes with an inert, high molecular weight gas like argon or krypton. According to recent research,
a window's gas fill can remain effective over its expected 20 year life.
The type of material used in spacers - the components that separate the sheets of glazing in a window -
also affects heat loss. For example, the optimum spacing for a window with argon fill is 12.7 mm (1/2",)
and 8.5 mm (1/3") with a krypton fill. Spacers should be made of a material that is strong enough to hold
the glazing apart and that provides high thermal resistance. Non-metallic spacers provide greater thermal
resistance than metallic spacers. In today's windows insulating spacers contribute significantly to the
unit's overall energy performance. These spacers also keep the edge of the glass warmer, reduce thermal
stresses and lessen the likelihood of condensation forming - which can rot wood frames and cause vinyl
frames to freeze and crack. Condensation inside the sealed unit indicates that the spacer has failed.
The frame material chosen for a window can be as important as the glazing itself. Frames are available
in a variety of materials - wood, metal (aluminum,) fibreglass and vinyl (PVC) - and combinations of
materials - metal-clad wood, vinyl-clad wood,etc. Each material has benefits and drawbacks in terms of
insulating value, durability, cost, aesthetics and maintenance requirements.
Generally, well-designed fibreglass frames provide the best insulating value, followed by PVC and wood
frames. Some materials require less maintenance than others. When fibreglass or wood is combined with
metal or vinyl in the same frame, the unit can offer the dual advantages of good insulating value and minimal
maintenance. Frames made completely of metal must have a good thermal break in the frame to slow the flow
of heat to the outdoors.
Because a window's potential solar gain is reduced by frame bulk - a frame can occupy up to 30% of a
window's area - narrower frames with higher insulation values are desirable. In Canada, a desirable
residential combination is a window with high solar- heat gain and a narrow frame profile that minimizes
blocking of the sun. Fibreglass frames offer insulation and strength in low-profile, contributing to
some of the highest energy ratings. The best way to compare the energy efficiency of all of the above
window design elements is through the ER number. (See below.)
When selecting windows, consideration must also be given to how they operate and how well they seal
when closed. The different operating types include fixed, casement, awning, and vertical and horizontal
gliders. Each type allows a varying degree of air leakage. Fixed windows are the most airtight, since
they have no moving parts or openings. However, they are not practical for all applications. Of the
windows with operating parts, casement and awning units usually seal better than sliding windows.
The placement of windows in the house, particularly in relation to the sun, is an important factor in
controlling heat loss and gain. In fact, windows are the only component of the building shell (outer walls,
floor and roof) that can both limit energy loss and enhance energy gain. When properly constructed, sized
and oriented, the solar heat gain a window provides to the home in the winter can exceed the heat it loses.
To minimize heat loss and maximize solar gain in the winter months, the majority of windows in a house
should be located in walls that face within 30 degrees east or west of due south, where possible. North-facing
windows should be kept to a minimum. To avoid overheating in the summer months, large areas of unshaded windows
facing west or south should also be avoided.
Manufacturers stress that their window products must be installed "square, plumb and level" in order to
assure long life and continuing high performance. Openings around locating shims must be filled with
insulating foam, larger spaces with insulation, and the home's vapor barrier sealed to the edges of the unit.
A new standard has been developed by the Canadian Standards Association to compare the thermal performance
of windows. It is starting to become recognized and more widely used across Canada.
Much like the EnerGuide program which compares the energy consumption of home appliances, the Energy
Rating (ER) number reflects a window's ability to admit solar heat, prevent heat losses and resist air
leakage losses. The ER value is measured in watts per square metre. It is calculated as the difference
between the heat gains and heat losses of the window over the entire heating season (for an average Canadian
home and location).
Window ER numbers generally range from -50 to +15. The higher the ER number, the better the window
performance. A standard, double-glazed window with an ER of about -30 is a substantial energy loser.
A window with an ER number of 0 is neutral in energy consumption - it contributes as much solar heat to
the house over the heating season as it loses. In contrast, any window with a plus number actually adds
more heating energy to a house than it expends. The table below offers some examples.
| |
ER Number |
ER Number |
| Window Description |
Fixed |
Operable |
| Thermally-broken Aluminum Frame, Double-glazed, Aluminum Spacer |
-35 |
-50 |
| Wood Frame, Double-Glazed, Aluminum Spacer |
-15 |
-30 |
| Wood Frame, Double-glazed, Insulating Spacer, Low-E, Argon Fill |
+5 |
-8 |
| Fibreglass Frame, Triple-glazed, Insulating Spacers, Lo-E, Argon Fill |
+15 |
+8 |
Today, windows should not be compared and selected without checking warranties, quality standard ratings
and performance ratings. Many quality high-performance windows are sold for little more than traditional
windows. Their modest price premium can be paid back many times over the life-span of the window.
The Insulating Glass Manufacturers Association of Canada (IGMAC) tests and certifies glazing units.
Certified products bear a label on the spacer between panes indicating: IGMAC, a date, a company name and
a place of manufacture. If the spacer is the insulating type, the label is etched in a corner of the glazing.
Manufacturers offer a range of warranties on the glazing unit (up to ten years) and the entire window.
R-2000 builders are trained and knowledgeable about window types, performance, placement and installation.
They can provide important guidance and should have direct input into the window selection process to make
certain that the house meets its specified energy performance target. As well, R-2000 builders apply their
special training and attention to detail when installing windows, and sealing frames against air leakage.
By working together, the R-2000 builder and home buyer can ensure that the windows will contribute to the
high levels of quality, comfort and efficiency that are trademarks of every R-2000 home.
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