Selecting The Right Energy Efficient Windows

big-windows.jpgA Source of Heat Loss

Windows can be major sources of heat loss in
the winter and heat gain in the summer.
In 1990 alone, the energy used to offset
unwanted heat losses and gains through
windows in residential and commercial
buildings cost the United States $20
billion (one-fourth of all the energy used
for space heating and cooling
). However,
when properly selected and installed,
windows can help minimize a home’s
heating, cooling, and lighting costs. One
option— energy-efficient
windows
— are available for reducing a
home’s heating and cooling energy
requirements.

Double or Triple Pane?

Double- or triple-pane windows have
insulating air- or gas-filled spaces
between each pane. Each layer of glass
and the air spaces resist heat flow. The
width of the air spaces between the panes
is important, because air spaces that are
too wide (more than 5/8 inch or
1.6 centimeters) or too narrow (less than
1/2 inch or 1.3 centimeters) have lower Rvalues
(i.e., they allow too much heat
transfer). Advanced, multi-pane windows
are now manufactured with inert gases
(argon or krypton) in the spaces between
the panes because these gases transfer less
heat than does air.

Multipane windows are considerably
more expensive than single-pane
windows and limit framing options
because of their increased weight.

Frame Materials and Spacers

Frame and Spacer Materials
Window frames are available in a variety
of materials including aluminum, wood,
vinyl, and fiberglass. Frames may be
primarily composed
of one material, or they may be a
combination of different materials
such as wood clad with vinyl or
aluminum-clad wood. Each frame material
has its advantages and disadvantages.
Though ideal for strength and customized
window design, aluminum
frames conduct heat and therefore lose
heat faster and are prone to condensation.
Through anodizing or coating, the
corrosion and electro-galvanic
deterioration of aluminum frames can be
avoided. Additionally, the thermal
resistance of aluminum frames can be
significantly improved by placing
continuous insulating plastic strips
between the interior and exterior of
the frame.
Wood frames have higher R-values, are
not affected by temperature extremes,
and do not generally promote condensation.
Wood frames do require
considerable maintenance in the form of
periodic painting or staining. If not
properly protected, wood frames can
swell, which leads to rot, warping, and
sticking.
Vinyl window frames, which are made
primarily from polyvinyl chloride (PVC),
offer many advantages. Available in a
wide range of styles and shapes, vinyl
frames have moderate to high R-values,
are easily customized, are competitively
priced, and require very low
maintenance. While vinyl frames do not
possess the inherent strength of metal or
wood, larger-sized windows are often
strengthened with aluminum or steel
reinforcing bars.
Fiberglass frames are relatively new and
are not yet widely available. With some
of the highest R-values, fiberglass frames
are excellent for insulating and will not
warp, shrink, swell, rot, or corrode.
Unprotected fiberglass does not hold up
to the weather and therefore is always
painted. Some fiberglass frames are
hollow; while others are filled with
fiberglass insulation.

Spacers

Spacers are used to separate multiple
panes of glass within the windows.
Although metal (usually aluminum)
spacers are commonly installed to
separate glass in multipane windows,
they conduct heat. During cold weather,
the thermal resistance around the edge of
a window is lower than that in the center;
thus, heat can escape, and condensation
can occur along the edges. To alleviate
these problems, one manufacturer has
developed a multipane window using a
1/8-inch-wide (0.32 centimeters-wide)
PVC foam separator placed along the
edges of the frame. Like other multipane
windows, these use metal spacers for
support, but because the foam separator is
secured on top of the spacer between the
panes, heat loss and condensation are
reduced. Several window manufacturers
now sandwich foam separators, nylon
spacers, and insulation materials such as
polystyrene
and rockwool between the glass
inside their windows.

Different combinations of frame style,
frame material, and glazing can yield
very different results when weighing
energy efficiency and cost. For example,
a fixed-pane window is the most air-tight
and the least expensive; a window with a
wood frame is likely to have less
conductive heat loss than one with an
aluminum frame; double-pane, low-e
window units are just as efficient as
triple-pane untreated windows, but cost
and weigh less.
No one window is suitable for every
application. Many types of windows and
window films are available that serve
different purposes. Moreover, you may
discover that you need two types of
windows for your home because of the
directions that your windows face and
your local climate. To make wise
purchases, first examine your heating
and cooling needs and prioritize desired
features such as daylighting, solar
heating, shading, ventilation, and
aesthetic value.

The NFRC Label

The National Fenestration Rating Council (NFRC) energy performance label can help you determine how well a product will perform the functions of helping to cool your building in the summer, warm your building in the winter, keep out wind, and resist condensation. By using the information contained on the label, builders and consumers can reliably compare one product with another, and make informed decisions about the windows, doors, and skylights they buy.

NFRC adopted a new energy performance label in 2005. It lists the manufacturer, describes the product, provides a source for additional information, and includes ratings for one or more energy performance characteristics.

NOTE: All energy performance values on the label represent the rating of windows/doors as whole systems (glazing and frame).

The information contained on the label is also available in the NFRC’s online Certified Products Directory.

 

U-Factor

U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits in the house.

Visible Transmittance

Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

Air Leakage*

Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

Condensation Resistance*

Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.

* This rating is optional and manufacturers can choose not to include it.

ENERGY STAR Qualified Windows, Doors and Skylights Eligibility

To be eligible for the ENERGY STAR, products must be rated, certified, and labeled for both U-Factor and Solar Heat Gain Coefficient (SHGC) in accordance with the procedures of the National Fenestration Rating Council (NFRC) Exit ENERGY STAR at levels which meet the following ENERGY STAR qualification criteria in one or more Climate Zone. Storm windows, window films and interior doors are not eligible for ENERGY STAR at this time.

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