Window Frames
From CWIs “Cool Windows for a Better
California”
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Introduction
Window frames do more
than simply hold the glass in the wall.
Their material,
finish, and style help to define the architectural
character of the home.
The frame allows us to open the window
and bring in fresh air
and close it tight to prevent intrusion. It
must resist rain and
wind and help prevent heat loss.
We will examine the
most popular frame materials currently
available. We'll
describe the characteristics and benefits of
each frame. Particular
attention is given to the energy
efficiency aspects of
window frames.
When comparing
windows, buyers should consider energy efficiency, cost, security, maintenance,
ease of cleaning, aesthetic appeal, color selection, durability and ease of
use. Knowledge of your window frame options is essential to understanding the
function and benefits of high performance windows.
Aluminum Frames
Design of the aluminum extrusions is critical to energy
performance and the structural and operational characteristics of the window
product. Dies are created for extrusion of each of the several different shapes
that may occur in a single window. Extrusion design requires a delicate
balance; larger, more complex extrusions may make a stronger frame but detract
from the energy performance and increase production cost.
Aluminum can be
fabricated to extremely close tolerances. This, coupled with the material's
durability and strength, insures that an aluminum window will provide years of
reliable operation.
Aluminum is a very
good conductor of heat, about 1 000 times more conductive than materials like
wood and vinyl. However, aluminum window frames are often designed using a
lower profile cross-section than their wood and vinyl counterparts. This
smaller surface exposure can help minimize frame heat loss and yield a larger
glass vision area
Thermally Improved
Aluminum Frames
Thermally improved
aluminum windows deliver the strength and durability of solid aluminum, but
with a reduction of heat loss through the frame.
A standard aluminum
frame window will allow heat to travel freely from the warm inner side of the
frame to the cold exterior side. This heat loss path can be short-circuited by
separating the aluminum in a section of the frame and replacing it with a
non-conductive material. A thermal break separation of only 1/4"
can substantially improve window performance.
It
is important that all frame parts which extend metal from inside to outside be
broken and that the breaks are placed correctly to avoid thermal bridges or
short circuits for heat loss. This type of frame requires additional
manufacturing steps and typically costs more than standard aluminum windows.
Some manufacturers make partially thermal broken products to meet mandated
energy code U-factors. Be sure to check the NFRC window U-factor rating since
not all thermal break windows perform better than well designed standard aluminum products.
Also, thermal break aluminum windows
may cost just as much as competing vinyl products with better U-
factor ratings. Thermally improved aluminum windows may be a reasonable choice when an aluminum frame and enhanced energy performance are both high on your list of window priorities.
Vinyl Frames
Vinyl's
low thermal conduction properties make it nearly ideal for use in window
frames. Like aluminum frames, vinyl window frames are extrusions, but because
vinyl is not as rigid as aluminum, several internal hollow chambers are often
added to provide strength. These chambers also trap air, increasing the energy
performance and improving the sound deadening qualities of the frame. In
general, the more internal chambers, the stronger and more energy efficient the
vinyl frame will be. As with aluminum windows, the design of the extrusion is critical
to the energy performance, structural strength, and economic success of the
product.
Metal
reinforcement can be added to the hollow sections of vinyl frames to add
rigidity and strength to
the frame. This reinforcement is typically installed where the fixed and operable panels meet (the interlock) in most sliding windows, and on selected sections of large frames. Metal reinforcement normally has but a minor effect on the overall energy performance of the window.
Vinyl windows can match the exterior
appearance and maintenance-free aspect of clad wood windows. This, plus their
fine thermal performance makes vinyl windows the fastest growing segment of the
U.S. window market.
Wood Frames
Wood
windows have been used for centuries. Despite the coming of metals and
synthetics, wood remains the traditionalist's choice for window frames. Due to
its inherent insulating qualities, wood frames deliver exceptional thermal
performance along with an attractive natural look.
Wood
windows with modern weather stripping and hardware are a vast improvement over
the drafty, poorly operating windows found in older homes. Unfortunately,
quality wood windows are among the higher priced fenestration products.
Wood/Clad Frames
Clad
wood windows should be considered if wood windows are desired but maintenance
is a major concern. Cladding is typically a thin aluminum or vinyl protective
cover that shields the wood exterior from the weather but leaves the natural
wood exposed on the inside.
The
thermal energy performance of clad products is about equal to plain wood windows. However, if metal
cladding extends under the sash and into the interior, heat can be transferred
and slightly lower U-factors should be expected.
Fiber Glass & Composite Frames
Relatively new to the window market, fiberglass frames provide a combination of benefits. Fiberglass frames are structurally strong, expand and contract very little with temperature changes, and are good thermal insulators. Fiberglass window frames achieve strength without bulk, and thus may be built with a lower frame profile -like traditional aluminum frames, but with better thermal performance. Several varieties of fiberglass frames have recently become available as some manufacturers of other frame types have entered the composite market. At least one wood window producer has introduced a frame with wood interior and composite exterior components. Rather than just a surface cladding, the fiberglass
portion makes up the structural, finished exterior section of the window
So far,
fiberglass windows have been priced above vinyl, approaching or matching the
cost of high-end wood windows. As more new window products become available,
the issue of price will determine which segments of the housing market will
accept, adopt, or demand fiberglass frames.
Other Composite Frames
Any of the frame material types
mentioned above may be combined to make hybrid frames. For instance, interior
wood strips are sometimes attached to an aluminum frame to combine the exterior
weathering, strength and weight properties of aluminum with a natural wood
interior finish. Although these products own a very small share of the current
market, new products combining vinyl with fiberglass, wood or other materials
are appearing as manufacturing processes evolve and improve.
Steel Frame Windows
Steel window frames were popular
in buildings in the early to mid 1900's but are only used now in rare
circumstances. Sometimes steel is specified where fire-protection and
extra-strength assemblies are required by code. In other cases architects
seeking a retro warehouse look will select steel frame windows. A few companies
manufacture steel windows in this country for this specialty market. Although
durable, steel frame windows are heavy, they rust and require serious
maintenance.
Publication Information
© Copyright 1998 California Window
Initiative
For more information contact:
California Window Initiative
604 Bancroft Way
Berkeley, CA 94710
(800) 600-9050
(510) 649-9593
Fax (510) 649-9593
Fact Sheet Source
The information in this fact sheet
was excerpted with permission from the California Window Initiative’s “Cool
Windows for a Better California”