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”