Window Frames, What are window frames

Window frames speaks to what are window frames, what are storm windows and types of window frames.

The traditional frame material for windows is wood but aluminum, steel, plastics and combinations of these four materials have also come into widespread use.


Wood is a fairly good thermal insulator and changes size relatively little with changes in temperature and if free of knots is easily worked and is consistently strong. But in service, wood shrinks and swells with changing moisture content and requires repainting every few years.

When wetted by weather, leakage or condensate, wood windows are subject to decay though their resistance to decay can be improved with preservative treatments. Knot free wood is becoming increasingly rare and expensive so composite wood products are increasingly used for frames.

These include lumber made of short lengths of defect free wood finger jointed and glued together, oriented strand lumber and laminated veneer lumber. These materials although functionally satisfactory are not attractive so they are normally covered with wood veneer on the interior and clad with plastic or aluminum on the exterior.

Frames made of solid wood may also be clad on the exterior for improved weather resistance and to reduce maintenance requirements. Clad wood windows presently account for the largest share of the market for wood framed windows.


Aluminum used in window construction is strong, easy to form and join and in comparison to wood much less vulnerable to moisture damage.

The extrusion process by which aluminum sections are formed results in shapes with crisp, attractive profiles and durable factory finishes eliminate the need for periodic repainting after installation. Aluminum conducts heat so rapidly however that unless an aluminum frame is constructed with a thermal break made of plastic or synthetic rubber components to interrupt the flow of heat through the metal, condensate and sometimes even frost will form on interior window frame surfaces during cold winter weather.

The majority of of commercial and institutional windows, as wells as many residential windows are framed with aluminum. Aluminum windows are usually anoded or permanently coated.


Plastic frames, though relatively new, now account for more than half of all windows sold in the US residential market. Plastic windows never need painting and they are fairly good thermal insulators. Many also cost less than wood or clad wood windows.

The disadvantages of plastics as window frame materials are that they are not as stiff or strong as other window materials and they have very high coefficients of thermal expansion. The most common material for plastic window frames is polyvinyl chloride which is formulated with a high proportion on inert filler material to minimize thermal expansion and contraction.

Glass fiber reinforced plastic (GFRP) windows frequently referred to as fiber glass windows are the newest product in the window market. GFRP frame sections are produced by a process of pultrusion. Continuous lengths of glass fiber are pulled through a bath of plastic resin, usually polyester and then through a shaped heated die in which the resin hardens. The resulting sash pieces are strong, stiff and relatively low in thermal expansion. Like PVC, they are fairly good thermal insulators.

GFRP windows are more expensive than those made of wood or plastic.

The thermal performance of both vinyl and GRFP frames can be enhanced with foam insulation injected into the hollow spaces within the frame sections.


The chief advantage of steel as a frame material for windows is its strength, which permits steel sash sections to be much more slender than those of wood or aluminum. Usually steel windows are permanently coated to present a pleasing appearance and prevent corrosion. If they are not they will need periodic repainting.

Steel is less conductive of heat than aluminum, so it is unlikely to form condensation under most weather conditions. Thus, steel window frames are rarely provided with thermal breaks, although, where greater thermal performance is required, thermal break frames are also available from some manufacturers. They are, however, more conducive of heat than wood or plastic frames.

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