Issues in Window's Frame Selection
The material used to manufacture the frame governs the physical characteristics
of the window, such as frame thickness, weight, and durability, but
it also has a major impact on the thermal characteristics of the window.
The window U-factor, as given on the NFRC certified rating or label,
incorporates the thermal properties of the frame, as well as the glazing.
Since the sash and frame represent from 10 to 30 percent of the total
area of the window unit, the frame properties will definitely influence
the total window performance.
Such frames are dimensionally stable and achieve good insulating value
by incorporating air cavities (similar to vinyl). Like vinyl, the cavities
can be filled with insulation for higher thermal performance. The strength
of fiberglass allows manufacturers to maintain the same sight lines
as aluminum windows while achieving significantly lower U-factors. The
low coefficient of thermal expansion maintains seal integrity and minimizes
warpage or leakage in high inside/outside temperature differentials.
Fiberglass pultrusions have a higher heat deflection temperature than
vinyl, permitting the use of dark colors unlike other thermoplastic
extrusions. They can be painted, powder coated, or finished with coextruded