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Fiberglass window

Also called GRP window

A window built from thin-walled pultrusions of glass fiber and resin.

A fiberglass window is made of glass fibers set in resin. It is much stiffer than vinyl, so its walls are thinner and its members narrower, and it insulates about as well.

Fiberglass expands and contracts very little, at close to the rate of the glass it holds, so the seals around the glass are under less strain. It can be painted. It costs more than vinyl, and because the resin cannot be melted the corners are joined with fasteners and adhesive instead of being welded.

Bottom rail of a fiberglass sash in section, outside at left

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Fiberglass sash, bottom rail in section

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