What to know
- Fiberglass expands with glass, which reduces seal stress through freeze–thaw cycles.
- It holds dark exterior colours better than vinyl.
- Expect fewer local dealers and a longer lead time than commodity vinyl.
Where fiberglass earns the extra money
Fiberglass expands and contracts at nearly the same rate as glass. That matters in Prairie and northern climates where a vinyl sash can work against the sealed unit all winter. Slimmer frames also mean more glass in the same opening, which is why it shows up on modern and mid-century replacements.
Dark colours on a south or west wall are another reason. Vinyl can oil-can or warp under heat load; pultruded fiberglass is more stable.
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