Fiberglass is the top performer in Utah’s conditions. Its thermal expansion coefficient closely matches glass — nearly eliminating the stress at the IGU seal and weatherstripping that vinyl frames accumulate across Utah’s 100°F+ annual temperature range. Fiberglass resists UV degradation without chalking or color fading, achieves the highest condensation resistance factor values, and maintains dimensional stability across decades of thermal cycling. Multi-chamber vinyl (3+ chambers, reinforced corner welds, TiO₂ UV stabilization) performs acceptably for most applications and remains the most common choice by volume. Single-chamber vinyl develops corner joint separation in Utah’s thermal cycling within 10–15 years — allowing air infiltration and progressive seal degradation. Aluminum frames without a thermal break produce significant interior condensation in Utah winters and are not recommended for residential use. The performance and durability premium for fiberglass over multi-chamber vinyl is real and meaningful in Utah’s specific environment.