When XPS sandwich panels are used in underfloor heating systems, why is the maximum service temperature limited? How to quantify thermal creep behavior?

XPS has a softening point of 85–95°C. Underfloor heating supply water temperatures often reach 60°C, but localized areas under coverings can trap heat at 70–75°C. At these temperatures, the XPS compressive creep rate accelerates: according to ASTM D2990, under 60°C/50kPa, strain reaches 1.5% after 1000 hours, five times that at 20°C. Quantitative design uses the “time-temperature superposition principle.” Extrapolating from 70°C short-term creep tests to 50 years, compressive strain should be kept <2% to prevent floor cracking. Recommendations: install a heat distribution layer (aluminum foil or cement mortar) on top of XPS to lower the temperature below 50°C, or switch to higher heat-resistant extruded board (e.g., crosslinked XPS or polyisocyanurate PIR).

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When XPS sandwich panels are used in underfloor heating systems, why is the maximum service temperature limited? How to quantify thermal creep behavior?

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