OTTV (Overall Thermal Transfer Value)

OTTV accounts for heat conduction through opaque walls, heat conduction through glazing, and solar heat gain through glazing. It is calculated separately for each facade orientation and then area-weighted. ASHRAE 90.1 and many Asian building codes (Singapore BCA, Hong Kong, Malaysia) set maximum OTTV limits, typically 45-50 W/m2 for commercial buildings. OTTV is more relevant than U-value alone in tropical climates because it captures the dominant solar heat gain component.

In cooling-dominated climates, envelope performance determines 30-50% of cooling energy. OTTV provides a single metric that captures all envelope heat gain pathways. Meeting OTTV limits during design ensures acceptable cooling loads without requiring compensatory measures. OTTV is a key compliance parameter in many tropical building codes.