Thermal effects of cavity ventilation in facades: Experimental measurement and comparison with standard calculation methods

• Outdoor-facing facade tests were compared to hot box tests and R-value calculations. • Cavity ventilation found to increase effective R-values in sunny, calm conditions. • Standard hot box tests are unable to predict this ventilative cooling effect. • R-value calculation methods do not take cavity ventilative cooling into account. • Hot box tests of facades with and without cavities are compared. Ventilation of cavities behind facade cladding can have varied effects on the facade thermal performance. However, not all standard R-value calculation and test methods can accurately predict these effects. In this study, facades with various cavity details were tested in a guarded hot box and in outdoor-facing experiments, and results were compared with R-value calculation methods prescribed in ISO 6946, NZS 4214, and AS/NZS 4859.2. While hot box tests and calculations indicated that facades with ventilated cavities have lower effective R-values compared to sealed cavities, the outdoor tests demonstrated that under sunny conditions with low wind speeds, cavity ventilation can increase the facade effective R-value; reducing membrane temperatures by an average of 2.0–3.2 °C (maximum measured reduction: 7.0 °C). Tracer-gas measurements revealed that such cooling was achieved by ventilation air flow rates in the order of 2 L s −1 m −1 (90 air changes per hour). R-value calculations overpredicted the magnitude of apparent R-value reduction due to cavity ventilation by almost an order of magnitude relative to hot box tests. However, in some cases, this compensated for other inaccuracies, resulting in overall mean absolute errors of 5.5–11 %. Hot box tests showed that, even without capturing the potential benefits of cavity ventilation, battened-out ventilated cavities increased facade effective R-values by 0.17–0.42 m 2 K W −1 , exceeding the improvement provided by R0.2 thermal break strips. This study demonstrates that current design methods misrepresent the effects of cavity ventilation under realistic conditions and should potentially be revised to more accurately model real facades.