Case study comparing thermal energy demand intensity and peak power intensity in a renovated historic masonry school using measured and estimated enclosure airtightness

Accurate estimation of building infiltration is critical for HVAC equipment sizing, yet many designs assume a single, uniform airtightness value, which can lead to oversizing or undersizing of systems. This issue is particularly important in the context of historic preservation and adaptive reuse, where maintaining the integrity of Canada’s substantial stock of masonry schools and institutional buildings is a priority. This case study quantified airtightness in a medium-sized historic masonry and curtainwall institutional building, measuring whole-building air leakage of 2.18 L/m²/s at 75 Pa, with the historic masonry section at 3.47 L/m²/s and the contemporary section at 1.17 L/m²/s. Façade-level measurements of twin classrooms showed 5.67 L/m²/s for masonry-and-plaster walls and 8.59 L/m²/s for exposed masonry walls, highlighting substantial variability in localized leakage. Simulations using empirically derived airtightness values revealed heating thermal energy demand intensity (TEDI) of 26.1 kWh/m²/a for the masonry-only façade and 20.4 kWh/m²/a for the plastered façade, and 10.5 kWh/m²/a for the whole building air tightness, while peak heating power for the HVAC system differed by 60% (masonry) and 29% (plastered masonry) above the measured whole building airtightness. Cooling demand was largely insensitive to façade airtightness and driven primarily by high occupant loads. These results demonstrate that façade-specific airtightness significantly influences thermal performance, energy consumption, and occupant comfort, underscoring the need to account for localized air leakage in the design, operation, and adaptive reuse of Canada’s historic and contemporary masonry buildings.