Internal Wall Cooling by Reversed Airflow in Ventilation Ducts After Building Retrofit: A Case Study of a Higher Education Facility

This study investigated an unusual yet practically significant side effect of the thermal retrofit of educational buildings, namely the intensive cooling of internal walls containing uninsulated ventilation ducts. Malfunctioning gravity ventilation led to a reversal of airflow direction, resulting in the inflow of cold air through internal ducts and a deterioration of thermal comfort in the rooms. The cooled zones were identified during thermographic surveys carried out using a FLIR E8-XT camera. Numerical heat transfer analyses were performed in THERM 8.1 (LBNL), while the thermal performance of the building and its seasonal heating demand were calculated in Audytor OZC (Sankom). Three variants were compared: (1) standard design conditions, (2) scenario accounting for the cooling effect of internal walls due to reverse flow of ventilation air, and (3) mechanical ventilation with heat recovery (MVHR). Under the analysed actual operating conditions, the transmission heat loss increased from 27,479 W to 52,473 W, while the annual heating demand rose by 57.6% from 71.3 to 112.4 kWh/(m3∙year). The indoor air temperature decreased by approximately 3 ÷ 7 °C (depending of the space), while the surface temperature near the ducts fell to 5.1 ÷ 12.6 °C at an outdoor temperature of −13 °C. The implementation of MVHR (η = 75%), compared to current situation, reduced the overall heat loss coefficient by 63.9% from 3934.225 to 1417.825 W/K and the annual heating demand by 70.3% from 112.4 to 33.3 kWh/(m3∙year). The results confirm that uncontrolled airflow through internal ventilation ducts may lead to substantial heat losses and localised cooling zones in building elements, effectively acting as thermal bridges.