Thermal performance of ventilation ductwork integrated in retrofit façade insulation in multi-family buildings in cold climates : Heat loss and insulation analysis

Sammanfattning

Existing multi-family buildings in cold climates often have significant ventilation heat losses, especially when they rely on natural or exhaust-only ventilation without heat recovery. Mechanical supply and exhaust ventilation with heat recovery can reduce these losses, but installing new ductwork inside existing buildings may be difficult due to space limitations, construction constraints and disturbance to occupants. This thesis evaluates the thermal performance of a retrofit ventilation concept in which preheated supply air from an attic FTX unit is distributed through exterior ducts embedded within added façade insulation. A simplified steady-state analytical model was developed for a representative four-storey multi-family building in a cold Swedish climate. The critical case considered a ground-floor apartment supplied through a 12 m vertical galvanized steel duct embedded in 180 mm of mineral wool façade insulation and located next to an existing autoclaved aerated concrete wall. The model included outward heat transfer through the duct wall and insulation, exterior surface resistance, and a simplified heat-gain correction from the indoor side through the existing wall. A short horizontal attic duct section and comparative duct alternatives were also evaluated. Under the assumed conditions of 21 °C indoor temperature, −5 °C outdoor temperature and 85% FTX temperature efficiency, the supply air temperature after the heat exchanger was 17.1 °C. After passing through the exterior façade duct, the calculated supply air temperature at the apartment inlet was approximately 16.1 °C, corresponding to a temperature decrease of about 1.0 K. The outward heat flow was 37 W, while the heat contribution from the AAC wall was 1.8 W, resulting in a net cooling effect of 35 W. The useful heat recovery efficiency at the apartment inlet was therefore reduced from 85% to approximately 81.2%. The results indicate that the exterior façade-integrated duct causes a measurable but limited reduction in recovered heat performance. For a representative cold month, the net air-cooling penalty was approximately 25 kWh per apartment, compared with about 540 kWh of useful recovered heat. The attic duct section was secondary, and the façade-integrated configuration performed better than an externally mounted cylindrically insulated duct. Overall, individual exterior supply ducts embedded in façade insulation appear thermally viable for cold-climate renovation projects, although future work should validate the results with measurements and more detailed dynamic modelling.