ACH & Infiltration Heat Loss Calculator

Convective heat loss from air infiltration is calculated as: Q_vent = 0.33 × n × V × ΔT. Where 0.33 is the volumetric heat capacity of air (ρ × cp / 3600 in Wh/m³K), n is the air change rate per hour (ACH), V is the heated internal air volume in cubic metres (m³), and ΔT is the temperature difference between inside and outside.

Under the widely accepted Sherman-Grimsrud / Kronvall 'Rule of 20' (LBL model), natural air changes per hour under normal ambient conditions is estimated as: ACH_nat ≈ n50 / N, where the divisor N typically equals 20 (adjusted to 15–25 depending on building height, wind shielding, and leak distribution).

A certified MVHR system extracts stale, humid air from kitchens and bathrooms while supplying fresh outdoor air to living rooms and bedrooms. A high-efficiency counter-flow heat exchanger transfers 85% to 92% of the heat from the exhaust air to the incoming fresh air, reducing ventilation thermal heat demand by ~85%.

Stack effect (or chimney effect) is the buoyancy-driven movement of air through a building. Warm indoor air is less dense than cold outdoor air, creating positive air pressure near the top of the house (pushing warm air into the attic) and negative suction pressure at the ground floor (drawing cold air in through floorboards and doors).

UK Building Regulations Part L (2021) sets a maximum limiting air permeability of q50 ≤ 8.0 m³/(h·m²) at 50 Pa (with typical new builds achieving 3.0 to 5.0). The international Passivhaus standard requires ultra-airtight performance of n50 ≤ 0.6 ACH at 50 Pa.

Buildings need continuous fresh air to evacuate moisture (from breathing, showering, cooking), carbon dioxide (CO₂), and volatile organic compounds (VOCs). If an older home is sealed without controlled mechanical ventilation (such as continuous MEV or MVHR), relative humidity climbs above 65%, causing toxic black mold (Stachybotrys) and condensation.