Heat Loss Calculator
Understanding where your home loses heat helps prioritize insulation upgrades and correctly size heating equipment. This calculator estimates heat loss through walls, windows, ceiling, and floor using the standard formula Q = U x A x ΔT, then recommends a furnace size with a safety margin for air infiltration.
Heat flows from warm areas to cold areas through every building surface. The rate of heat loss depends on three factors: the U-value (thermal transmittance of the material), the area of the surface, and the temperature difference (ΔT) between inside and outside.
Windows are typically the weakest point in a building envelope. Single-pane windows have a U-value of 1.0, meaning they lose 1 BTU per hour per square foot per degree F, roughly 13 times more than R-13 insulated walls. Upgrading from single to double-pane windows cuts window heat loss in half. Triple-pane low-E windows reduce it by 75%.
The calculator adds a 20% safety margin to the calculated heat loss to account for air infiltration, the warm air that escapes through gaps around doors, windows, electrical outlets, and other penetrations. In drafty older homes, infiltration can account for 25-40% of total heat loss. A professional blower door test can measure your actual infiltration rate.