Heat Loss & U-Value Calculator

Heat flows through a wall, roof, floor or window in proportion to its area, its U-value (thermal transmittance, W per m² per kelvin) and the temperature difference across it: Q = U × A × ΔT. The page sums that over every element you list, shows each element's share so the weak point is obvious, and reports the total UA — the loss per degree — which lets you scale to any other design temperature by multiplication. One inside and one outside temperature apply to all elements; a floor on the ground or a wall to an unheated space sees a different ΔT in practice and would need its own entry with an adjusted U-value or temperature.

Air that leaves the space takes heat with it: Q = airflow × density × specific heat × ΔT. Density and specific heat of air vary with temperature, pressure and altitude, so they are inputs; the values shown at first (1.2 kg/m³ and 1,005 J/kg·K) are a labelled sample for dry air near 20 °C at sea level. Enter the airflow from the Air Changes page or from your ventilation design; leaving it at zero is flagged because real buildings always exchange some air.

A U-value depends on the exact build-up, the insulation product, the frame, the glazing and the age of the element, and building regulations set limits that differ by jurisdiction and year. Take U-values from the construction specification, the product declaration or your standard's calculation method (ISO 6946 for opaque elements, ISO 10077 for windows). The page will not guess them.

This is the steady-state heat loss used to size heating. A cooling load needs solar gains through glazing, internal gains from people and equipment, and the thermal mass of the building; thermal bridges, intermittent heating and safety margins are design decisions outside this page. In imperial mode U is in BTU/(h·ft²·°F), area in ft², temperatures in °F and results in BTU/h. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.