Interior Wall Surface Temperature Calculator

Enter the indoor air temperature, the wall U-value, the inside film heat-transfer coefficient, and the indoor-to-outdoor temperature difference. The result helps you compare the predicted surface temperature with the indoor dew point, which is the practical test for surface condensation. It also reports heat flux, the inside-film temperature drop, the implied outdoor temperature, and a dimensionless temperature factor for transparent engineering checks.

What the calculated surface temperature means

A room may feel comfortably warm while a poorly insulated wall remains much colder than the indoor air. Heat flowing through the wall crosses a thin boundary layer of relatively still air at the inside face. That layer creates a temperature drop between the room air and the physical surface. The calculator determines steady heat flux as the wall U-value multiplied by the indoor-to-outdoor temperature difference. It then divides that heat flux by the inside film coefficient to obtain the room-side film drop, and subtracts the drop from the indoor temperature. The resulting interior surface temperature is the value to compare with the indoor dew point. If the surface is warmer than the dew point, liquid condensation is not predicted under the entered steady conditions. If it is equal to or colder than the dew point, moisture can condense. Because relative humidity or dew point is not one of the four inputs, the calculator does not invent a pass or fail result. It provides the thermal value needed for an honest condensation assessment.

Choose inputs that describe the same condition

Use an indoor air temperature in degrees Celsius and a non-negative temperature difference defined as indoor temperature minus outdoor temperature. Celsius degree differences and kelvin differences have the same numerical size, so either convention works for the difference. The U-value must represent the complete wall assembly, including all layers and relevant surface resistances according to the method used in your project. Enter the inside film coefficient in watts per square metre-kelvin. It depends on heat-flow direction, surface orientation, air movement, and the standard or design method being followed; it is not a universal material property. Keep all inputs from the same design case. Combining a winter design temperature difference with a U-value or film coefficient based on incompatible assumptions can create a precise-looking but misleading answer. The calculator rejects zero or negative thermal coefficients, non-finite numbers, negative heating-season differences, and a U-value greater than the inside film coefficient because those values do not form a physically consistent resistance model for this calculation.

Assess condensation risk and understand the limits

First calculate or measure the indoor dew point from indoor temperature and relative humidity, then compare it with the reported interior surface temperature. A positive margin, with the surface warmer than the dew point, indicates no surface condensation under the assumed steady state. A small margin deserves caution because humidity, outdoor temperature, air movement, furnishings, and workmanship vary over time. Corners, thermal bridges, window reveals, structural penetrations, and areas behind cabinets can be colder than the one-dimensional clear-wall estimate. The method also assumes steady heat transfer and does not model thermal storage, rain, solar gain, air leakage, moisture diffusion, or intermittent heating. Use the heat-flux and film-drop outputs to audit the arithmetic, and use the temperature factor to compare cases that have different indoor and outdoor temperatures. For design approval, mold analysis, or investigation of an occupied building, supplement this screening calculation with measured humidity, thermal-bridge analysis, suitable safety margins, and the requirements of the applicable building standard. The API price is $0.002 per calculation, while the same deterministic calculation can run in the browser.