The thermal admittance of a material can be described as its ability to exchange heat with the surrounding environment under cyclical temperature variations.

Admittance is dependent upon a number of material properties, most notably density, thermal capacity and the thermal conductivity. It is also dependent upon the time period of temperature oscillations and is subject to the resistance of the film of air at its surface.

## Units and Measures

It is the ratio of the heat absorbed by a material per unit area and deviation of temperature from its mean. If the temperature of a material in a space at any time deviates from the mean by an amount Ts, then the surface absorbs Y x Ts heat per unit area. Therefore, the peak absorption of heat in a space is equal to: AY Ts; Where A and Y are the area and the admittance respectively of each surface and Ts is the temperature swing about the mean.

Following are the units of thermal admittance:

• SI Units: watt per square-meter kelvin (W /m²•K)
• Imperial Units: British thermal unit per hour Square feet degree Fahrenheit (Btu /ft²•°F•h)