Last edited 28 Sep 2020

Thermal admittance of building materials

Thermal admittance (Y) is a measure a material's ability to absorb heat from, and release it to, a space over time. This can be used as an indicator of the thermal storage capacity (thermal mass) of a material, absorbing heat from and releasing it to a space through cyclical temperature variations, thus evening out temperature variations and so reducing the demand on building services systems.

Thermal admittance is expressed in W/(m^2K), where the higher the admittance value, the higher the thermal storage capacity.

Thermal admittance is calculated as the heat transfer (in watts W) / area (m^2) x the temperature difference between the surface of the material and the air.

Typical admittance values based on a 24-hour cycle might range from 1.0 for a timber frame wall with brick outer leaf, to 2.65 for a cavity wall with 100 mm dense aggregate block (ref. The Concrete Centre).

The admittance time lead, ω (expressed in hours), is a measure of the time delay between the peak heat flow between the material surface and the space and the time of the peak temperature in the space.

Admittance is dependant primarily on a material's density, thermal capacity, thermal conductivity, surface resistance and the time cycle of the temperature variation.

As the thickness of a material increases, so the admittance approaches a constant value. It is generally considered that in the UK, with a 24-hour thermal cycle, heat energy can only penetrate up to 100 mm into materials such as concrete and masonry.

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