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.
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 100mm 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.
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 100mm into materials such as concrete and masonry.
 Related articles on Designing Buildings Wiki
 External references.
- ISO 13786:2007. Thermal performance of building components -- Dynamic thermal characteristics -- Calculation methods.
- The Concrete Centre: Thermal Mass Explained (2012 update).
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