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.
[edit] Related articles on Designing Buildings Wiki
[edit] External references.
- ISO 13786:2007. Thermal performance of building components -- Dynamic thermal characteristics -- Calculation methods.
- The Concrete Centre: Thermal Mass Explained (2012 update).
Featured articles and news
50th Golden anniversary ECA Edmundson apprentice award
Showcasing the very best electrotechnical and engineering services for half a century.
Welsh government consults on HRBs and reg changes
Seeking feedback on a new regulatory regime and a broad range of issues.
CIOB Client Guide (2nd edition) March 2025
Free download covering statutory dutyholder roles under the Building Safety Act and much more.
AI and automation in 3D modelling and spatial design
Can almost half of design development tasks be automated?
Minister quizzed, as responsibility transfers to MHCLG and BSR publishes new building control guidance.
UK environmental regulations reform 2025
Amid wider new approaches to ensure regulators and regulation support growth.
The maintenance challenge of tenements.
BSRIA Statutory Compliance Inspection Checklist
BG80/2025 now significantly updated to include requirements related to important changes in legislation.
Shortlist for the 2025 Roofscape Design Awards
Talent and innovation showcase announcement from the trussed rafter industry.
OpenUSD possibilities: Look before you leap
Being ready for the OpenUSD solutions set to transform architecture and design.
Global Asbestos Awareness Week 2025
Highlighting the continuing threat to trades persons.
Retrofit of Buildings, a CIOB Technical Publication
Now available in Arabic and Chinese aswell as English.
The context, schemes, standards, roles and relevance of the Building Safety Act.
Retrofit 25 – What's Stopping Us?
Exhibition Opens at The Building Centre.
Types of work to existing buildings
A simple circular economy wiki breakdown with further links.
A threat to the creativity that makes London special.
How can digital twins boost profitability within construction?
The smart construction dashboard, as-built data and site changes forming an accurate digital twin.