Sustainability
Dynamic thermal modelling and overheating assessment
Dynamic simulation for overheating risk, comfort and energy — TM52, TM59, Approved Document O and TM54 — run while the facade can still change.
- Discipline
- Sustainability
WHAT IT IS
Dynamic thermal modelling simulates a building hour by hour across a weather year, rather than at a single design condition. It is what answers the questions a steady-state calculation cannot: whether the building overheats in summer, how often and by how much, whether a free-running strategy is viable, and what the building is likely to actually consume.
The assessment methodology depends on what is being demonstrated. CIBSE TM52 covers free-running non-domestic buildings against adaptive comfort criteria. TM59 covers homes. Approved Document O sets the requirement for overheating mitigation in new residential buildings in England. CIBSE TM54 is the separate methodology for predicting operational energy use, which compliance modelling does not do.
WHEN YOU NEED IT
At concept, while glazing ratio, orientation, shading and ventilation strategy are all still open.
This is the single most common sequencing failure in building services. An overheating assessment run after the facade is fixed can only report the problem, and by then mechanical cooling is usually the only remaining answer — which adds plant, plant space, electrical load and running cost to solve something that a shading decision would have solved for nothing.
OUR PROCESS
The model is built from the design information with the weather file appropriate to the location and the assessment — a design summer year for overheating risk, a test reference year for energy. Options are then tested against it in the order they can still be changed: form and shading first, then glazing and ventilation, then plant.
Where the result is marginal, the sensitivity is stated. A building that passes on one weather file and fails on the next is not a building that passes.
DELIVERABLES
What you receive
- Dynamic thermal model of the building
- Overheating assessment to TM52, TM59 or Approved Document O
- Comfort analysis by space and by orientation
- Options appraisal in the order the design can still change
- Operational energy prediction to TM54 where required
STANDARDS
Standards applied
The editions in force at the time of design. Where a standard is amended mid-project we confirm which edition the appointment is based on.
- CIBSE TM52The limits of thermal comfort — avoiding overheating in European buildingsCIBSE
- CIBSE TM59Design methodology for the assessment of overheating risk in homesCIBSE
- CIBSE TM54Evaluating operational energy use at the design stageCIBSE
- Approved Document OOverheating (England)GOV.UK
- CIBSE Guide AEnvironmental DesignCIBSE
- BS EN 16798-1:2019Indoor environmental input parametersBSI
QUESTIONS
Common questions
What is the difference between CIBSE TM52 and TM59?
TM52 is the methodology for assessing overheating risk in free-running non-domestic buildings — those not mechanically cooled — using adaptive comfort criteria that vary with the outdoor running mean temperature. TM59 is the methodology for homes, and applies fixed criteria to bedrooms and living spaces. They are not interchangeable, and a residential assessment carried out to TM52 will not satisfy a reviewer expecting TM59.
When is dynamic thermal modelling needed rather than a steady-state calculation?
Whenever the question involves time: overheating frequency and severity, the viability of natural or mixed-mode ventilation, thermal mass, or predicted energy use. A steady-state calculation answers the peak design condition and nothing else, which is sufficient for sizing plant and insufficient for demonstrating comfort.
Does Approved Document O apply to my project?
Approved Document O covers overheating mitigation in new residential buildings in England, including dwellings, student accommodation and care homes. It does not apply in Scotland, where overheating is addressed through the Technical Handbooks, and a Scottish project pursuing an overheating assessment is usually doing so to TM52 or TM59 at the request of a client, funder or planning authority.
Which weather file should an overheating assessment use?
A design summer year for the nearest representative location, rather than the test reference year used for energy calculations. Which design summer year, and whether a future-climate variant is also run, should be agreed at the outset — the answer changes the result, and a pass obtained on the mildest available file is not a robust one.