Delivering CIBSE TM59 dynamic thermal modelling across current and future climate scenarios to test the full cooling hierarchy before justifying air conditioning in existing homes.
Project Overview
- Client: Mimetico
- Sector: Private Residential (Existing Dwellings)
- Location: Kensington & Chelsea and Knightsbridge, London
- Services Provided: CIBSE TM59 Dynamic Thermal Modelling, Part O Overheating Assessment
- Scale: Multiple individual London properties, each assessed separately across current and future climate scenarios
- Target: Assessed against CIBSE TM59, Approved Document Part O (2021), and CIBSE Guide A (2015a) fixed/adaptive temperature criteria – testing passive measures and mechanical ventilation before justifying air conditioning
The Challenge
London faces a genuine and worsening overheating problem, driven by the Urban Heat Island effect and a housing stock built to retain winter heat rather than dispel summer heat; City Hall’s Heat Ready London Strategy identifies over 1 million homes across the capital as facing high heat vulnerability. In response, the London Plan sets a clear cooling hierarchy: passive measures, then mechanical ventilation, must be shown insufficient before mechanical cooling is accepted – precisely to avoid active cooling compounding the very heat problem it’s meant to solve. That leaves homeowners and their architects in a genuine bind: a real, worsening comfort problem, but a planning system that expects every rung of that hierarchy to be tested, not assumed. Mimetico needed exactly this evidence, across a series of projects on period and mews properties across Kensington & Chelsea and Knightsbridge.
The Solution
CIBSE TM59 Dynamic Thermal Modelling
For each property, we built a dynamic thermal model using DesignBuilder EnergyPlus under CIBSE TM59 and Approved Document Part O methodology, testing against current (2020) and future (2050, 2080) climate scenarios so any conclusion would hold up across the building’s expected lifetime. Critically, every assessment worked through the cooling hierarchy in order rather than jumping straight to a cooling conclusion.
On one project – a mews property undergoing refurbishment alongside a new basement – this staged approach was tested most thoroughly. Modelled with passive measures only, several rooms failed badly, with one study space showing overheating for over a third of occupied hours. Adding mechanical ventilation alone brought some improvement, but the same rooms still failed. Only once both stages had been tested and shown insufficient did the modelling turn to mechanical cooling – at which point nearly every room passed, with only a slight residual risk remaining in one bedroom.
Elsewhere, the picture was more mixed. At one period flat, cooling applied to the worst-affected kitchen resolved that room under current conditions, but bedrooms continued to fail an entirely different criterion – night-time comfort – that kitchen cooling alone couldn’t touch. At a larger property nearby, this same night-time bedroom problem persisted even under future climate projections, with cooling reducing but not eliminating the risk in every case.
The Outcome
Mimetico came away with property-specific, quantified evidence for each assessment – not a generic assertion that a space needs cooling, but a genuine, staged test of the entire cooling hierarchy planning policy expects to see. Critically, the modelling was honest wherever the picture was mixed: where cooling in one room didn’t resolve a different room’s night-time overheating, that was reported as clearly as the properties where the full hierarchy resolved every space. That kind of evidenced, non-uniform conclusion – showing passive measures and ventilation genuinely tested and found wanting, not assumed – is exactly what gives this modelling credibility as planning evidence.
