TM59 vs TM52: What’s the Difference in Overheating Assessments?  TM59 and TM52 sound similar but assess different buildings. Learn the real difference, when to use each, and how to stay compliant.It’s easy to mix them up.

TM59. TM52. Two small codes two very different purposes.

The world of building performance and overheating compliance, these two CIBSE guides are essential. But they’re often misunderstood, misapplied, or even overlooked entirely.

So, let’s set the record straight. In this guide, we’ll explain what is the difference between TM59 and TM52, why they matter, and most importantly which one your project needs.

What is TM59?

TM59 is the go-to overheating assessment for residential buildings. It was published by CIBSE (Chartered Institution of Building Services Engineers) in 2017 to respond to a clear problem: new homes in the UK were getting too hot to live in during summer.

It focuses on:

• Flats and apartments
  
  
• Student accommodation
  
  
• Care homes
  
  
• Urban developments with limited ventilation
  
  

What Does TM59 Calculate?

A TM59 assessment uses dynamic thermal simulation modelling (DSM) to test:

• How high internal temperatures get?
  
  
• How long they stay above acceptable levels?
  
  
• How well ventilation and design features reduce heat buildup?
  
  

TM59 Uses Criteria:

• Bedrooms: must not exceed 26°C for more than 1% of night-time hours
  
  
• Living Rooms: must not exceed 26°C for more than 3% of daytime occupied hours
• Any Room: Temperature >26 °C must not occur for more than 3% of annual occupied hours

What is TM52?

TM52 is older than TM59 and is used for non-domestic buildings think:

• Offices
  
  
• Schools
  
  
• Libraries
  
  
• Hospitals
  
  
• University buildings
  
  

Instead of focusing on living comfort, TM52 is about thermal comfort during working or learning hours.

What Does TM52 Calculate?

• How much indoor temperature drifts above comfort levels
  
  
• How often it happens
  
  
• How extreme the heat gets when it does
  
  

TM52 Uses Criteria:

1. Hours of exceedance
   
   
2. Daily weighted exceedance
   
   
3. Maximum temperature difference

To pass, you must meet at least two out of three.

TM59 vs TM52 What’s the Real Difference?

Here’s a quick side-by-side to clear the confusion:

                Feature

                TM59

              TM52

Applies to

Residential homes/flats

Offices, schools, non-residential

Purpose

Prevent overheating in homes

Ensure thermal comfort during work

Assessment criteria 2 thresholds

Bedrooms & living areas

3-point performance criteria

Based on occupancy

Day and night use

Daytime/working hours

Main usage

Building Regs Part O, planning

BREEAM, educational settings

Introduced

2017

2013

If people are living and sleeping in the building, use TM59.
If they’re working, learning, or healing in the space, use TM52.

Why Does it Matters (Beyond Compliance)?

This isn’t just about ticking boxes.

Poor overheating control impacts:

• Health: heat stress in vulnerable residents
  
  
• Wellbeing: poor sleep, fatigue, reduced concentration
  
  
• Energy use: more demand for fans or retrofitted cooling
  
  
• Legal compliance: especially with Building Regulations Part O
  
  

The risk is real. According to the UK Climate Change Committee, overheating risk will double by 2050 if homes aren’t designed better.

How Gradwell Group Supports You?

At Gradwell Group, we don’t just run numbers we help you pass with confidence.

We offer:

• TM59 assessments for residential schemes
  
  
• TM52 assessments for non-domestic projects
  
  
• Fast, detailed reports ready for submission
  
  
• Design advice to reduce risk and improve comfort
  
  
• Affordable pricing and no generic templates
  
  

FAQs

1: Can I use TM52 for residential projects?

A: No. TM52 is for non-domestic buildings. Use TM59 for homes.

2: Is TM59 a legal requirement?

A: It’s not law, but it's widely required to meet Part O of Building Regulations and pass planning.

3: What if my project fails the TM59 assessment?

A: Gradwell Group will work with you to tweak design features ventilation, glazing, shading to help you pass.

4: Do both TM52 and TM59 use thermal modelling?

A: Yes, both use dynamic simulation modelling but with different thresholds and goals.

Final Thoughts

• TM59 is for residential comfort and safety
  
  
• TM52 is for daytime work and learning spaces
  
  

Choosing the wrong one can cost you time, money, and compliance. But getting it right? That’s where we come in.

At Gradwell Group, we help you select the right assessment, pass it efficiently, and build spaces people actually enjoy living or working in. Let’s future-proof your next project without the headaches.