What is TM 52 Analysis?

TM 52 analysis refers to the methodology outlined in CIBSE TM52: The Limits of Thermal Comfort: Avoiding Overheating in Summer, a guidance document from the Chartered Institution of Building Services Engineers (CIBSE). This analysis is a standard method used to assess the likelihood of overheating in buildings, particularly during warmer summer months, and to ensure occupant thermal comfort.

Understanding TM52 Overheating Assessment

The primary goal of a TM52 analysis is to determine if internal building temperatures exceed acceptable limits for comfort and health during periods of high external temperature or internal heat gains. Unlike simpler static temperature checks, TM52 uses a dynamic approach, considering varying internal and external conditions over time.

The Three Criteria for Overheating Assessment

Mirroring CIBSE TM52, there are three criteria to assess when overheating is likely to be an issue: total hours of exceedance, daily weighted exceedance, upper limit temperature. Buildings are assessed against these criteria based on dynamic thermal simulations or monitored data.

Here's a breakdown of the three criteria:

Criterion 1: Total Hours of Exceedance

• Description: This criterion measures the total number of hours during the occupied period of the analysis period (typically May 1st to September 30th for summer) where the operative temperature exceeds the upper limit of the acceptable thermal comfort range.
• Requirement: This criterion is mandatory and must be less than 40 hours. If the total exceedance hours reach 40 or more, the building or space is deemed to have a high risk of overheating.

Criterion 2: Daily Weighted Exceedance

• Description: This criterion assesses the severity of overheating. It uses a weighted calculation where exceedances by larger margins or for longer durations within a single day are given more weight. The weighting is based on the temperature difference above the comfort limit.
• Requirement: This criterion passes if the daily weighted exceedance value is less than or equal to 6.

Criterion 3: Upper Limit Temperature

• Description: This criterion checks if the operative temperature ever exceeds a fixed absolute upper limit. This is typically set significantly higher than the adaptive comfort limit and represents a temperature that is almost certainly uncomfortable or potentially unsafe.
• Requirement: This criterion passes if the operative temperature never exceeds the fixed upper limit temperature (often defined as the comfort temperature plus 4°C, although the specific limit can vary based on building type and context).

Why TM52 Analysis is Important

Implementing TM52 analysis is crucial for several reasons:

• Health & Wellbeing: Prolonged exposure to high indoor temperatures can lead to discomfort, reduced productivity, and health risks. TM52 helps identify and mitigate these risks.
• Building Regulations & Standards: Compliance with TM52 is often required or recommended by building regulations and design standards in many regions to demonstrate acceptable thermal performance.
• Energy Efficiency: Designing buildings to avoid overheating naturally, through passive measures like shading and ventilation, can reduce the reliance on energy-intensive air conditioning systems.
• Occupant Satisfaction: Comfortable occupants are happier and more productive. Avoiding overheating is key to achieving high levels of occupant satisfaction.

Practical Steps in a TM52 Analysis

A typical TM52 analysis involves:

1. Creating a Detailed Building Model: Using thermal simulation software, a digital model of the building is created, including its geometry, construction materials, glazing, shading devices, internal heat gains (occupants, equipment, lighting), and ventilation strategies.
2. Defining Occupancy and Use Profiles: Realistic schedules for when spaces are occupied and how internal heat gains vary are set up.
3. Selecting Weather Data: Representative weather data for the building's location, typically including a 'design summer year' or future climate scenarios, is used.
4. Running Simulations: The software simulates the building's thermal response over the defined period (e.g., May to September).
5. Analyzing Results: The simulation outputs (operative temperatures) are processed against the three TM52 criteria to determine if the building passes or fails the assessment.
6. Identifying Solutions: If the building fails, the analysis helps identify which areas are most affected and evaluate potential solutions, such as:
   • Improving natural ventilation
   • Adding external shading (e.g., brise-soleils, shutters)
   • Using high-performance glazing
   • Adjusting thermal mass
   • Considering mechanical cooling (as a last resort if passive measures are insufficient)

Table Summary of TM52 Criteria

Understanding and applying TM52 analysis is essential for designing and assessing buildings that provide comfortable and healthy environments, particularly in the face of rising global temperatures.