Overheating in Buildings | Part O, CIBSE TM52 & TM59 Explained (Ireland & UK Guide)

Overheating in buildings has become a serious issue across Ireland and the UK due to improved airtightness, increased insulation levels, and rising summer temperatures linked to climate change. While energy-efficient construction reduces heat loss in winter, it can unintentionally trap excess heat in summer if not properly designed.
Modern building regulations now require overheating risk assessments to ensure occupant comfort and safety.
In this guide, we explain
• What building overheating is
• Why is it increasing in new homes
• Part O of the Building Regulations
• CIBSE TM52 & TM59 overheating criteria
• How overheating is assessed
• How to design compliant, comfortable buildings

What Is Overheating in Buildings?

Overheating occurs when indoor temperatures rise above acceptable comfort thresholds for prolonged periods, causing discomfort, disturbed sleep, and potential health risks.
It is especially problematic in:
• Highly insulated dwellings
• Airtight buildings
• Apartments with limited cross ventilation
• South- and west-facing glazing
• Urban developments affected by the heat island effect
Overheating risk is now a critical consideration in low-energy and NZEB-compliant buildings.

Why Overheating Is Increasing in Modern Homes

Several factors contribute to rising overheating risk:

1. Improved Airtightness
   Modern construction reduces air leakage to improve energy efficiency. However, without an adequate ventilation strategy, heat becomes trapped indoors.
2. Large Areas of Glazing
   Extensive south- or west-facing windows increase solar gains, especially without shading.
3. Climate Change
   Summer temperatures in Ireland and the UK are rising, increasing peak indoor temperatures.
4. Apartment Design Constraints
   Single-aspect apartments often struggle to provide adequate purge ventilation.

Part O Overheating in Buildings Regulations

Part O was introduced in England in 2021 to limit unwanted solar gains and ensure adequate means of removing excess heat.
Although Ireland does not yet have a direct equivalent to Part O, overheating risk assessments are increasingly requested in:
• Apartment developments
• Planning submissions
• Passive House projects
• Large residential schemes

Part O Compliance Routes

Part O provides two main approaches

1. Simplified Method
   Uses prescriptive glazing limits and minimum-free-area ventilation requirements.
2. Dynamic Thermal Modelling (DTM)
   Uses simulation software (IES VE, TAS, DesignBuilder) to assess overheating risk against CIBSE criteria.
   For complex or high-density developments, dynamic modelling is typically required.

CIBSE TM52 : Overheating in Non-Residential Buildings

CIBSE TM52 applies to

• Offices
• Schools
• Commercial buildings
• Non-domestic spaces
It sets three overheating criteria based on adaptive thermal comfort theory.
A building fails TM52 if two of the following three criteria are exceeded:

TM52 Criteria

1. Hours of Exceedance
   Indoor operative temperature exceeds comfort threshold for more than 3% of occupied hours.
2. Daily Weighted Exceedance
   Measures the severity and duration of overheating across a day.
3. Upper Limit Temperature
   The absolute maximum temperature should not exceed the comfort threshold by more than 4°C.
   TM52 uses adaptive comfort models from BS EN 15251 / EN 16798 standards.

CIBSE TM59 : Overheating in Homes

CIBSE TM59 applies specifically to residential buildings, including:
• Houses
• Apartments
• Student accommodation
• Care homes
TM59 uses dynamic thermal modelling to evaluate overheating risk under realistic occupancy scenarios.
TM59 Key Criteria
Criterion A – Living Rooms & Bedrooms (Adaptive Method)
Temperature should not exceed the adaptive comfort threshold for more than 3% of annual occupied hours.
Criterion B – Bedrooms (Night-Time Limit)
Bedroom operative temperature should not exceed 26°C for more than 1% of annual night hours (10pm–7am).
This is particularly important because sleep disruption is a major concern for overheating.
TM59 assessments account for:
• Internal heat gains
• Solar gains
• Ventilation strategy
• Window opening behaviour
• Urban noise constraints

Relevant Standards & Guidance

Overheating assessments may reference
• CIBSE Guide A – Environmental Design
• BS EN 16798-1 (Indoor environmental input parameters)
• Part L (Energy Efficiency Regulations)
• EPBD (Energy Performance of Buildings Directive)
Overheating in buildings analysis is often carried out alongside energy modelling to balance winter efficiency with summer comfort.

How Is Overheating in Buildings Assessed?

1. Dynamic Thermal Modelling (DTM)
   Software simulation model:
   • Solar radiation
   • Thermal mass
   • Ventilation rates
   • Occupancy patterns
   • Internal gains
   • Future climate weather files (e.g. 2030, 2050 scenarios)
   Static SAP or DEAP calculations do not accurately predict overheating risk. Dynamic modelling is required for compliance with TM52 and TM59.

Strategies to Prevent Overheating in Buildings

Effective overheating in buildings mitigation includes
Solar Control
• External shading devices
• Overhangs
• Brise soleil
• Solar control glazing
Window Design
• Balanced glazing ratios
• Avoid excessive west-facing glazing
Ventilation Strategy
• Cross ventilation
• Night purge ventilation
• Mechanical ventilation where natural ventilation is insufficient
Thermal Mass
• Concrete floors
• Exposed structural elements
• Materials that absorb daytime heat
Landscape & Urban Design
• Tree shading
• Reduced hard landscaping
• Green roofs

Why Overheating in Buildings Matters for Developers

Ignoring overheating can lead to:
• Planning delays
• Building control rejection
• Occupant complaints
• Reduced property value
• Future retrofit costs
With increasing climate pressures, overheating in buildings compliance is becoming as important as energy efficiency compliance.

Overheating & NZEB |Finding the Balance

Highly insulated NZEB buildings reduce winter heating demand but can increase the risk of summer overheating if poorly designed.
A balanced approach is required:
• Optimised insulation
• Controlled solar gain
• Effective ventilation
• Accurate modelling at the design stage
Energy efficiency without comfort is not sustainable design.

Do You Need an Overheating Assessment in Ireland?

While Ireland does not yet have a direct equivalent to Part O, overheating assessments are increasingly requested for
• Apartment blocks
• Large residential schemes
• Passive House projects
• Planning applications
• Investor-backed developments
Forward-thinking developers now assess overheating risk early to avoid costly redesign.

Conclusion

Overheating in buildings is no longer a future problem it is a current design risk in modern buildings.
By complying with Part O principles and assessing projects against CIBSE TM52 and TM59 criteria, developers can:
• Protect occupant health
• Improve long-term building performance
• Future-proof developments
• Reduce regulatory risk
As climate patterns continue to change, overheating analysis will become a core part of building compliance across Ireland and the UK.

Need an Overheating Risk Assessment?

If you are planning a residential or commercial development and require:
•TM52 assessment
• TM59 modelling
•Dynamic thermal simulation (IES-VE)
•Part-O compliance support
• Integrated energy & overheating modelling
Contact BES Consultants today for professional guidance and compliance support.
Let’s design buildings that are efficient in winter and comfortable in summer.