Overheating in UK Homes

The Stats That Demand Action

Overheating is no longer a rare issue—it’s a growing public health risk. The introduction of Part O of the Building Regulations was driven by clear evidence of widespread heat-related discomfort in UK homes.

Part O Key Statistics: 80% of homes overheat

80% of UK homes experienced overheating

A 2022 study by the University of East London found that 80% of UK homes experienced overheating during recent heatwaves—up from just 18% in 2011.

This isn't just discomfort - it's a design and compliance failure.


75% of Living Rooms and 57% of Bedrooms Overheated

In a study of post-2010 low-energy homes, three-quarters of living rooms and over half of bedrooms exceeded comfort thresholds (>25°C for more than 5% of occupied hours). Many of these homes included MVHR systems.


•    Overheating persisted despite high insulation and air tightness
•    MVHR systems without summer bypass or adequate airflow failed to cool
•    Rooms with poor cross-ventilation or sealed windows performed worst

Read our blog on overheating for more information

Blog
75% of Living Rooms and 57% of Bedrooms Overheated

MVHR Systems Can Exacerbate Heat Without Proper Design

Surprisingly, homes fitted with MVHR sometimes showed:


Higher average indoor summer temperatures

While MVHR systems are designed to retain warmth in winter, they can unintentionally retain heat during the summer if not properly equipped with a summer bypass. Without this feature, the system continues to recover heat even when it’s not needed, causing warm air to recirculate indoors.


Reduced temperature fluctuations

MVHR systems create a sealed, airtight environment with controlled ventilation. While this improves energy efficiency, it can also mean that internal heat—whether from cooking, appliances, or solar gain—has nowhere to escape. Without natural ventilation or active cooling, the heat becomes "locked in," leading to consistently warm indoor conditions with little variation between day and night. 


Night-time overheating

Bedrooms are particularly vulnerable to overheating at night if windows are closed for noise, pollution, or security reasons. In these cases, an MVHR system must provide adequate "purge" ventilation—short bursts of high airflow that help flush out warm air and bring in cooler outside air. Without this functionality, heat lingers in bedrooms overnight, leading to uncomfortable sleeping conditions.

Part O of the Building Regulations was introduced to reduce the risk of overheating in new residential dwellings in England. With homes becoming more airtight and summers growing hotter, overheating is a growing health risk — especially in flats and urban developments.

Key aims of Part O:
•    Limit unwanted solar gain
•    Ensure effective removal of excess heat
•    Support health, safety, and comfort of occupants


Applies to: New-build homes, including flats, houses, and student housing
Doesn’t apply to: Extensions, conversions, or non-residential buildings

Doesn’t apply to: Extensions, conversions, or non-residential buildings

See Building Regulations


What causes overheating

Overheating is typically caused by a mix of design and environmental factors, including:

•    South- or west-facing glazing without shading
•    Lack of cross-ventilation or secure nighttime openings
•    Inadequate ducting or undersized ventilation systems
•    High internal heat gains from people and appliances
•    Over-reliance on sealed MVHR without bypass options

In some cases, flats have recorded internal temps exceeding 28°C for more than 10% of occupied hours — well above comfort guidelines.

Blog
What causes overheating

Simplified Method for Routes to Compliance (For Low-Risk Homes)

•    Limits glazing by orientation
•    Requires shading on large glazed areas
•    Night cooling assumed via open windows
•    Suitable for suburban homes with good cross-ventilation

Dynamic Thermal Modelling for Routes to Compliance (For Complex Homes)

•    Required for flats, single-aspect units, or urban areas
•    Assesses risk using CIBSE TM59 modelling
•    Offers design flexibility — but needs expertise

ResiDUCT™

Part O:Overheating 
Where overheating planning conditions are in place for residential dwellings, ventilation airflow rates are typically increased. More airflow = larger ducting. 

The ResiDUCT™ range offers a low profile, high performance ducting option which is ideal for typical ceiling void sizes.

Installed Performance
Air leakage from duct connections within an MVHR system installation can be signficant. The MVHR unit  ends up having to compensate when it is commissioned, increasing operational noise and energy consumption for end users. 

The ResiDUCT™ system addresses air leakage with an innovative design that simplify connections, adds strength and prevents bowing as parts are connected together. 

At a time when installed performane is crucial to maintain comfort, ResiDUCT™ can offer performance advantages.

View our ResiDUCT Page for more information

Mechanical Ventilation: MVHR Design for Part O Compliance

In developments where natural ventilation alone cannot prevent overheating — especially urban apartments, single-aspect flats, or noisy locations — mechanical systems like MVHR (Mechanical Ventilation with Heat Recovery) become essential for compliance.


Key MVHR Considerations:

Increased Airflow Rates for Cooling

  • Use boosted summer ventilation rates during warm periods to flush out heat.
  • Controls can allow for manual override or temperature-triggered boosts.
  • Ensure the unit supports summer bypass mode to prevent heat recovery from adding to internal temperatures.

Larger Ducting for Higher Flow with Less Noise

  • To deliver high airflow without excessive fan speed or noise, consider larger diameter ducting (e.g. 125–160 mm vs standard 100 mm).
  • Larger ducting reduces air velocity, minimizing pressure drops and operational noise.
  • Oversized main ducts with branch ducts to individual rooms can optimize system performance.

See ResiDUCT Brochure 

Low-Profile Ducting for Shallow Ceiling Voids

  • Many modern low-rise or modular homes have limited ceiling voids.
  • Use oval or flat-oval ducting systems (e.g. 204x60mm or 220x90mm) to maintain high airflow in tight spaces.
  • These systems can be routed easily within service zones, behind plasterboard, or above bathrooms/kitchens.

What is the maximum allowable temperature under Part O?

There’s no absolute max temp. Instead, overheating is assessed using metrics from CIBSE TM59, e.g. internal temps exceeding 26°C for more than 3% of occupied hours.

FAQs

Does Part O apply to existing homes or extensions?

No. Part O only applies to new residential buildings, including flats, houses, and student housing. It does not apply to extensions, conversions, or non-residential buildings.

Does Part O set a maximum indoor temperature?

No. Part O doesn’t set a fixed maximum temperature. Instead, it uses thermal modelling (e.g. CIBSE TM59) to assess overheating risks, such as indoor temperatures exceeding 26°C for more than 3% of occupied hours

What is a summer bypass in an MVHR system?

A summer bypass allows the MVHR unit to divert warm air around the heat exchanger, so it doesn’t recover heat during warmer months. This helps expel hot indoor air and bring in cooler outdoor air to reduce overheating.

What type of homes require dynamic thermal modelling for compliance?

Flats, single-aspect dwellings, or homes in urban and high-risk areas usually require dynamic thermal modelling (CIBSE TM59) rather than the simplified method, to accurately assess and mitigate overheating risk.

Why are MVHR systems sometimes linked to overheating?

MVHR systems can trap heat if they lack a summer bypass, boost airflow, or adequate ducting. Without these, warm air may circulate instead of being expelled, particularly in sealed, airtight homes. Read more about this earlier in the page

Why is duct size important in MVHR design?

Larger ducting (e.g. 125–160mm) allows for higher airflow with less resistance and noise. Smaller ducts may cause pressure build-up and force the MVHR unit to work harder, increasing energy use and reducing comfort.
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