Part O Overheating Calculations

Part O Overheating Calculations

With the ever-increasing fabric performance and higher levels of air tightness in new buildings being driven by Part L, Overheating is an unintended consequence that has to be addressed now. In 2021 the UK Government welcomed Part O to the Building Regulations Party, the first of its kind, which will tackle overheating in new residential buildings.

Having taken effect in June 2022, Approved Document O was introduced to safeguard the health and welfare of occupants within a building by setting out new rules to protect them from overheating within new homes.  The principal aims of Part O is to encourage the design and construction of buildings that mitigate overheating risk by;

  • Limiting unwanted solar gains within summer.
  • Provide adequate means of quickly removing excess heat from indoor environments.

Do I need a Part O Overheating Assessment?

Yes, all proposed dwellings that submitted for Building Regulations approval after 15th June 2022 (England) and 21st November 2021 (Wales) need to comply with the new Part O regulations.  If your proposed building falls into these categories, Part O compliance will be required.

Purpose for which the building is intended to be used.

Residential (dwellings)
Dwellings, which includes both dwelling houses and flats

Residential (institutional)
Home, school, or other similar establishment, where people sleep on the premises.

  • The building may be living accommodation for the care of any of the following;
  • -Older and disabled people, due to illness or other physical or mental condition.
  • -People under the age of 5 years old.

Residential (Other)
Residential college, hall of residence and other student accommodation, and living accommodation for children aged 5 years or older.

Note, shared communal rooms and common spaces that contain more than one residential unit are also covered by Part O.

If your development falls into any of the above categories and you would like to discuss further with our in-house Part O specialists, please get in touch here.

What information do we require to complete your Overheating Assessment?

  • Floor plans
  • Elevations showing which windows/doors are openable
  • Sections
  • Precise orientation angle (usually on a site plan)
  • Construction specifications and u-values
  • Window u-values and g-values (we can advise what helps to achieve compliance)
  • Heating/cooling specification
  • Ventilation specification (we can advise if mechanical ventilation is required)

So, how do we calculate Part O?

There are two methods of determining the overheating risk of a new building…

Simplified Method

Approved Document O details a Simplified method than can be used to provide a different way to assess the likelihood of overheating so that a more complex TM59 assessment doesn’t have to be done where it isn’t necessary. 

The simplified method focuses on two areas, limiting solar gains and removing excess heat.  The requirements to achieve compliance with these two areas differs depending on the location of the building and whether or not it has provision for cross ventilation.

Are you ready for the ‘Simplified’ bit?

The simplified method looks at the area of glazing in proportion to the floor area and proposes a minimum free area (the geometric open area of a ventilation opening) that must be achieved.

Please note:  This is called ‘simplified Method’ for a reason and is not always an appropriate method to use in all buildings, please get in touch. One of our specialist colleagues can advise whether a simplified method is applicable.

How is compliance assessed under the ‘Simplified Method’?

Part O Appendix B provides a checklist that can be filled out and then provided to building control to demonstrate compliance.

Dynamic Thermal Modelling: CIBSE Thermal Model 59 (TM59)

Using world leading, state of the art thermal modelling software, we can offer TM59 Dynamic Thermal Modelling Assessments to assess your buildings risk of overheating and whether you comply with Part O.

CIBSE’s TM59: Design Methodology for the assessment of overheating risk in homes is the backbone of the thermal modelling method, it defines the criteria for when overheating will/will not occur as set out below:

How can I guarantee Part O compliance via ‘Dynamic Thermal Modelling’?

To ensure compliance with the dynamic approach all the following must be true…

  • CIBSE TM59 is used
  • All TM59 limits have been used from section 2.5 & 2.6 of part O.
  • Acceptable items have been used to reduce overheating as defined with section 2.7-2.11 of Part O

Our assessor can then provide you with a fully compliant report to pass onto Building control. 

Our team of dedicated Part O experts will provide you with CIBSE (Chartered Institution of Building Services Engineers) TM59 assessment showing the unit has complied with Part O Building Regulations.

We’re proud to have been offering TM59 assessments long before they became a mandatory requirement.

Get in touch here and our dedicated team can guide you through process.

If you would like some bedtime reading, we have put together a detailed technical breakdown of the Simplified & Dynamic Thermal Modelling methods, please visit here.

What if my building fails Part O?

We would be lying if all projects pass at their first attempt. There are a number of factors that impact a Part O assessment, including SAP outcomes, air tightness, noise, pollution and security.  If the building doesn’t pass, we relish the opportunity to find a solution. We’ll combine our extensive experience and knowledge, communicating with all members of the design team to ensure that our proposed solutions work for everyone involved.

Get in touch

Melin Energy Consultants

01554 571010

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