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  • Cliff L'Aimable _Chartered Surveyor & Building Engineer

Part O - Overheating

Updated: Nov 29, 2022

Part O of the Building Regulations: Overheating Rules for New Homes
Blog Post May 2022 by Cliff L'Aimable - at Building Control Surveyors Corporate Approved Inspectors

My company and team offers private Building Regulation Approvals, technical design checking and site inspections of all building projects. Tel 01992 710 763 EmaIL support@bcsurv.com


In 2022, Building Regulations Part O will introduce new requirements to safeguard new home occupants from overheating.


In the past, many professionals did not pay much attention to overheating when designing and constructing new homes, but with the publication of Building Regulations Part O: Overheating, this changes.


The new Building Regulations (Part 0) took effect on 15th June 2022 and while the provisions in Part O are not designed to guarantee comfort, they are there to protect the occupants’ health and welfare.


Here’s what you need to know about Building Regulations Part O as a developer builder, following its introduction this year.


What is Part O of the Building Regulations?

Part O: Overheating -primary objective is to restrict excess solar gain in buildings and to offer mechanisms for rapidly removing the surplus heat from the interior environment.





Solar gain is the increase in space or building temperature, caused by the sun's heat, which can lead to overheating in the summertime in particular.


In addition, the regulations will address overheating caused by uninsulated heating pipes, cylinders, or a lack of heating controls.


Part O is indeed important. It means that we can make more informed decisions about how we design homes and buildings, as well as address design issues that can affect overheating risks, which will become more pronounced as a result of climate change.


It is important to note that Part O does not apply to glazed extensions, but if you are constructing an extension and want it to be habitable and healthy, you should follow the Part O guidelines.


Why is Overheating Important?

Historically, most concerned in homebuilding placed great focus on generally insulating and draft-proofing buildings to retain heat throughout the winter.



Photo: SIPS Panelled building

However, when construction involves lighter weight materials such as timber frame and Structural Insulated Panels (SIPs), the lack of thermal mass that might contribute towards limiting overheating is problematic.


Since thermal mass stores the thermal energy, which locks the heat away and prevents overheating.


A limited portion of the Standard Assessment Procedure (SAP), used to evaluate the energy performance of dwellings, previously examined overheating, but only as an average for the entire dwelling.


Here is an illustration of why Part O will be advantageous: If a 15m² glass box was built on the side of a 350m² solid wall property then it is highly likely that the overheating potential would not be flagged up during the build because there would not be enough heat generated in the ‘glass box’ to lift the average temperature in the whole house beyond the risk level. Therefore, the box may be uninhabitable.


Now, Part O specifies that rooms must be evaluated in isolation and that suitable cooling arrangements must be provided to safeguard the inhabitants' health.


How do you calculate the risk of overheating?

There are two primary methods for calculating the probability of a home overheating:


• The Simplified Procedure

• Computational Thermal Dynamics


Dynamic modelling is likely to be a costly procedure, whereas the basic solution would be far more intuitive and straightforward to implement in a regular home design.


When should the Simplified Method be used?

The Simplified Method can be utilised in moderate and high-risk locations for overheating, providing homebuilders with options for minimising solar incidence and removing excess heat.


You can download the Part O Approved Document from the governments website (https://www.gov.uk/government/publications/overheating-approved-document-o), it offers a step-by-step guide to using the Simplified Method, postcode lists for high-risk locations, and actions required if you have provisions for cross ventilation. The checklist in Part O Appendix B can be completed and submitted to us in Building Control to demonstrate your compliance.


When would dynamic thermal modelling be used?

The method of Dynamic Thermal Modelling can be utilised for more complex buildings and when higher precision and interpretation are required.

The CIBSE Thermal Modelling programme (TM59) is used to precisely duplicate the building's shape and plot it against the possibility for seasonal or hourly overheating.


This tool can then be used to calculate the possible danger of overheating in particular rooms and at specified times of day. It is also possible to model various mitigation strategies, including as window coatings, shading, shutters, and blinds, to determine their effectiveness.


What Should the Temperature of my Rooms Be?

A maximum of 1% of yearly sleeping hours, or 32 hours per year, may be spent in bedrooms with temperatures exceeding 26 degrees Celsius. This assessment does not include daytime hours, thus sleeping in on a hot summer day does not count.


The remainder of the residence and daytime hours are subject to various computations with temperature limitations ranging between 20 and 25°C. This is the so-called "comfort temperature," and the rest of the home (including bedrooms during the day) cannot exceed it by more than 1°C for more than 3 percent of the year.


Targets distinguish between dwellings with natural ventilation and those with mechanical ventilation. There must be a window (or windows) that can be opened at least 1/20 of the floor area for the space to be naturally ventilated.


If there are sufficient openable windows to cover the area but there is a viable reason not to open them, such as a noisy road, train track, or airport, or if they cannot be opened for security or safety reasons, an exception to this rule exists.


If there are sufficient openable windows to cover the area but there is a viable reason not to open them, such as a noisy road, train track, or airport, or if they cannot be opened for security or safety reasons, an exception to this rule exists.


Is compliance with overheating regulations mandatory?

Well Yes. The Building Regulations stipulate that suitable provisions must be made for a dwelling, including one or more rooms for residential uses in order to reduce undesired solar gains in summer and provide adequate ways of removing heat from the interior building environment.


The simplified method or TM59 models are not mandatory, as they are merely the method to demonstrate compliance with Part O, but TM59 could be a very useful design tool, not only for new build properties, but also for building extensions that are at risk of high solar gain and could become an extremely uncomfortable living area.


Before construction begins, any overheating risks can be assessed and mitigated if these calculations are carried out prior.



When does Part O become active?

Part O went took effect on 15 June 2022, with a grace period for structures that are still under construction lasting until 2023.


If you submitted an initial notice prior to 15 June 2022, your project will continue to be evaluated under the old Building Regulations as long as construction commences prior to 15 June 2023.


If you acquired Building Regulations approval under the old regulations, however, you must commence construction by June 15, 2023, otherwise the new regulations will apply.

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