Part O of the Building Regulations 2010 (Part O), which deals with preventing overheating in new build residential buildings, came into force on 15 June 2022.
It is an important new regulation which reflects concerns over a lack of regulatory controls regarding the impact of hot summers and ever-increasing heatwaves on the health and wellbeing of residential occupants.
Developers and designers of residential buildings need to factor in the requirements of Part O at an early stage of the design and development process. Guidance for complying with Part O is found in “Approved Document O: Overheating” (ADO).
Which buildings does Part O apply to?
Part O applies to “residential buildings”. As set out in the ADO, this definition covers:
- residential dwellings (including houses, flats and apartments);
- institutional residential buildings such as schools and care homes; and
- other residential buildings including student accommodation.
A unit that contains accommodation as well as space for a commercial purpose (e.g. a workshop) may also be caught by the scope of Part O if certain conditions apply.
Regulation O1 - What developers must do
In designing and constructing new residential buildings, developers must make reasonable provision to:
- limit unwanted solar gains in summer; and
- provide an adequate means to remove heat from the indoor environment.
ADO distinguishes between buildings in a “high risk location” and a “moderate risk location”. A table of postcodes sets out the two categories (with “high risk” locations being those densely populated urban locations like central London). The risk location in which a building is based affects the steps which must be taken to limit solar gains and remove excess heat.
Regulation O1 - Measures for compliance
Compliance with the regulations can be demonstrated by either:
- the Simplified Method; or
- Dynamic Thermal Modelling.
Alternative approaches to compliance are possible, but any such approach should be worked through and agreed with a building control body before any construction works begin.
1. The Simplified Method
The Simplified Method deals with the size and design of the windows on the building, minimising the amount of glazing on all non-north facing facades while making sure that there is satisfactory openable window area to mitigate against overheating.
2. Dynamic Thermal Modelling
Dynamic Thermal Modelling uses a model of the building, employing dynamic thermal modelling software and the standard set of data inputs from the CIBSE TM59 methodology.
The model will need to demonstrate that the internal temperature within the building space does not exceed a certain temperature for a certain number of hours.
Dynamic Thermal Modelling takes a holistic approach to the building and is expected to be the method of choice for developers on larger-scale projects given that it allows for greater design flexibility.
Regulation O2
The second section of Part O provides that in meeting the obligations of Regulation O1:
- account must be taken of the safety of any occupant and their reasonable enjoyment of the residence; and
- mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it.
According to the APO, developers can demonstrate that they have taken account of the occupants’ safety and enjoyment of the residence if they considered all of the following:
- noise (in particular the likelihood that windows may be closed during sleeping hours if external noise is an issue that affects the building);
- pollution;
- security;
- protection from falling; and
- protecting from entrapment.
Implications of Part O for developers?
There is considerable uncertainty surrounding the implementation of Part O, and developers should be aware of the following issues.
Impact on design
Reducing the risk of overheating must form part of the design process in tandem with other key elements including views, daylight, ventilation and optimal winter solar gains.
Early intervention
Developers should consider compliance with Part O right from the start of the design process. Engagement with the designers and any specialist consultants (e.g. noise / air quality) will be vital to ensure a compliant integrated design solution and avoid the need for changes down the line / planning-associated amendments.
Cost
Obtaining specialist consultant advice at an early stage will be an additional cost, but this cost is likely to be offset if the need for additional ventilation or cooling equipment, or retrospective changes to the design, are avoided.
The cost of Dynamic Thermal Modelling is also a cost for developers to bear in mind where this is the compliance method of choice, and demand for such assessments is expected to rise steeply.
Sustainability
The APO is clear that passive means of cooling / ventilation are preferred, and these have the advantage of being more sustainable in the long term.
Where mechanical cooling or ventilation is required, its energy-generating impact will have an adverse impact on projects aiming for a ‘net-zero’ status.
Modular construction, often praised for its ‘greener’ credentials including less waste and a quicker, more energy-efficient production process, will need to factor in the need for each unit or module to be Part O compliant. Attention must be paid to the location of the unit on site, including geographical facing and proximity to noise or pollution sources.
Conclusion
Do not get caught out by Part O! It is a deceptively modest regulation, which may have far reaching effects, particularly on city centre residential developments.
