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What is Part O of Building Regulations?

Part O? Overheating Assessment? Dynamic simulation modeling? all very familiar words for a building physics and sustainability consultant, however since the new Part O of Building Regulations has been implemented, a huge number of questions have been raised for the rest of the industry. We will do our best to answer these questions below.

What is Part O?

Part O is a new Approved Document released as part of the Building Regulations for England. The regulations for Wales and Scotland have been separated and these will be implemented at later dates. The Approved Document Part O covers overheating within dwellings and living accommodations.

Overheating in dwellings has a wide range of implications that should be considered and eliminated during the design stage. These range from reduced thermal comfort for the occupants during the summer months resulting in a lack of sleep, which can have ‘knock-on’ effects on the occupant’s health, to increased carbon emissions and energy usage through the implementation of mechanical cooling or ventilation.

What needs to comply with Part O?

The Approved Document Part O currently applies to new developments, the document does not currently expand to change of use or extension to existing dwellings. if you are constructing a new dwelling, you will need to ensure compliance to Part O through one of the 2 sections available for compliance.

The implementation date of Part O: Overheating is the 15th of June 2022, if you are submitting your project to Building Control after this date, then Part O will be mandatory.

How do you comply with Part O?

Within the Part O: Overheating Approved Document, two options for showing compliance are listed

Section 1 – Simplified Method

For the simplified method, the strategy to reduce overheating risk should be selected according
to the location of the new residential building and whether it has cross-ventilation, following
paragraphs 1.3 to 1.5.

1.3 For the purposes of following the simplified method, the building’s overheating risk category is
determined by its location in one of the following areas.

a. ‘Moderate risk’ location – England, excluding high risk parts of London in (b).
b. ‘High risk’ location – urban and some suburban parts of London detailed in Appendix C.

NOTE: Appendix C also provides guidance for some parts of central Manchester.

1.4 For the purposes of following the simplified method, it should be identified whether the
dwellinghouse or each residential unit, shared communal room and common space is able to have
cross-ventilation, i.e. it has openings on opposite façades.

NOTE: Having openings on façades that are not opposite does not meet the approved document
definition of cross-ventilation, e.g. in a corner flat.

NOTE: A multi-occupancy residential building should not be categorised as having or not having
cross-ventilation. Each residential unit, shared communal room and common space should be
categorised separately.

1.5 The building’s overheating risk category based on location and whether it is cross-ventilated should
be used to select the relevant guidance for both of the following purposes.

a. To limit unwanted solar gains in summer – follow paragraphs 1.6 to 1.9.

b. To provide an appropriate means of removing excess heat from the indoor environment – follow
paragraphs 1.10 to 1.13

Section 1 Summary

In summary, the dwelling must meet specific requirements to be able to show compliance through section 1, these are based on the Location of the site, if the dwelling has Cross Ventilation design & specific restrictions on the size of glazing on the dwelling. If the dwelling meets all of these requirements, then a checklist can be completed showing compliance to Part O via Section 1.

if any of the criteria are not met, Section 2 must be completed.

Section 2 – Dynamic Simulation Modelling

2.3 To demonstrate compliance using the dynamic thermal modelling method, all of the following
guidance should be followed.

a. CIBSE’s TM59 methodology for predicting overheating risk.

b. The limits on the use of CIBSE’s TM59 methodology set out in paragraphs 2.5 and 2.6.

c. The acceptable strategies for reducing overheating risk in paragraphs 2.7 to 2.11.

2.4 The building control body should be provided with a report that demonstrates that the residential
building passes CIBSE’s TM59 assessment of overheating. This report should contain the details in
CIBSE’s TM59, section 2.3.

NOTE: Appendix B of this approved document includes a compliance checklist. The designer may
use this checklist to demonstrate compliance to the building control body.

Section 2 Summary

If Section 1 criteria can not be met, the site is within ‘High Risk’ locations or has restrictions on window openings i.e. acoustic requirements, Dynamic Simulation Modelling (Section 2) should be undertaken.

Dynamic Simulation modelling consists of developing a 3D model of the development and running simulations based on the development specification and parameters to further understand the internal temperatures of living, kitchen, and bedroom areas. the model delivers much higher accuracy than section 1 and can assist in designing out any overheating risks.

Can I show compliance without appointing a consultant?

Section 1 can be completed by a designer on the project and doesn’t have to be done by a sustainability consultant for example. however, the designer must be competent in the design of dwellings to accurate provide and review the information within Part O. Alternatively an external consultant can assist with this.

Section 2 must be undertaken by a consultant with adequate skills and qualifications to undertake an accurate dynamic model.

How much does an overheating assessment cost?

The cost of an overheating assessment can vary based on the size, location, number of units, and design of the proposed development. the costs for a dynamic thermal model can range between £250 to £5000+

What are the first steps to getting an overheating assessment?

Reviewing compliance to Part O is always best to be completed as early in the design stage as possible. If you are currently designing or working on a project, we would suggest getting in contact with a sustainability/building physics consultant to discuss the scheme and determine the best route and costs for ensuring compliance. If Vision Energy & Sustainability is your consultant of choice, get in touch on the below details

Email –

Phone – 01625 315040

Sustainability & Building Compliance

Vision Energy are a leading Sustainability & Energy consultancy working nationwide. We specialise in calculations & reports covering Building Regulations, Planning Permission and all other sustainability or energy needs within the construction sector. With a vast amount of knowledge and experience having worked on thousands of projects across the UK one of our engineers would be more than happy to assist you on your project. SAP Calculations | SBEM Calculations | Energy Statements | BREEAM Pre-Assessments | M&E Design | EPC's | Water Efficiency | Air Permeability

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