The Simplified Method of ‘Part O – Overheating’ Made Simple

2021 edition- for use in England

Part 1: The Simplified Method

Over the last few years, summer temperatures have been rising, and with high levels of insulation and air tightness, combined with a high percentage of glazed areas, the risk of overheating in our homes is increasing.

Pro Sustainability thermal comfort team, has produced this simple table based on the Part O document to assist you during the design process, the simplified method is actually simple to follow, but for sure restrictive. It mainly targets reducing solar gains and making sure that any excess heat can be purged.

Part O document takes effect on 15 June 2022 for use in England.

Application of Part O

The guidance in the Overheating O document applies only to new residential buildings only;

  • Residential (dwellings)
  • Residential (Institutional): Home, school or other similar establishment, where people sleep on the premises.
  • Residential (Other): Residential college, hall of residence and other student accommodation, and living accommodation for children aged 5 years and older.

Moreover it applies to the following:

  • Shared communal rooms and common spaces of buildings containing more than one residential unit
  • Live/work units
  • Mixed use developments (Part O would only apply to the residential units and their serving corridors)


O1 Overheating mitigation

O1(2)(a) Ensuring the overheating mitigation strategy is usable

What is the aim of O1?

The aim of requirement O1 is to protect the health and welfare of occupants of the building by reducing the occurrence of high indoor temperatures.

What are the methods to demonstrate compliance?

a. The simplified method

b. The dynamic thermal modelling method

When can the O1 Simplified Method be used?

The simplified method provides guidance on ways to limit solar gains and methods of removing excessive heat.

This method would be appropriate for most units, except:

  • Buildings with more than one residential unit which use a communal heating or hot water system with significant amounts of horizontal heating or hot water distribution pipework.
  • O1(2)(a) ‘Ensuring the overheating mitigation strategy is usable’ has different standards, if any of those mean that the simplified method cannot be met, dynamic thermal modelling should be used. For example, if external noise or safety is an issue.
  • Certain locations that are not well represented, for example Manchester City Centre (Appendix C of the original document)

What are the buildings’ categories used in the simplified method?

The strategy to reduce overheating risk is based on the categories defined in the simplified method;

  1. Location of the new residential building.
    1. ‘Moderate risk’ location – England, excluding high risk parts of London in (b).
    1. b. ‘High risk’ location – urban and some suburban parts of London detailed in Part O Appendix C as shown from the image below.
  2. With or without cross-ventilation. 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.

Limiting solar gains guidance

The building’s overheating risk category based on location and whether it is cross-ventilated should be used to select the relevant guidance, where the following should be followed:

a. The maximum glazing area of the building or part of the building should be determined using the orientation of the façade that has the largest area of glazing.

b. The maximum glazing area of the most glazed room should be determined using the orientation of the façade that has the largest area of glazing.

c. Shading for buildings in the high risk location

Residential buildings in the high risk location should, in addition to following the maximum glazing areas, provide shading for glazed areas between compass points north-east and north-west via the south.

Shading should be provided by one of the following means.

a. External shutters with means of ventilation.

b. Glazing with a maximum g-value of 0.4 and a minimum light transmittance of 0.7.

c. Overhangs with 50 degrees altitude cut-off on due south-facing façades only.

Removing excess heat guidance

Minimum requirements of free areas are given for the different buildings’ categories.

The total minimum free area is the free area for the whole dwelling house, residential unit, shared communal room or common space, including any bedrooms.

Buildings or parts of buildings with no cross-ventilation should equal or exceed the minimum free

areas shown in the guidance.

Openings should be designed to achieve the free areas in paragraphs 1.10 and 1.11. The equivalent

area of the opening should be assessed by either of the following means.

a. Measurement of the product to BS EN 13141-1.

b. Calculation using Appendix D.

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