Part O Simplified Method – A Quick Guide

Approved Document O of the Building Regulations (Part O) sets out how building designers should minimise the risk of overheating in new dwellings. Most homes with a reasonable amount glazing should be able to pass Part O using the simplified method.

The simplified method is a checklist of different criteria that changes based on the dwellings risk to overheating. So, how do you determine the overheating risk of your building?

1. Location
2. Ventilation Type

Part O – Moderate Risk vs High Risk Areas

The majority of the UK is classified as a ‘moderate risk’ for summer overheating.

There are two areas within the UK where this is elevated to a ‘high risk’ of overheating.

• Urban parts of London
• Some suburban parts of London

England and Wales use their own individual versions of Part O and Scotland uses Building Standard 3.28. The guidance in these documents is similar, but compliance targets are different. Here we are looking at the English version of Part O only.

The Part O simplified method is still available to projects in high risk areas, with a higher pass criteria. However, local planning guidance, such as the London Plan, mandate the more complex CIBSE TM59 route to overheating compliance for all dwellings. So although Part O may allow the simplified method, local regulations may not.

How Does Ventilation Affect Part O Compliance?

Part O recognises two ways of managing overheating within a building.

1. Cross Ventilation
2. Mechanical Cooling (air conditioning)

Why does this matter? Dwellings that can be naturally ventilated with cross ventilation have a more relaxed target for passing the Part O simplified method. This is to discourage designers from specifying air conditioning units in every close-to-the-wire unit.

For a building to be classified as being cross ventilated, it needs to have openings on opposite facades. The guidance specifically states that a corner flat with two external facades is not classified as being cross ventilated.

Simplified Part O Glazing Levels

Once the risk factor and ventilation strategy are determined for the building, we will have target glazing levels to achieve.

We then need to identify two key areas of the design

1. Which facade has the highest level of glazing,
2. Which room has the highest level of glazing and it’s orientation.

In Table 1.1 below, you can see the maximum glazing levels allowed to be able to pass Part O for a cross ventilated building in each risk category. Part O defines glazing area as the area of transparent material, not including the window frame.

For dwellings that use mechanical ventilation (air conditioning), these maximum glazing levels are reduced. Table 1.2 shows what these maximums are.

For dwellings in high risk areas additional overheating mitigation needs to be implemented for glazed areas between compass points north east through north west via south. Shading should be provided by one of the following methods.

• External shutters with ventilation
• Glazing with a maximum g-value of 0.4 and minimum light transmittance of 0.7
• On due south facades, overhangs with 50 degrees altitude cut off

Part O and Opening Sizes

Referred to as ‘free area’ within Part O, the amount that windows open needs to achieve a minimum number in relation to the floor area. Again, these targets change based on risk factor and ventilation strategy.

Part O defines free area as

The geometric open area of a ventilation
opening. This area assumes a clear sharp-edged
orifice that would have a coefficient of discharge
(Cd) of 0.62.

For a cross ventilated dwelling the free areas need to achieve minimum amounts. Table 2.1 below describes these minimums for moderate and high risk projects.

Again for mechanically ventilated dwellings, these numbers are much harder for compliance. Table 2.2 shows these figures.

Above, you can see that compliance with the simplified method to pass Part O in a high risk location in a dwelling that is mechanically ventilated requires total glazing across the dwelling to have a free area of 95% where a cross ventilated dwelling in a moderate risk area only requires 55%.

This pattern is repeated across every Part O compliance metric. It will be significantly easier to pass a dwelling with the simplified method that is outside of a high risk location and is naturally cross ventilated.

The final point in Part O with relation to the simplified method is that where a communal heating or hot water system is used within a residential unit with multiple dwellings, the simplified method cannot be used. Heat gains for the shared pipe work cannot be modelled correctly and the dynamic simulation method must be used.