Part O Glazing Limits – The Simplified Method

In this quick article we’ll cover the glazing limits needed to pass the Part O simplified method. The limits change depending on location and ventilation method. For overheating compliance, there are two glazing metrics a property needs to satisfy which change with their orientations.

• Facade with the most glazing – Maximum of 11%-18% of total floor area
• Room with the most glazing – Maximum of 11%-37% of the room’s floor area.

How Does Location Affect Part O Glazing Limits

Part O designates urban and some suburban parts of London as being at high risk of overheating. This is due to the urban heat island effect which causes localised elevated temperatures.

In these areas the maximum areas of glazing in dwellings has been reduced to limit the amount of solar heat gain.

In London, in addition to the Part O designation of being a high risk area, local planning guidance – the London Plan – specifically mandates that all planning applications require a full overheating assessment using dynamic simulation following CIBSE TM59. This rules out the use of the Part O Simplified Method for most new dwellings in Greater London.

Other areas in England also encourage TM59s as part of the application process, particularly in dense, urban areas of Manchester, Birmingham and Bristol and for properties will high amounts of glazing or multi unit developments. None have the blanket requirement like London though.

If your project is located within a high risk location within London, and a TM59 is not required, the glazing limits for the Simplified method are

• Facade with the most glazing – Between 11%-18% of the total floor area
• Room with the most glazing – Between 11%-37% of the room’s floor area

In reality, pretty much every new dwelling in London will need a full TM59 Dynamic Simulation Model (DSM) and the simplified method will not be relevant.

Ventilation and Part O Glazing Limits

Approved Document O of the Building Regulations (Part O) sets different glazing limits for dwellings that are cross ventilated and those that are not.

For a dwelling to be cross ventilated, it needs to have openings on opposite facades. Corner flats that have openings on adjacent facades are not cross ventilated.

To discourage the use of mechanical cooling, like air conditioning units, Part O reduces the maximum glazing areas to make these properties more difficult to pass the simplified method.

The table below shows the glazing limits for both cross ventilated and assumed mechanically cooled properties in a moderate risk location. As discussed above, The simplified method is highly unlikely to be relevant for a project within a high risk area.

The table above shows that overall glazing remains the same in relation to the floor area of the dwelling across both ventilation types. However, there are enhanced restrictions on the room with the most glazing. If the room is orientated in the south or west directions, the total glazing permitted in these rooms halves. North and east orientated ‘most glazed’ rooms see their maximum levels of glazing drop by 30%.

The Part O ‘glazing in the most glazed room’ restriction is particularly challenging in a south facing kitchen dining area with large bifolds in a property with mechanical cooling.

Window opening sizes for Part O

Within Part O, the amount a window opens is measured in ‘free area’. This is the actual amount of area open to the outside. In the same way as window sizes, targets change for cross ventilated and non cross ventilated dwellings. For a property in a moderate risk area (the majority of the UK), these minimum free areas are in the table below

The first action needed here is to work out what works out as the greater number, 9% (12% for mechanically cooled) of the total floor area or 55% (80% for mechanically cooled) of the total glazing area. The free area of the windows in the entire home needs to add up to at least the greater of these two numbers.

Part O has an additional check for Bedrooms, where the free area needs to be a minimum of 4% of the floor area.