Overheating: Part O & Your New Home

Overheating isn’t often the first thing that comes to mind when thinking of British housing stock. Traditionally UK houses were not built with thermal efficiency in mind – they were uninsulated, ‘draughty’ and relied on pumping heat into the structure through fires and then central heating in more modern times.

For newer houses, especially those built in the last decade, greater focus on thermal efficiency in the Building Regulations has resulted in much more efficient houses. Some of these houses have the opposite issue: they are too thermally efficient, they can get too hot, and are difficult to cool down to acceptable levels when they do get too hot.

In response to these problems the government has introduced a new set of regulations governing new builds in England: Part O, taking effect from June 2022 as part of a larger package of changes which is being called FLOS: Parts F and L have changed, and Part O and S are new.

The new regulations are designed to make sure that new buildings don’t (or don’t often) get too hot, and when they do the occupiers are able to cool them down easily and ‘securely’.

There are two routes to achieve compliance with the new regulations:

• the ‘simple’ method
• dynamic thermal modelling

The route your Architect ultimately chooses will have a large impact on your design. Here is a breakdown of each route:

The ‘Simple’ Method

‘Simple’ is in quotation marks for a reason! This method assumes that the building will be cooled by natural ventilation (so no MVHR allowed, windows that open and ventilation panels if necessary).  It then allows a certain amount of glazing in proportion to the floor area of the building, and mandates a minimum amount of ‘free areas’ (openings where the outside air can get in) based on several conditions:

• Which area of the country the building is in.
• Which of the four main compass points the most glazed façade is closest to.
• Whether cross-ventilation is possible.
• In certain cases(!) – whether any kind of shading has been used.
• On top of this – it limits the amount of glazing allowed in the most glazed room of the most glazed façade (dependent on whether cross ventilation is possible or not).

Cross ventilation is defined as allowing windows/vents to be open on opposite facades of the building on every storey, so houses with basements/partially underground storeys are very difficult to cross-ventilate.

To work all this out, the glazing area of all the windows and doors needs to be measured, and the opening area and maximum opening angle of each openable window needs to be calculated. If the number of glazing/openable areas falls outside the criteria listed above, then the amount of glazing needs to be reduced and/or the amount of ventilation needs to be increased until the criteria for a pass are met.

The one advantage with this method is that it doesn’t require a specialist to produce a thermal model to show the building will ‘pass’ Part O, the calculations (though a little complicated to wrap your head around) can be done by the architect.

There are several large disadvantages with this method though:

• No MVHR allowed.
• The combination of maximum glazing and minimum openable area limits is very constrictive.
• Only a very basic shading is considered under a very specific set of circumstances.

Dynamic Thermal Modelling

When the simple method fails, we need to turn to Dynamic Thermal Modelling. This is a method of modelling the real-world predicted heating/cooling cycle of buildings using a method developed by the Chartered Institute of Building Services Engineers (CIBSE): TM59.

The Dynamic method allows all the things the Simple method does not. Such as, shading and MVHR, allows flat roof lanterns and takes a more detailed account of the orientation of the building. This allows for greater design flexibility, elements which fail in the Simple method can be offset in other areas. This is a huge advantage.

The major disadvantage of this method is of course that it needs specialist modelling software and therefore an outside consultant is required, which is an additional cost to the client. It does however allow design creativity to flourish away from the restrictions of the simple method.

Part O (along with the rest of the regulations changes) is already having a large impact on the way we design buildings to be more energy efficient and comfortable to live in and use. As a practice we will always ensure that we use the appropriate method and tools to meet our clients’ wishes.