Part O: Overheating, What you need to consider

summer overheating in buildings new UK building regulations passivhaus passive house

A new building regulations document, Part O, has been added to the England & Wales Building Regulations. This came into force 15th June 2023 and has been introduced to prevent new residential buildings overheating in the summer.

Instinctively you may think this is due to climate change and temperatures rising but this isn’t the only reason. Improvements in energy efficiency standards in both new and refurbished homes means that residential dwellings are better insulated and much more airtight than before. Gone are the leaky windows and gappy floorboards allowing a constant draft of fresh air.

Over the past two decades the Building Regulations have focussed on the prevention of heat losses in winter and failed to address the overheating in summer. The new Part O regulation requires that summer overheating is mitigated via passive means as far as possible, and mechanical cooling (air conditioning) only used where the requirement cannot be met by using window openings. It applies to all residential buildings, and includes not only dwellings (flats and houses) but also care homes and student accommodation.

The Passivhaus standard has its own energy efficiency criteria which address the energy balance through calculating the heat losses and gains and making sure ventilation is controlled with an energy efficient mechanical ventilation system. This principal applies to any building type for Passivhaus. However even then the reliance on occupants to open windows and utilise external shading in summer to prevent overheating is still a risk.

There are two methods to achieve compliance with Part O: the ‘simplified’ method or dynamic thermal modelling.

The simplified method limits solar gains and maximises natural ventilation through window sizing and window design, and takes account orientation and geographic location. These are the same principles of Passivhaus design in relation to solar gains, shading and ventilation. Passivhaus designers also used the PassivHaus Planning Package (PHPP) to be able to calculate the risk of overheating with the criteria being that internal temperatures do not exceed 25°C for more than 10% of the year. However, the simplified method from Part O only requires the designer to calculate that the total glazed area within the dwelling does not exceed a limit based on floor area and orientation of the façade, and that total glazed area does not exceed the prescribed percentage limit of the floor area. The designer is also then required to check that the maximum glazed area is sufficient to remove excess heat. 

The alternative route is dynamic thermal modelling, based on methodology developed by CIBSE in TM59 and involves creating a model of the residential building in thermal modelling software and using the data inputs from TM59 and climate data. The model can be used to show whether a scheme is compliant. To demonstrate compliance the model will need to show that the internal temperature does not exceed a specified temperature for a certain number of hours. 

The simplified method is considered to be most useful for architects designing a one-off home in a low risk area, otherwise the dynamic thermal modelling is expected to be the route taken by the majority as it provides more design flexibility.

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