What is a Part O Overheating assessment?
Whilst overheating was historically assessed as part of the SAP calculations the updated building regulations, which came into effect on the 15th June 2022, saw the introduction of Part O which deals with overheating in domestic dwellings in more detail.
Approved Document O: Overheating provides guidance on how to comply with the new requirements. Alongside the prevention of overheating in domestic dwellings, the new Part O also applies to overheating in properties, such as care homes and student accommodation.
How do I comply?
There are two routes to compliance:
1. The simplified method – a simplified calculation which firstly calculates solar gain based on the amount of glazing, orientation and floor areas and ventilation based on the configuration and amount of openable windows in the property.
2. DTM or dynamic thermal modelling based on CIBSE’s TM59 Design methodology for the assessment of overheating document.
What is the simplified method?
The simplified method is based on the principle that overheating is exacerbated by excess solar radiation and insufficient ventilation.
The methodology sets out to limit the amount of glazing on the south, west and east facades while ensuring there is sufficient openable window area to prevent overheating. To do this, the simplified method requires details about the glazed area and opening area of every single window in the dwelling to be calculated in order to check:
• That the total glazed area within the dwelling does not exceed a limit based on the floor area and orientation of the most glazed facade.
• And that the total area in the most glazed room does not exceed a percentage limit, based on the floor area of that room.
Maximum allowable glazed area varies depending on whether a dwelling has windows on opposite sides to allow cross-ventilation. As might be expected, the allowable glazed area is reduced for buildings without cross-ventilation.
Having established the maximum glazed area, the simplified method then requires designers to check the area of openable windows is sufficient to remove excess heat.
Approved Document O also makes a distinction between buildings in high-risk urban locations, such as central London, and moderate-risk locations.
The simplified method is best suited to ‘typical/standard’ houses or one off builds without an excessive amount of glazing.
What is dynamic thermal modelling?
The dynamic thermal modelling route is based on the methodology developed by CIBSE in TM59. This involves the creation of a model of the dwelling, or development, using dynamic thermal modelling software and using the standard set of data inputs from TM59. Based on these inputs and using climate modelling from CIBSE 2020s Design Summer Year (representative of 2010-2040), the model can be used to show whether a scheme is compliant.
For those familiar with building simulation tools, the modelling route will be relatively straightforward and CIBSE’s TM59 provides standardised input for internal gains, occupancy profiles etc.
To demonstrate compliance, the model will need to show that the internal temperature within a space does not exceed a specified temperature for a certain number of hours, with an added requirement for bedrooms than for other areas of the dwelling.
The dynamic thermal modelling approach could be particularly advantageous to a developer because it will allow them to target solutions for specific rooms or dwellings where problems have been identified, rather than having to introduce more generic solutions across the entire development, potentially allowing for more creative solutions.
A dwelling should be constructed to meet the requirements of Part O using passive means as far as reasonably practical. Mechanical cooling is not totally excluded but should only be used when all other practical options have been explored.
CIBSE TM59 was introduced in 2017. Since then, it has been used extensively by industry, including as a means of demonstrating compliance with the London Plan, with positive feedback.
That said, it is impossible to say that a compliant scheme will never overheat because there is a lot of uncertainty. Weather patterns are not completely predictable – and neither is the way people use their homes. Nevertheless, with the introduction of Part O, CIBSE can take comfort from the fact that designers will now have to demonstrate to building control bodies that they have taken all necessary steps to address the overheating risk in a dwelling at the design stage.
What route should I take?
We can conduct a simplified method, which will usually be the more cost effective approach to take, but there is no guarantee that the design will pass, especially the more complicated builds where there is a lot of glazing involved.
If the simplified method fails then there are a number of ways to achieve compliance. Either modify the design – i.e. reduce the amount of glazing or increase the amount of openable windows. Alternatively, you can opt for a DTM approach which will typically cost more but will give you the design flexibility to overcome issues which may be harder to address using the simplified method.
Unfortunately, we will only know if a building will pass or fail the simplified method by actually carrying out the calculations.