Part O couldn’t have been more timely. With this month’s record-breaking temperatures and the first ever Red warning about extreme heat issued by the Met Office, overheating mitigation is front of mind for many people. As temperatures increase due to global warming, we are finding that our UK housing stock simply isn’t prepared for hotter climes. However, simple measures such as limiting window size and using shutters – just as you see in the architecture of countries with higher summer temperatures – can help us to maintain safe internal temperatures.

The new Building Regulations Part O came into force a month ago, on 15th June 2022, and is designed to protect the health and welfare of residential property occupants by helping to ensure high indoor temperatures are avoided where possible. It covers all new residential buildings in England, including care homes and student accommodation (though not hotels). Some geographic areas are defined as ‘high risk’ (most of London), while others are ‘moderate risk’ (the rest of the country).

Part O: overheating mitigation

This new legislation is designed to ensure solar gains are limited, and that there are means of removing excess heat, in high summer temperatures – although that’s not to say Part O is aimed at creating ‘comfortable’ internal temperatures, just safe ones.

Compliance under the simplified method is relatively straightforward; a maximum area of glazing for the overall dwelling is indicated as a percentage of floor area (note this is not related to the size of the block, but the individual dwelling), and there is a maximum percentage of glazing in the most glazed room within the dwelling. The table indicates values for high-risk and moderate-risk homes (which is according to geographical area), and also for buildings with and without effective cross ventilation.

Shading for high-risk areas

Residential buildings in high-risk areas also require compulsory shading, which can be achieved through external shutters with ventilation, glazing with maximum g-value of 0.4 and a minimum light transmittance of 0.7, or overhangs with 50 degrees altitude cut-off (on due-south-facing facades only). In addition, designers must check that the area of openable windows is sufficient to remove excess heat; there is a minimum allowable free area calculated as a percentage of floor area or glazing area, whichever is larger, and these differ according to whether the dwelling is in a high-risk or moderate-risk location.

Bizarrely, those dwellings with cross ventilation in high-risk locations are able to have a higher overall maximum area of glazing (and lower opening areas in some cases) on the west elevation than those in moderate-risk locations. When you consider that the west elevation can be more difficult to shade due to the lower sun angle, this does suggest a lack of consistency in the new Part O requirements.

Dynamic thermal modelling for design flexibility

For those schemes that either don’t achieve the prescriptive glazing areas under the simplified method or want design flexibility, a dynamic thermal modelling method using CIBSE’s TM59 methodology can be employed.

The updated methodology does not permit the inclusion of internal blinds and curtains, which can significantly reduce the amount of direct solar heat gain into buildings, and focuses solely on shading measures which are not occupant controlled. This doesn’t mean that occupants shouldn’t install internal blinds and curtains (in fact, they should be positively encouraged to do so and shut them to reduce undesired solar gains), but just that these should be seen as simple retro-fit items for occupants for further improvement.

What do we think of Part O?

In summary, the simplified method looks time consuming and is perhaps unlikely to be followed by many designers, particularly where the same house type is proposed with various orientations, or where a greater design flexibility is required. The dynamic modelling method doesn’t fully follow the established CIBSE TM59 methodology, and the effect of this variation will be an increased reliance on cross ventilation and external shading – not a bad thing considering future climate predictions.

Until we’ve been able to get to grips with a few projects, it’s difficult to know what impact Part O is going to have – but we expect a number of tweaks to the Approved Document moving forward, to deal with inconsistencies and to incorporate the lessons learned from applying it to real-life builds.

To find out more about how we can help you to achieve Building Regulations compliance in the light of the latest Part O changes, call us on 01206 266755 or email