Building Regulations Part O - Cross Ventilation and cooling of interiors

‘O’ is for Overheating

‘O’ is for Overheating – It’s certainly no coincidence that I’m considering the new Building Regulations Part O (2021 edition) – hence the title ‘O is for overheating’ – just after parts of the UK have experienced record, or near record, high temperatures. Haven taken effect last month, this new portion of the Building Regulations suite of documents concentrates on the mitigation of potential overheating in new build dwellings.

Many of us will be aware that there are new dwellings, larger apartment schemes in urban areas in particular, that regularly overheat, subjecting their occupants to very uncomfortable internal temperatures, in turn impacting on their quality of sleep and life in general. The urban heat island effect adds to this problem, encouraging the gradual escalation of temperatures in our urban centres as summertime heat soaks into the plethora of man-created hard surfaces.

The issue with some dwellings is that they do not allow cross-ventilation, for a host of reasons, such as briefing and site constraints. If internalised circulation corridors are used as the primary form of hot water and heating distribution via centralised plant, these spaces also gradually become overheated. This is not helped by the fashion for lots of full-height glazing. These are issues that have been well understood when designing office layouts, but this thinking has not been extensively carried over into the residential sector.

Part O considers whether dwellings can provide cross-ventilation and asks the designer to interrogate the areas of glazing to each façade according to compass orientation, as well as calculating areas of openable glazing. Coming out of this analysis is the option for a simplified form of meeting the requirements of Part O, and this has definite benefits in not requiring the designer to undertake dynamic thermal modelling.

I think the most crucial aspect of this is a reconsideration at early stages of the initial design principles. Plan layout options are available. We have used vertical strategies where mirrored pairs of apartments are accessed via a single lift / stair core to the northern façade or, sandwiched between the two. This form has the benefit pf allowing cross ventilation within the apartments with sleeping accommodation on the most northerly (cool) façade. This is also an easier form to locate onto sloping or fragmented sites and can provide a good solution to lower-scale development. This form can also deliver efficient circulation from stair to apartment door and can be internalised or external, if pushed out to the rear.

We have also used plan forms where the access corridor is external on the northern side, also termed as ‘balcony access.’ This can support a very similar apartment arrangement to that proposed above whilst reducing the number of lift and stair cores. This doesn’t have to convey the stereotypical ‘council flat’ type of design feel. These spaces can be of a good quality and provide spaces for unplanned social interaction, places for plants and the opportunity for personalisation and ownership.

Consideration can also be given to the master planning of larger developments, and we should start thinking about the cross-flow of air, natural summertime shading and landscaping proposals, in relation to providing shade and reduced heat absorption. Maybe even introducing water into these spaces. We should consider if all building spaces need to be internalised, or instead whether circulation routes are better provided in the open air, albeit sheltered from the rain.

In the first instance these arrangements may not seem like they naturally provide our developer clients with what they envisaged but I believe that we will have to use innovative solutions to provide accommodation that is fit for habitation year-round. In the wake of Covid and the realisation that we can exist outside in the fresh air, I think this has become a much easier ‘sell’.

Part of the issue is the amount of hard landscaping that surrounds many of our buildings. Large car parks can present huge areas of exposed, unrelenting black tarmac that soaks up the heat during the day. Any dark coloured hard surfacing to the perimeter of our residential buildings will do the same, releasing heat into open bedroom windows at night when the occupant is instead trying to purge the heat that’s already inside. Car parks are most efficient of course if simply hard-surfaced, but we could reconsider this approach, instead providing spaces where cars, trees and plants coexist, the tress providing shade and the planting reducing heat absorption and rainwater run-off. Surfacing choices will also make a difference. The adoption of reinforced grass and gravel systems will generally provide less heat absorption and radiation back to the surrounding air than block paviours, tarmac, and concrete. These spaces could create the opportunity for PV arrays coordinated into canopies, providing shade, night-time lighting opportunities and maybe even rainwater collection.

The planning system is encouraging the introduction of much more nature into development sites, controlled by the need for net biodiversity gain. In terms of controlling potential overheating in our urban centres and residential buildings this is a good thing. Plants provide necessary shade and moisture through transpiration as well as improving air quality and supporting general wellbeing. As a society we have known this for centuries, utilising courtyard gardens with water into the spaces where we live. This type of ‘model’ is more often seen in hotter climates, but we may well need to start looking at equatorial solutions in the UK if we want to deliver places where people can live comfortably in the future.

In the coming years we will potentially have these sorts of issues to consider more and more, but let’s not think disproportionately. We will still need to deal with the heated season, providing access to sunlight and passive warming, rain and the effects of wintertime weather. In England and Wales an estimated 20,000 to 50,000 excess winter deaths occur annually, dependent on specific weather events each year and taken as pre-covid figures. Whilst excess summer deaths pre-covid are generally limited to recognised heatwave events and amount to between 500 and 1000 people per annum, again dependent upon the specifics of weather each year. So, in terms of health, the UK Government statistics show that you are many times more likely to die of cold than heat. The question is whether that trend will dramatically change, forcing us to design our buildings, and live in them, in a very different way.

Mortality rates aside, we all want to feel as comfortable as possible in our homes, whatever the time of year or specific weather conditions. This provides UK designers with a difficult but not insurmountable task to overcome, and one that Part O is attempting to address.

Dale's experience spans a wide range of project sectors, sizes and stages and has particular interests in environmental design and earth-sheltered construction. His favourite projects reuse and breathe new life into under-loved existing buildings.