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How will Approved Document O transform window design and ventilation strategies?

New Building Regulations about overheating are now in effect

New Building Regulations came into force on 15 June 2022. Among them, a completely new Approved Document, covering overheating in dwellings, has come into force: Approved Document O.

The new requirements are likely to have a major impact on housing design in terms of designing and specifying façades and windows – especially in locations deemed to be “higher risk” rather than “moderate risk”. A table of postcodes designating which are higher risk is provided.

Part O applies only to new dwellings. This covers residential buildings of most kinds, including care homes and student residences - the most significant exception being hotels. Significantly, it applies to common parts as well as habitable rooms.

Any schemes where full plans or a building notice had been submitted ahead of 15 June 2022 can be built to previous regulations if work starts within 12 months.

Finding functioning design solutions for windows could be challenging under the new regulations, warns Tony Hall, Technical Design Director at Levitt Bernstein. He explains that two methods of compliance are permitted:

  1. A dynamic thermal modelling method based on the CIBSE TM59 approach to predicting overheating risk
  2. A ‘simplified method’, which allows practices to avoid assessments by specialist consultants

The simplified method

The simplified method is in some ways a misnomer, according to Hall. In practice, it is actually quite complicated, requiring several step calculations and sets of measurements to be taken. The result generated is a simple pass or fail: in the case of a fail, the architect has no choice but to repeat the process until the elements causing the fail have been identified and rectified.

Stephen Barnshaw, Associate Director of Technical Services at jhai Ltd, explained in a recent RIBA webinar that the simplified method seeks to limit glazed areas. Part O sets out tables that establish a maximum amount of glazing in the external fabric relative to the total floor area of the building.

The simplified method is restrictive on window sizes where orientation or location are issues. Hall suspects that the simplified method may not always be practical in higher-risk areas in instances such as:

  • single-aspect dwellings
  • west-facing dwellings
  • common areas
  • areas where there are noise and pollution issues (both of which have to be mitigated under Part O)

He counsels that every building in a high-risk location is likely to need shading on windows between its north east and north west aspects. Dual-aspect dwellings could prove challenging because under Part O, cross ventilation only counts if it is on opposite sides of the building.

For many projects, satisfying Part O's requirements will mean bringing in a consultant with expertise in the CIBSE TM59 approach

If window sizes are reduced to pass the test, this may compromise daylight requirements. Hall points out that rooflights are a potential solution here as they can increase the amount of daylight but are not specifically included in overheating calculations.

Barnshaw advises that, if you are in a high-risk location, in addition to the maximum glazing considerations, you must also provide one of the following:

  • external shutters with means of ventilation
  • glazing with a maximum G value of 0.4 and minimum light transmittance of 0.7
  • an overhang with 50 degrees altitude cut off, on due south facing façades only

The dynamic thermal modelling method

In contrast, dynamic thermal modelling may offer far greater design flexibility because it takes a holistic approach to the building, Hall points out. This must be carried out by an expert in the CIBSE TM59 methodology.

He suggests that specialist thermal modelling might be seen as the only practical route to compliance for:

  • internal communal spaces
  • flatted developments
  • single aspect developments and high-risk locations

Overheating mitigation measures will clearly have planning implications, so relevant design work must therefore be carried out at pre-planning stages.

Earlier this month, the government’s Chief Planner Joanna Averley warned planning authorities to expect revised planning applications where shading was being increased and/or the amount of glazing changed to satisfy Part O.

Implications for compliance with other Building Regulations requirements

The new regulations also require openable windows that pose a risk of falling from height to have a minimum guarding height. If sills become higher or wider, or guarding measures are added externally, the resultant visible changes will become planning considerations.

Marrying overheating mitigation measures with additional Part O requirements relating to noise and pollution (closed windows have overheating impacts) and security from falling and entrapment is likely to present several technical challenges.

The same can be said for Part M requirements for access and Part B for fire safety. Hall admits that he and fellow architects have been struggling to come up with designs for functioning windows that can still be cleaned, for instance. A webinar presented by Hall for the RIBA, in which he proffers some detailed design solutions to some of the issues will shortly be made available to RIBA Members.

The Department for Levelling Up, Housing and Communities (DLUHC) is aware that further guidance is needed, and this is being prepared by the Future Homes Hub. It has already published guidance on Part L: where to start. Hall advises architects to check the Future Homes Hub for updates: it is likely to answer many of the technical questions.

Print copies of Approved Document O (and other Approved Documents) are available from RIBA Books.

Thanks to Tony Hall, Technical Design Director, Levitt Bernstein; and Stephen Barnshaw, Associate Director of Technical Services at jhai Ltd.

Text by Neal Morris. This is a Professional Feature edited by the RIBA Practice team. Send us your feedback and ideas

RIBA Core Curriculum topic: Legal, regulatory and statutory compliance.

As part of the flexible RIBA CPD programme, professional features count as microlearning. See further information on the updated RIBA CPD core curriculum and on fulfilling your CPD requirements as an RIBA Chartered Member.

First published Thursday 23 June 2022

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