Approved Document O is up and running. Mark Taylor looks at the pros and cons of routes to compliance and what it means for apartment and house designs
The transitional window for Requirement O of the Building Regulations closed in June and must now be complied with. We can face our inevitable future of annual heatwaves with the regulations to give occupants of new homes some protection against the worst of the emerging climate disruptions. So what are the methods of compliance, and the stumbling blocks to clear calculation – and what might the regulations spell for the future of apartment and house design?
The aim of requirement O was to include all aspects of passive overheating design, including limiting unwanted solar gains in summer and providing adequate means of removing excess indoor heat, while ensuring such means are safe. Location, orientation, cross ventilation, shading, glazing and ventilation areas are all under scrutiny. Provision of information to the owner completes the list of considerations.
As with all the regs’ functional requirements, there is the usual approved document containing means of meeting Requirement O. AD-O outlines two methods for achieving the performance targets: a simplified and a more thorough dynamic thermal modelling method embodied in CIBSE TM59. The dynamic method is more refined as the building is modelled and more inputs are required. There is also a third method, not included, that could be fruitful to work with.
Many a brow will have been furrowed by the complexities of the ‘simplified’ method. Its cocktail of criteria can be a headache for designers of houses (it is not deemed suitable for buildings of more than one residential unit with distributed hot water).
Textual and tabular hurdles start with assessment of floor areas and opening sizes. Minimum levels are set for free area, the geometric open area of a ventilation opening. There is also equivalent area, a measure of the aerodynamic performance of an opening. The equivalent areas of the actual openings should meet or exceed the geometric free area targets relevant to the total floor areas of the dwelling and also those of the bedrooms. There are three ways of checking equivalent area: the tables in the appendix, an online calculation tool or EN 13141-1.
One ostensibly puzzling oddity is that the total free area targets for buildings in high risk areas are lower than those in low risk areas. This seems to have partly given rise to the publication of a web page of frequently asked questions by the Department for Levelling Up, Housing & Communities. It is justified by buildings in high risk areas in inner London postcodes (and some Manchester postcodes) needing one of the following: external shutters with means of ventilation, limits on glass G value/light transmission, or overhangs with 50° altitude cut-off on due south-facing facades. The targets for bedrooms are mysteriously the inverse: those with free area in high risk areas being higher than those in low risk areas. There is no explanation for this apparent anomaly.
Using the dynamic method should allow greater flexibility in design as the modelling considers a range of mitigation measures, building geometry, locational features and shading types. Specific limits are placed on modelling inputs covering occupancy and window/door opening and closing times. Mechanical ventilation is considered as well as active cooling but the latter only as a last resort when preventative and passive measures cannot work.
Good Homes Alliance
There is another method of assessing overheating, although not mentioned in the Approved Document. The Good Homes Alliance has produced a simplified tool and a guidance book. This approach appears to sit somewhere between the AD-O simplified and dynamic methods and may prove useful, as the Approved Documents are not the only allowable way of meeting the requirements of the Building Regulations. Designers should check whether their building control body and warranty provider will accept its outcome as evidence of compliance before relying on it.
The total free area targets for buildings in high risk areas are lower than those in low risk areas
Design hurdles arise from the mandate to ensure reasonable enjoyment of the building for its occupants. This part of the AD covers operability of openings, noise at night, pollution, security, protection from falling and entrapment.
One of the biggest challenges arises from the need to discount any openings accessible to the casual burglar from ventilation areas for security in the simplified method and treat them as closed at night in the dynamic method. This could include ground floor openings and those near flat roofs, which can be a significant proportion of the openings on a typical house.
The presentation of the AD is sometimes confusing and leaves some questions unanswered.
First, why are there no provisions for alterations and extensions to existing dwellings? Part O misses a significant opportunity to control over-glazing and inappropriate treatment of house extensions. Approved Document L similarly underachieves as the UK stock of existing dwellings volumetrically expands but is insufficiently constrained to reduce its carbon footprint in doing so.
Secondly, why are there no specific limits for roof lights? Sloped or near horizontal glazing can give rise to significantly higher summertime insolation at British latitudes than vertical glazing. This appears to be a glaring oversight, especially given such exhaustively detailed methodology covering equivalent ventilation area calculation.
Where will AD-O take design of apartments?
Single-aspect units can be avoided; dual and deck access can be encouraged; geometry and self-shading can be explored, orientation can be optimised; large areas of glazing (especially south and west facing) of high-level corner units can be moderated; windows may be better designed with more opening area; louvred panels may be introduced to help balance glazing with ventilation areas; ambient loop water heating could limit overheating from common areas and adjacent risers; shading from balconies can be exploited; and glass can be specified for maximised light transmission with minimised G-value. If these don’t achieve the targets, supplementary ducted ventilation can be introduced.
Blocks near busy roads, railway lines and industrial sites will need more specialised treatment, potentially with acoustic attenuation to opening vents. If comfort cooling has to be used, it should not be allowed to operate while windows are open and must still comply with Requirement L of the Building Regulations. It should be treated with caution and much discussion with the building control body and the warranty provider.
Sloped or near horizontal glazing can give rise to significantly higher summertime insolation
Where will AD-O take the design of houses?
Houses are in theory less likely to overheat than apartments, and use of the dynamic modelling method should ease the burden of change to the way the typical house is envisioned. However, the discounting of ‘accessible’ openings will take its toll on freedom of design, so creativity will be needed – possibly around the use of grilles, louvres or shutters to meet security provisions.
Much of the outcome depends on the approach of the designer: early engagement with the foibles of AD-O should produce a well-rounded, future proofed and occupant-optimised building of social worth. Insufficient or late engagement could result in a hotchpotch.
The use of the dynamic modelling method at early stage can be seen as a true aid to design, with results that encourage windows with maximised, secure and inclusive openings without imposing great restrictions on their area.
As the UK climate trajectory heads inexorably towards the Mediterranean, will our new housing stock begin to resemble the shuttered house on the cover of TM59? Only time will tell.
Mark Taylor is director of technical design at Allies and Morrison