While environmental considerations are now – rightly – higher on the agenda in building control, this does not always mean that UK regulation takes a harmonised approach.
For instance, how can the Building Regulations ensure that indoor air is fresh and free of pollution, while maintaining the energy efficiency and performance of the building fabric?
Although revisions were made to ventilation rates and other guidance in Ventilation: Approved Document F in 2021, the overall aims remain ensuring that excessive humidity and pollutants are removed from a dwelling and providing a background fresh air supply throughout.
To do so, the document recommends three basic alternatives:
Although MEV and MVHR systems have been available for many years, most existing houses use traditional methods because whole-house mechanical systems are costlier and more complex to install than local extractor fans. For the same reasons, many housebuilders also prefer traditional systems.
Growing awareness of the need to conserve energy and reduce carbon emissions is, however, prompting a shift to MEV and MVHR. Some local and regional government – in particular, the Greater London Authority – also require performance to exceed compliance with the Building Regulations.
As a result, more MEV and particularly MVHR systems are being installed. Despite the initial cost, mechanical fans use relatively little energy compared to what they save, provided that the building design minimises uncontrolled ventilation in the form of leaks and draughts.
For instance, most dwellings that Levitt Bernstein has designed in London in recent years have combined MVHR with high-performance building fabric, air-source heat pumps and photovoltaic panels.
Meanwhile, a revised version of Conservation of fuel and power: Approved Document L was also published in 2021. This increases the required building fabric performance standards, meaning that higher levels of airtightness must now be met.
It is not uncommon to design an air permeability of less than 5m3 per hour per square metre of envelope area (h/m2) at 50 Pascals (Pa). Indeed, this is the notional dwelling specification set in Approved Document L – that is, the performance the building is compared against. Some high-performing dwellings achieve much lower rates.
This increased airtightness has implications for the choice of ventilation system, though. Approved Document F states that traditional systems can only be used if the design air permeability is higher – that is, worse than 5m3/(h.m2) at 50Pa or an as-built measured performance higher than 3m3.
Because a traditional ventilation system relies on leaky construction, 5m3 is the designed worst case, while the 3m3 figure accounts for construction not allowing as much background ventilation as this. Yet these levels exceed Approved Document L's specification.
The latter's restriction on traditional systems is required to achieve high levels of airtightness – but there is a risk of condensation and mould build-up if there is insufficient ventilation. This is a particular risk when there are dead spots with little air movement in the dwelling.
MEV or MVHR systems enable a continuous flow of air out of the dwelling, thus reducing the risk of moisture build-up. Therefore, these are the only systems that should be installed when good levels of airtightness are required.
Other guidance in Ventilation: Approved Document F discourages the use of traditional systems, for example to minimise ingress of external pollutants where the building is near road traffic or if air quality exceeds the values from Schedule 2 of The Air Quality Standards Regulations 2010.
Unless the building is away from the road and an air quality assessment determines that there are no external pollutants, therefore, controlled ventilation intake will be needed.
The only way of achieving this is by using an MVHR system, where ventilation intake can be placed on the side of the building away from traffic or other sources of pollution.
Given the energy efficiency targets in Conservation of fuel and power: Approved Document L – particularly relating to fabric performance – and the restrictions on system choice in Ventilation: Approved Document F, the simplest way of complying with both is using an MVHR system for ventilation, unless further surveys and assessments are carried out and expert engineering advice is sought.
Any approach that does not follow the guidance would need to be fully justified. This would pose a risk for the project – and on small schemes could even be more expensive than the cost of installing MVHR. Given this and the benefits of MVHR, it could become the default ventilation system in dwellings instead of a traditional approach from now on.
The construction industry will thus need to prepare for this – both in terms of changing its approach to design and regulatory compliance, and having sufficient suitably trained installation engineers in place with the necessary skills and experience to install these systems as demand increases.
'Any approach that does not follow the guidance would need to be fully justified'
Throughout this series on the unintended consequences of the changing regulatory environment, I have – as in this case – drawn attention to conflicting requirements or the knock-on effects of guidance that may not at first have been apparent. I hope, though, that these articles have raised awareness, so future changes are considered holistically and not as isolated regulations.
With the design and construction of buildings becoming ever more complex and interconnected, we as an industry therefore need to be prepared for any future reforms to avoid exposure to unintended consequences.
Tony Hall is a retired former technical design director at Levitt Bernstein
Contact Levitt Bernstein: Email
Related competencies include: Construction technology and environmental services, Sustainability
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