Linking airtightness and ventilation for better building design

This year's CIBSE Napier Shaw medal was awarded to Jenny Crawley, Jez Wingfield and Cliff Elwell for their paper entitled ‘The relationship between airtightness and ventilation in new UK dwellings’. Jenny Crawley gives us an overview of their work. 

New homes in England must comply with a large set of building regulations including requirements outlined in the approved documents for energy efficiency (Part L) and ventilation (Part F). These two approved documents both include aspects relating to airflow through the building. Part L has limits on building air permeability, which is uncontrolled air leakage through the entire building fabric. Part L regulates the supply of enough fresh air through designed apertures such as vents and fans. The expectation is that these uncontrolled and controlled airflows will add up to provide enough total fresh air for occupants, whilst at the same time not wasting energy through excessive ventilation heat loss. This process needs to work across a range of ventilation strategies including mechanical and natural (passive) approaches. One might expect that different airtightness strategies are combined with different ventilation strategies. So for example, homes with mechanical ventilation and heat recovery (MVHR) would normally be expected to be very airtight (lower air permeability) to maximise the benefit of the heat exchanger, whereas naturally ventilated homes may rely on a degree of air leakage through the fabric (higher air permeability) to supplement fresh air from trickle vents.

Now, what is happening in practice?

We examined the design air permeability and ventilation strategies of 140,000 new homes built in the UK between August 2015 and December 2016. This was a unique opportunity to obtain a national-scale picture of ordinary building design – not specifically eco-homes – using data recorded for compliance purposes and provided to us by the Air Tightness Testing and Measurement Association. Note that here we focus on design – we have written elsewhere about the resulting actual airtightness arising from the compliance process.

We found that design airtightness levels for different ventilation strategies are remarkably similar. In other words, air permeability level is not being tailored to ventilation strategy. In quantitative terms,

-          Across 5 different ventilation strategies, mean design air permeabilities all fall between 5.02 and 5.48 m3/m2h, a total difference of only 0.46 m3/m2h; (this and all subsequent airtightness values are given at 50 Pascals)

-          Almost three quarters of homes with MVHR had design airtightness at or above 5 m3/m2h;

-          Almost a fifth of naturally ventilated homes have design airtightness under 5 m3/m2h.

This outcome is problematic. For homes with MVHR systems, the relatively high design air permeabilities means the MVHR system heat exchangers would not be used efficiently and there will be more ventilation than needed. For naturally ventilated homes at the lower end of design air permeability there is a risk of under-ventilation. On this point, it is permissible to design naturally ventilated homes with air permeability under 5 m3/m2h subject to providing extra trickle vent area for background ventilation. However, research suggests that in such cases indoor CO2 levels are higher than advisable levels (Tim Sharpe et al., 2015); this could be because the vents are often closed (Tim Sharpe et al., 2015), or because the extra vent area is often not actually provided (AECOM, 2019).

Why have ventilation and airtightness strategies emerged as practically independent of one another? It is likely because of their presence within two different approved documents, motivated by different purposes. Furthermore, airtightness is only one aspect of energy efficiency compliance and has to be traded off against other measures in order to meet regulatory carbon emission targets in SAP, so ventilation is not the only concern when choosing an airtightness target.

Our view is that this problem can be remedied in regulation quite simply by considering airtightness and ventilation strategy together. There are different ways of doing this; we suggest one approach as an example below. This method sets out ranges of design air permeability which are permitted for use with different ventilation strategies. Ranges of air permeability are used instead of exact numbers, due to the difficulty and absurdity of constructing a building whose leakiness is specified to two decimal places!

< 3 m3/m2h

3-5 m3/m2h

5-7 m3/m2h

7-10 m3/m2h

Natural vent.

Allowed

Allowed

Allowed

MEV

Allowed

Allowed

Allowed

MVHR

Allowed

Allowed

A joined up approach to airtightness and ventilation seems possible at the design and compliance stage. However, this introduces important considerations when the homes are built and occupied. Firstly, if MVHR is only allowed in buildings with low air permeability, it is very important that the system functions properly and does indeed deliver the amount of fresh air promised. There is evidence that this is not always the case (Balvers et al., 2012; Zero Carbon Hub, 2013). From the other perspective, if MVHR does deliver the right amount of fresh air, it is crucial that the low air permeability specified with it in our approach does consistently materialise. This has not been the case in recent research in the UK (T. Sharpe et al., 2016)

In conclusion, if airtightness and ventilation are linked more explicitly in building regulations for new homes, this can lead to getting the most out of both strategies, maximising energy performance and minimising the risk of low air quality.

AECOM. (2019). Ventilation and Indoor Air Quality in New Homes.

Balvers, J., Bogers, R., Jongeneel, R., van Kamp, I., Boerstra, A., & van Dijken, F. (2012). Mechanical ventilation in recently built Dutch homes: technical shortcomings, possibilities for improvement, perceived indoor environment and health effects. Architectural Science Review, 55(1), 4-14.

Sharpe, T., Farren, P., Howieson, S., Tuohy, P., & McQuillan, J. (2015). Occupant interactions and effectiveness of natural ventilation strategies in contemporary new housing in Scotland, UK. International journal of environmental research and public health, 12(7), 8480-8497.

Sharpe, T., McGill, G., Gupta, R., Gregg, M., & Mawditt, I. (2016). Characteristics and performance of MVHR systems. Retrieved from UK: http://www.fourwalls-uk.com/wp-content/uploads/2016/03/MVHR-Meta-Study-Report-March-2016-FINAL-PUBLISHED.pdf

Zero Carbon Hub. (2013). Mechanical Ventilation and Heat Recovery in New Homes

Jenny Crawley is Academic Manager of LoLo Centre for Doctoral Training in Energy Demand

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