Has cavity wall got a future in low energy building projects?

Love it or loathe it (and some people really loathe it – see @BanTheCavity on twitter), cavity wall construction is still very much an intrinsic part of UK housebuilding, comprising 75% of new housing in 2010.  To meet climate change targets, buildings in the future will need to have wall U values of 0.15 to 0.1 W/m²K., with airtightness 10 times better than current building regulations, minimal thermal bridging and thermal bypass (air movement around insulation).  How do we as a nation get from where we are to this low energy nirvana? Do we jettison the cavity and embrace timber frame and solid masonry with rendered external insulation? Or do we work with what we have in the short to medium term, while maybe moving towards alternative methods?

Timber frame or solid masonry with external insulation and render might offer an easier route to Passivhaus but are less familiar to UK builders. I am a pragmatist and there are a number of historical, economic and pragmatic reasons why cavity wall might have a role in ultra low energy construction in the future, including:

• familiarity to the UK construction industry
• cost and availability of materials
• planning department requirements
• lack of skills/ expertise in alternative construction methods

Cavity wall is so entrenched in this country that it is a big ask for us to move away from cavity wall immediately or completely.

In the year since our Passivhaus Diaries blog, we have been surprised by how much interest the project is still generating – from as far afield as Egypt and Australia – including a significant amount of enquiries from Eire.  We have also been in contact with numerous architects and designers in the UK, designing low energy projects based on Passivhaus principles, which are all using cavity wall construction.

Architect and certified Passivhaus designer, Mark Siddall, is currently designing a private house in Northumbria and has gone down the cavity wall route for very similar reasons to those at Denby Dale: planning requirements for external stonework, cost-effectiveness over timber frame and local building skills.

Conrad Till of JMP Architects (www.jmp-architects.com) is working on a private dwelling in Lancaster, which is about to go on site, following the Passivhaus principles of the Denby Dale project in cavity wall construction. Again, the cost-effectiveness of cavity wall as an established technology was a big factor in this decision along with the fact that builders understand cavity wall.

Andrew Yeats of EcoArc (www.ecoarc.co.uk) is working on the 40-unit Lancaster Cohousing Passivhaus project due to go on-site in July. He opted for cavity wall construction after asking the partner construction firm for quotes and feedback on seven different walling systems (including a variety of timber frame and masonry alternatives). The builders – a traditional Lancashire firm called Whittle Construction – were asked to rate the different construction methods on:

• Theoretical cost
• Buildability
• Ease of construction 
• Programme

It was found that, although timber frame construction would have allowed the project to be completed 10 weeks earlier in the programme, it was actually still over £80K more expensive than cavity wall options. The builders also preferred the cavity wall method because of cost certainty and the fact that they could use their own labour rather than out-sourcing it with the timber frame options. Interestingly, cavity wall was the least favourite construction option for the Passivhaus consultants on the project  – anxious about the inherent airtightness and cold-bridging issues with cavity wall. The builders’ greater confidence with cavity wall, combined with cheaper costs and cavity wall’s flexibility with external facing materials, ultimately won the day.

As we found at Denby Dale, cavity wall is not without its disadvantages, including:  difficulties checking for quality of workmanship (particularly on larger projects); potential airtightness problems with electrical wiring chased into blockwork of external walls; airtightness cannot be tested until plasterwork is complete – later than other construction methods and is potentially more difficult to rectify; and lastly thickness of walls adds to a building’s footprint. The forthcoming UK projects have found other aspects of low energy cavity wall design challenging – for example Conrad Till has been working carefully on the detailing on thresholds – particularly balconies – and Mark Siddall has stressed the importance of planning sequencing of the cavity wall construction carefully – such as when in the construction process to apply the parging airtightness coat. Andrew Yeats has had difficulties with detailing around larger door and window openings and is developing new detailing regarding positioning of plywood boxes within the cavity for the co-housing project.

From our experience over the last year, cavity wall doesn’t seem to be disappearing any time soon. Andrew Yeats says: “I would really like to work on a project using solid masonry and external insulation, which I believe is easier and more robust than cavity wall and fewer problems with thermal bridging, However, cavity wall is embedded within the UK building industry, so will still have a future role.” Conrad Till argues that the buildability of cavity wall and its wider choice of aesthetics – for example allowing the easier fixing of timber cladding – will ensure its future role in the UK. As Mark Siddall puts it: “There is no single solution for delivering low energy Passivhaus buildings in the UK. Cavity wall has no more flaws or failures than any other building technique. It just has challenges and obstacles - all of which can and have been overcome.  For example, cavity wall and masonry with external wall insulation (EWI) both face airtightness and convective thermal bypass challenges. It is too early to say that cavity wall is dead. Cavity wall – as long as it has full fill insulation (to avoid the risk of thermal bypass) - will still play a role in sustainable construction in the UK”.

Despite having championed cavity wall at Denby Dale, I am by no means wedded to the technique and am keen to see Passivhaus take off in the UK by whatever means possible. I hope this will help open up a debate on the future of the cavity wall and the best direction the UK construction industry needs to go in to achieve demanding sustainability targets. Climate change necessitates better building now – cavity wall might be a pragmatic choice in the short/ medium-term and might need to stay in the mix so long as it is combined with improved detailing and quality construction.

More information: www.denbydalepassivhaus.co.uk

Bill Butcher and Mark Elton, from ECD architects, will be writing a diary from the International Passivhaus conference this year. Follow them on twitter @billbutch and @mark_elton and on www.building.co.uk/sustainability