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Masonry Cavity Wall Construction: Embodied Energy & Buildability

Since completing our cavity wall Denby Dale Passivhaus project in 2009/10, we’ve always been curious about the embodied carbon and whole life carbon of the project.

In this blog Green Building Store Directors Bill Butcher and Chris Herring – drawing on research findings from PHribbon developer Tim Martel – explore the embodied carbon for cavity wall construction in comparison with other construction types. This data was first shared in an AECB webinar on 15 March 2022

Denby Dale Passivhaus

The Denby Dale Passivhaus was built by our construction team in 2009 and in 2010 it became the UK’s first certified cavity wall Passivhaus. Much of the detailing developed for the project had to be developed from scratch as there were no readily available templates to work from.

At the time of its construction, the focus was very much on reducing the project’s operational (in-use) energy by fabric first approaches and building to the Passivhaus standard, which was, at the time, challenge enough in itself. Since then, embodied carbon calculation methodologies, such as the PHribbon add-on to PHPP, have developed and evolved.

Why cavity wall?

At the time there were three main drivers for us to go down the cavity wall route on the Denby Dale project:

  • Cavity wall was the method that we at Green Building Company were most familiar with. We also recognized that cavity wall construction still represented the majority building method in the UK, and as such recognized that it was important to offer an avenue to reach the Passivhaus standard with this familiar methodology.  Effectively, we were demonstrating that any builder could do Passivhaus without changing their construction methodology while as far as possible using materials you could find in any local builders’ yard. Approximately 75% of UK housing construction is cavity wall.
  • West Yorkshire planning rules require natural stone facing on the exterior of most new buildings and so ruled out options such as masonry construction with external insulation and render, or timber frame with render or timber rainscreen.
  • At the time we understood that the high thermal mass of cavity wall blockwork could play a significant role in enabling us to achieve the best internal conditions and to mitigate overheating. While the thermal mass does contribute a little to mitigating overheating risks, we now know that this is far less significant than we had understood in those early days.   We also now know that the additional acoustic benefits of high mass are negligible, and for example timber frame construction with the right insulation materials can also be very effective in acoustic attenuation.

    Our key focus at the time was  on operational energy and being able to actually manage to build a Passivhaus. The Denby Dale Passivhaus project was one of the very first Passivhaus certified projects in the whole of the UK. When we started the planning of the project there were no certified buildings in the UK, and minimal Passivhaus expertise.  So we were working it out for ourselves, very largely.  Of course, our awareness and understanding of the issues of embodied carbon was not as developed as it is now. The project was also being built to a very tight budget and we simply had to focus on achieving the Passivhaus operational target, which alone was very challenging for us at that time. At the time the pricing of cavity wall construction was definitely cheaper than timber frame alternatives available, particularly with the need for Yorkshire stone facing.  While we have not analysed this in detail, it does appear that this may still be the case.

    PHribbon calculates Whole Life Carbon, that is Embodied CO2 from Cradle to Grave, covering RIBA stages A-C (and D where information is available). It is suitable for initial estimates for the RIBA 2030 Challenge v2 and LETI. This therefore includes:
    • Stage A, A1-A3 Manufacture including A4 Transport to site and A5 Construction
    • Stage B, Use of the building including B4 Replacement
    • Stage C, Demolition and Disposal of the building
    • And quoted separately Stage D, Reuse, Recycling potential (outside the usual scope of the main calculation). It calculates the credit from reuse of an element, assembled part such as a frame, or something that is reused offsite.
    A comparison of embodied carbon using PHribbon
    • We engaged Tim Martel, creator and developer of the AECB’s PHribbon, to undertake a comparison of construction types for us.  PHRibbon is an easy-to-use add-on for PHPP and as its creator Tim was obviously the perfect project partner for us.  The construction types were based on  Passivhaus newbuild project which itself was based on the PHPP and specifications for Green Building Store’s Kirkburton Passivhaus project.The results indicated that the timber I-beam wall + stone offered the lowest and best embodied carbon with timber I-beam + render a close second. Cavity wall construction with block/ brick offered the highest and worst embodied carbon, with cavity wall block/ stone the next worst. The use of Greenbloc in the inner leaf (Option 3) did help bring down the embodied carbon of cavity wall somewhat. Combining brick outer wall with either cavity wall or timber-Ibeam construction also noticeably increased the embodied carbon over the use of stone as the outer wall or rainscreen.However, it is worth pointing out that the embodied carbon figures do not vary wildly and that all construction types still fall within the RIBA 2030 Climate Challenge target for embodied carbon. That being said, timber frame construction clearly does offer a modestly lower overall embodied carbon over the cavity wall and heavier construction types.
    Heat pumpification
    • Tim Martel also looked at the impact of installing heat pumps for all construction types in the research and found that heat pumps made a massive difference to reducing the whole life carbon relating to all building types studied, because of the decarbonising Grid.
    What would we do now?
    • Armed now with this information, the Green Building Store in-house building team would now always tilt towards timber frame as a construction option if it fitted with our customers’ budget and wishes. We would also encourage our clients to install a heat pump.However, it is worth reiterating that operational energy is still the biggest priority and the most important element of the building to be addressed. Furthermore, a dramatic reduction in operational energy also optimizes the use of heat pumps for a number of reasons.
      • A highly insulated Passivhaus building fabric allows a constant, stable internal temperature. This is ideal for optimal heat pump operation and efficiency.
      • The greater challenge for energy supply in the UK may not be total demand, but rather peak demand. A highly insulated building fabric allows for heating input at off-peak times, due to the constancy of temperature and comfort in the building.
      • Small heat pumps with low heating demand can be optimized for hot water production, resulting in overall higher Coefficients of Performance.
      Once operational energy (building to the Passivhaus standard) and low carbon heating (installing a heat pump) have been tackled then it makes sense to look at embodied carbon of building materials.
    Our approach now
    • Building to Passivhaus makes a big difference to operational energy. We always address operational energy and aim for Passivhaus for our newbuild clients.
    • Switching to heat pumps makes a big difference to carbon emissions. We would now encourage our clients to choose heat pumps, optimised for low heating demand.
    • Once both of those are achieved, looking at the embodied carbon of building types is the next logical step…. Remembering local planning constraints and cost issues, we consider lower embodied carbon building options.
    Buildability: Is there still a place for cavity wall?
    It has been useful to see masonry cavity wall in the context of other construction methods and this has helped us to try to answer the question of whether cavity wall construction still has a place. Given the modest difference in the embodied carbon between all of these 6 construction types, at this point, the priority must be to adopt Passivhaus standards of new build operational efficiency combined with heat pumps for all new builds starting right now.  Currently the majority of our new build is still cavity wall construction, so it makes sense to us to make the transition to Passivhaus as familiar as possible for builders.  So at the moment we believe that cavity wall probably does still have a place in UK Passivhaus construction, due to familiarity, buildability and practical considerations.
    • Planning restrictions in some locations insist on certain vernacular construction types, which might favour cavity wall construction.
    • Consumer perceptions of longevity might influence the choice of cavity wall construction and use of brick and stone.
    • Cost is obviously still a factor in the choice of materials and our experience is that masonry may still often be considerably cheaper than timber frame construction.
    • Construction industry experience in England is heavily weighted to masonry construction. Cavity wall is the predominant construction type in UK – should we be suggesting that the industry to abandon it wholesale and at speed and go all out for timber frame?
    • The availability of products & supply chain might favour cavity wall construction in some situations.
    • There is a big question as to whether there is enough timber to build everything out of it. What would be the implications for biodiversity and carbon with relation to old growth forests if all new housing was built from timber? [See the AECB’s The Wood from the Trees report].
    • Practical site issues might make offsite timber frame solutions difficult eg if it is difficult to get cranes on site.
    In a counter-argument, however, it is worth asking whether the construction industry can realistically move from poor quality cavity wall to high quality cavity wall? This will mean a quantum jump in quality and major change in methodology for builders.  Will this be more or less alien than a largescale move to timber frame?  Monitoring and policing the quality of build for largescale masonry might make it difficult to ensure that the performance gap is minimized.  There is clearly little point in a wholesale move to Passivhaus if our dominant methodology cannot be adapted at scale to minimize the performance gap and achieve the potential for emissions reductions that Passivhaus offers.

    When we started trying to answer our original question, we never expected to reach conclusive answers.  Our intention was to ask the question and to give some indicative answers which could stimulate debate.  We hope that this study contributes to the debate about construction typology choices in the urgent drive to reduce the carbon emissions from buildings.
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