Continuing our climate adaptation series, building performance consultant Ricardo Moreira explains how extending his Victorian terraced house presented the opportunity to implement an extensive series of energy-saving modifications
Our industry has a large impact on the climate crisis and, as a result, the focus has strongly shifted towards net zero carbon efforts. Over the next decade, we will need a large-scale retrofit programme for the nation’s existing building stock if we are to reach our carbon targets.
As architects and engineers, many of us are trying to apply our professional experience to our own lives by also improving the performance of the homes in which we live. But how realistic and cost-effective is it to maximise energy reduction through a domestic retrofit that is on a much smaller scale than our typical projects?
I am halfway through retrofitting my Victorian terrace home in north London. To improve its performance, I took advantage of an outrigger and first-floor rear extension and renovation. Admittedly this is not the scenario for most homes in the nationwide retrofit roll-out we need, but nor is it an uncommon one. Although not seeking EnerPhit certification, I am trying to determine how far one can get to on operational performance within a constrained budget.
I started by developing a retrofit plan based on a whole-building approach. Even if measures are phased over a few years, as is the case here, it will allow for the impact to be considered holistically to avoid unexpected consequences on ventilation, thermal comfort and indoor air quality. As discussed in my RIBAJ article in November 2022, I used the Sketch-up plug-in DesignPH and Passivhaus planning tool PHPP to analyse the potential impact of measures.
Following a fabric-first approach, I started by addressing insulation, thermal bridging and air tightness. The existing solid walls on my home’s rear façade will be externally insulated and rendered, a low-disruption solution that does not require a major renovation. Phase 2 of the project will see the front façade internally insulated, to maintain continuity of the brick facades on the streetside. This solid-wall build-up requires special attention due to the risk of moisture retention within the wall, calling for breathable materials, in this case, wood fibre and diathonite, a lime-cork-clay plaster.
Similarly, the existing ground-level suspended timber floors have been insulated between joists, using wood fibre for breathability and low embodied carbon. Although removing floorboards is disruptive compared with the alternative of using a robot to blow insulation from the crawl space, it does offer the opportunity to install vapour and air-tightness barriers.
Avoiding thermal bridging is particularly challenging in existing homes. Careful detailing is crucial, and the adage ‘you can’t build what you can’t draw’ is particularly true, since resolving issues as they appear on site may not be possible.
Where achievable, ‘wrap-around’ solutions such as the junction between external insulation and the extension’s warm roof are preferable. Other situations are more challenging, such as at the corner of the external insulation with a new extension cavity wall (since the party wall agreement called for an external brick finish), where aerated concrete blocks were used as a satisfactory compromise.
To maintain continuity at the junction between the new slab insulation and the insulation within the cavity wall, load-bearing insulation Thermoblock was used at the bottom of the inner cavity leaf.
Sometimes, however, thermal bridges still occur on site, such as when metal wall ties were installed instead of the specified low-conductivity Teplo-BFR ties and had to be replaced halfway through.
Detailing is also important for where the external insulation meets windows, in this case, ultra-high-efficiency Passivhaus-certified triple glazing (Eksalta timber-aluminum composite, since they would achieve the Uw values we were after, ranging from 0.79 to 0.88). Those are to be installed in the insulation line, supported by a pre-built timber box, in a low-cost alternative to EPS.
This low-cost approach is particularly relevant for the airtightness strategy, another focus of the project. High-cost membranes can sometimes be replaced with Grade 3 OSB board taped at junctions, such as for the floor. Similarly, the builders’ preference for plasterboard, which is not a suitable airtightness barrier, in lieu of wet plaster, was accommodated by installing a parge coat on the brickwork behind it. The parge coat here works as the airtight layer, with the added benefit of being protected by the plasterboard from getting chipped and damaged.
In general, the role of site staff is crucial in achieving good airtightness. I decided to have a site ‘toolbox talk’ on the subject, covering the principles and some of the solutions. This attracted much interest and participation from the builders, with questions and suggestions. This highlighted to me the readiness for low-carbon upskilling in our industry, but also the lack of opportunities to do so.
Further to fabric, some of the building services have started to be implemented. Although the air-source heat pump (which qualifies for a grant under the government’s Boiler Upgrade Scheme) is yet to go in, the first fix of the MVHR (mechanical ventilation with heat recovery) is in place. The unit, a 92 per cent recovery efficiency, Passivhaus-certified Zhender 200m3, will go in a 1 x 0.6m bathroom cupboard, while the ductwork runs in a few ceilings but mostly along the house’s ‘spine’ walls on each floor – not for the faint-hearted, but worry not, these will be boxed in!
Following the fabric efficiency measures, the MVHR is expected to provide an additional 32 per cent reduction in heating demand, with an estimated 10-year payback. Another piece of technology is already in place to facilitate waste water heat recovery (WWHR): a Showersave QB1-21 vertical system with 65 per cent recovery efficiency, which recovers the heat from the shower waste for the shower water supply. It is expected to achieve a 34 per cent reduction in hot water demand, with an estimated three-year payback.
All the above will eventually be monitored for its operational performance and compared against previous performance and software estimates. Before then, of course, the project still needs to be completed, which my family is eager to see happen!
So, is it viable? To be sure, a renovation is a good pretext for the disruption needed for some of these performance measures. Nonetheless, solutions such as insulation retrofit (carefully considering moisture), some level of airtightness improvement and glazing upgrades are viable for most homes. Some of these measures can also help with thermal comfort, air quality and acoustics. And, ultimately, they will go a long way in getting homes ready for heat pumps and a low-carbon future.
Ricardo Moreira is managing director of building performance consultancy XCO2