Witness the Phitness

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Building: Deep retrofit to 144 m2 detached home
Method: External insulation on cavity brick wall
Location: Barton-on-Sea, Hampshire
Standard: Enerphit (certified)
Space heating cost: £36 per month (see ‘In detail’ for more)

David Dacombe and Julie Botham spent the best part of three decades living in a cold and damp four-bedroom 1930s detached house, in Hampshire. Passionate environmentalists, they set out to insulate the 144 square metre brick property a few years ago. But somewhere along the way, their ambitions soared and a new goal emerged to achieve the Enerphit (passive house) standard for retrofits.

The project ended up costing 50 per cent more than they initially envisaged, but their “new” home achieved Enerphit and has transformed their living experience.

“We spent a scary amount of money in the end, but we’re very pleased with the house and definitely glad we did it. We’ve ended up with almost a brand-new home,” says David, a retired engineer (the project was not fully costed until the contracting stage, and bids from builders came back higher than anticipated). “We get lovely comments from visitors and passers-by about the outside. The tiles have been replaced with slates which together with white and grey render gives a contemporary appearance outside. Inside, the main advantage is the comfort all-year round as it used to be cold, draughty and damp in winter.”

Before consulting an architect in 2016, the couple spent a long time thinking through their options. One ambitious plan was to knock down their existing house and build three townhouses in its place, then sell them and use the profit to build a passive house. “But we couldn’t find a suitable plot anywhere in the local area, and we felt it wasn’t a green thing to do because of all the embodied energy,” says David. Neither did they want to tear themselves away from the local area, where they have lived since 1994.

The house is 500 metres from a beautiful stretch of coastline, near Bournemouth, and close to the border of the New Forest National Park.

This article was originally published in issue 39 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €10, or click here to receive the next issue free of charge

Another option was to attempt a DIY approach to insulation. David read up on how to do this and started by stripping off the plaster in two rooms. But the reality proved more complex than the manuals had suggested.

“Cold bridges were a big issue. If you don’t get rid of them, you end up introducing even bigger ones,” David says [editor’s note: when you insulate internally, any remaining cold bridges — uninsulated paths that cut through the insulation layer — are exacerbated and potential points for condensation].

At this stage, they opted to seek the help of a professional architect.

RIBA recommended three local architects and the couple were impressed with Ruth Butler, who also lives in Hampshire. She had previously built four passive houses, including her own home, previously featured in Passive House Plus issue 33.

When David and Julie met her in 2016, their intention was simply to improve the insulation. But it was obvious that significant internal changes would in any event be essential to upgrade the airtightness. The couple had become familiar with the Enerphit standard and decided to go for it.

Ruth worked with the couple to develop options in 2017 and showed them three reconfigurations using 3D models. David and Julie wanted an open-plan design and agreed to get rid of a conservatory. Ruth convinced them to give up plans for an extension in favour of spending more money on the existing structure. The original roof structure was kept too, and all-in-all the house provides a first-class example of how to undertake a deep retrofit while preserving as much of the existing building as possible, in order to reduce the embodied carbon of the build. An analysis in PHribbon calculated the embodied carbon of the retrofit at 111.7 kgCO2e per square metre, for building life cycle stages A (product stage) through to C (end of life). For comparison, this smashes even the older, more stringent RIBA 2030 target for new build homes of 300 kgCO2e, emphasising the importance of preserving existing structures.

There was no opposition from New Forest District Council to the application for planning. But finding the right builder proved far more challenging. When David and Julie’s initial choice pulled out just as the project was about to get underway in mid 2018, they nearly gave up.

“It was very frustrating and we were on the verge of quitting,” David says. “There were a few builders with passive house experience in other parts of the south, but no one was prepared to travel for a small job.”

Ruth persuaded them to change their strategy and give it another shot. Instead of looking for a specialist, they put out a tender for skilled local builders and hoped to train them up in passive house methods. There were two bids and Ruth advised them to go with Tuakana Construction, who are based nearby. “I thought they’d be a good fit for an Enerphit project as they’d worked on a lot of historic buildings,” Ruth says. “I think there’s a synergy with the attention to detail you need for passive house projects.”

The builders began work in February 2019 while the couple rented a property for eight months. Tuakana’s operations manager Neil Woodley says the company likes to be stretched by new challenges, but admits to a few nerves at the outset. “We were a bit daunted at first because we weren’t sure what would happen if we didn’t meet the Enerphit criteria. We even joked that if we don’t get passive house accreditation, do we not get paid? But when you break it down, if you follow all the necessary details, you reap the rewards,” he says.

Construction in progress

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The key to success was to get the experienced team of builders thinking a little bit differently. “The foreman in charge had been doing things in a certain way for 30 years,” Neil says. “Suddenly he was asked to do things differently, the standard methods and processes he had been adopting for years were suddenly not permitted, basic things like the running of electrical and plumbing services were carried out in a completely different way. The few times we slipped up were where we did things out of habit that we have always considered good practice in a standard build, but you have to think differently in passive house. It comes down to understanding the reason behind the different ways of doing things. If you realise why all the little details are important, it becomes a much much more logical process.”

To challenge their pre-existing habits, all the builders in the team received half a day’s hands-on airtightness training from Bill Butcher, director of Green Building Store. That proved invaluable: the project went on to achieve an airtightness of 0.48 air changes per hour, beating not just the Enerphit standard of 1.0 but also the passive house new build standard of 0.6. “The success was in many ways down to the training. The building team really related to Bill as a fellow builder. It meant that they all started off on the project with the same language and understanding,” says Ruth.

Naturally, one of the main difficulties of achieving Enerphit was working around the original structure. “You’re stuck with all the thermal bridges, and it can be complicated to get the airtightness line around the existing pieces of construction,” Ruth says. “It’s a nice challenge to have and you have to get your brain working in how to resolve issues in the simplest, most practical way possible. We chose to use plaster parge on the walls [as the airtightness line] and put membranes over the existing floor and ceiling.”

The experience of living in the house is radically different now.

To create an open-plan space the team removed a number of walls. The builders knocked through the old entrance lobby and pulled down the wall between the kitchen and dining room. The kitchen was re-modelled entirely. “Opening up the living and dining areas and having the stairs as part of the house makes it feel like a much bigger house. We’ve taken away a rather ratty old conservatory on the south-facing side which was preventing direct access to the garden. They’re keen gardeners so that was important for them,” she says.

Ruth’s plans for the retrofit included a new thermal jacket for the house. This involved installing a rendered external insulation system on the outside of the cavity brick walls, raising the timber flooring to add insulation and installing a Brink mechanical ventilation with heat recovery unit (MVHR). All the old heating pipework was stripped out, leaving just one 1.5 kW fitted electric radiator, and one portable 1.5 kW radiator that is now used in the lounge (separate to the open-plan area). This ultra-minimal heating setup is testament to David, Julie and Ruth’s confidence that the house will perform as it is designed to (of course, the mild climate on the south coast of England also made it easier to achieve the Enerphit standard).

The house had recently had most of its old windows replaced with triple glazed units from Green Building Store. But during the retrofit these had to be re-positioned to accommodate the external wall insulation, and some windows were moved too. Thankfully, this this was reasonably easy to accomplish as straps had been used to fit the windows. They were moved 200 mm out into the insulation zone, and deeper extension sills were fitted.

There were not as many structural changes to the four bedrooms upstairs. But the design enlarged the bathroom, allowing both a shower and bath to be fitted. The builders also reconfigured the roof slopes and removed a dormer with two windows, so the bedrooms became less constrained.

To be awarded an Enerphit certificate, retrofitted homes must achieve a maximum space heating demand of 25 kWh/m2/yr. PHPP, the passive house design software, showed that the remodeled house’s space heating demand was 19 kWh/m2/yr, and it was certified in 2020. It was a finalist in the 2021 UK Passivhaus Awards in the small projects category.

David and Julie have monitored the house’s energy consumption over the past two years and they are using approximately one third of what they were pre-retrofit, although this has only resulted in marginally lower energy bills – likely due to comfort-taking and the switch from comparatively inexpensive gas to direct electric for heating and hot water. (see ‘In detail’ for more). In summer, they generate a small amount of electricity from their 12 m2 solar PV array and never switch the heating on. David says the lounge can sometimes get too warm, but that they manage this with an internal blind and by opening the bi-fold doors. In winter, the fitted electric radiator in the open place space provides whole house heating; the portable radiator in the lounge is only used on very cold winter days. The experience of living in the house, David says, is radically different now.

For Ruth Butler, the New Forest Enerphit project proved equally as life-changing. Although passionate about passive house new build, she points out that retrofitting old houses is more critical to the climate crisis.

“When they said they were interested in a deep retrofit I jumped at the chance. It turned out to be a landmark project in my career as an architect. It helped me to realise that deep retrofitting was my primary concern as an architect.”

The New Forest Enerphit turned out to be her final design for her own company, Ruth Butler Architects. In February 2020, she took a new job with Pritchard Architecture, in Portsmouth. The company carries out a lot of conservation and conversion work and Ruth’s role is to work on low energy refits. Her current projects include retrofitting a home in Prinsted, near Chichester and converting a barn into offices for Seven Sisters Country Park.

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