Phit the bill

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Building: : Deep retrofit & extension of 151 m2 detached 1960s house
Method: External wood fibre insulation, timber frame, air-to-water heat pump
Location: Harpenden, Hertfordshire
Standard: Enerphit Plus (certified)
Heating cost: £23/month*
* Based on monitored energy use for space heating – adjusted to current UK energy price cap and ignoring PV reduction.
See ‘In detail’ panel for more information.

A radical retrofit in a conservation area sounds like a recipe for trouble, but despite challenges including planning and a general lack of understanding in the national market, one project in Harpenden, St Albans, Hertfordshire has led the way, demonstrating just how much energy performance can be improved in old houses.

Owners Jonathan and Emma Dixon were inspired to take on the challenge by a number of factors including their own professional backgrounds as well as experiences abroad.

“I guess, it came from a few different aspects. Since we bought our first house in 2005, I’d always been interested in generally reducing energy use and using decarbonised energy sources, solar and so on,” Jonathan Dixon said.

Both engineers by profession, the Dixons were not only keenly aware of issues around energy consumption and embodied energy, but were also inspired by travels around Europe, and in comparison to what they saw, they found UK housing stock lacking.

“We saw better quality houses, particularly in Switzerland. We [also] lived in California and saw how terrible houses were there: drafty and not well-insulated,” he said.

The differences in mainland Europe were twofold: on the one hand a greater understanding of energy issues and sustainability, but also better production processes and onsite controls.

“In countries like Germany there is a basic level of building control and standards. They’re years ahead of us in the UK,” he said.

On returning to Britain the couple decided to do something about it. However, on moving back the couple were living in London but planning to move out to Hertfordshire to stay within a relatively easy commute of the capital. A partial rebuild was considered, but proved too costly in terms of time, so the couple settled on a retrofit and small extension on a 1960s house.

Before

(inset) Before

This article was originally published in issue 43 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €15, or click here to receive the next issue free of charge

What was not yet obvious, however, was just how comprehensive this project would be. Experience of low energy housing and a chance conversation drove the project toward a deep retrofit and passive house standard.

“We envisaged a relatively small project, but it grew in the process as I started to think about what they do in other countries. I hadn’t heard of the term passive house in 2018, but a friend of mine was doing a retrofit and I started to research it,” Dixon said.

Finding skills and support

The single most important part of the research was finding an architect who could create the design and complete the project, no small task given how rare such deep retrofits have been in Britain.

The answer to this conundrum was Heather McNeill of A D Practice in St Albans, to whom the couple turned having considered nine different practices.

“We found Heather, a qualified passive house architect. She spearheaded it and helped us to get it over the line,” said Dixon.

“We spoke to nine different architects, four local without passive house experience [and] four regional ones with passive house experience. We were really struggling to choose and then we happened to see Heather on Facebook. We really liked the things she said to us, pointing out aspects of the way the house was built and ways the quality of the build could be improved. She was really thinking outside of the box,” he said.

Indeed, McNeill is an experienced architect with a background in sustainability. She also has unusual experience in retrofits. After taking her first job working in CAD at the A D Practice she studied for her Part 1 at the University of Bath, and had a stint in practice before completing her Part 2 at the Centre for Alternative Technology in Machynlleth in Wales, demonstrating a strong interest in sustainability.

“I did my Part 2 there because that was the direction I wanted to go,” she said. Now a certified passive house designer, the Dixons’ Harpenden house formed the basis for McNeill’s Part 3 case study at Kingston University, a happy coincidence that drove the project forward.

“You learn all the theory, but there’s a lot more to it than that. I got the job almost by accident: we were the ninth architect they’d spoken to and this being a retrofit it’s arguably more difficult to do. As it was my case study for my Part 3 exam, I lived and breathed it,” she said.

The first challenge McNeill faced was planning: the house is in the Harpenden Common conservation area.

“It’s a very notable house where it is, so people would notice it [and] it was the end of a row of six, so it had to keep similar proportions to the existing houses,” she said.

As is typical with 1960s buildings, a lot of work was required to get the house up to modern standards of energy efficiency, let alone Enerphit Plus.

(inset) Before

I might well make an operating profit on energy generation in the next year

“When we turned up outside, we found it had a very thin curtain wall and our first thought was to put in proper, insulated walls.”

In the end, the existing brickwork walls were externally insulated using wood fibreboard and finished with a painted render, and the curtain walls were removed and replaced with new highly insulated timber I-beam walls with cellulose insulation and clad in larch. The rear single-storey extension was also constructed in this way and finished with either render or timber cladding. The existing ground floor was removed and replaced with a new highly insulated slab with underfloor heating.

Planning was not a problem, with McNeill describing the planners as relaxed. This despite the entire roof being a solar array.

“The classic [planning] problem is where renewables come in, but they [the authorities] were really keen,” she said.

“It was about aesthetics as well as getting maximum performance. We’re in a conservation area and the south-west pitch directly faces the road and no one else had solar installed,” said Dixon.

Like many aspects of the project, this array did require some extra care, but it was nonetheless a success.

“For the contractors it was their first retrofit [and] the fully integrated solar needs a fully square and true roof. The undulations meant it needed that bit of extra work,” said Dixon. “They’ve [the local planning authority] now got a default position of favouring sustainable measures, but we never wanted the retrofit look.”

Unexpected stop

However, before any of this work could be completed, external factors intervened in the form of the world’s largest spanner in the works: the coronavirus pandemic.

“We started in January 2020 and then, in March, the pandemic hit, and everything shut down. We had just done our demolition and had a house with no front wall, everything had to stop. That was a slightly hairy moment,” said Dixon.

As a result of strict regulations, not only did work have to stop, but the owners were prohibited from visiting the site until distance restrictions were lifted.

• Original rear elevation with curtain wall panels removed and extension slab laid Original rear elevation with curtain wall panels removed and extension slab laid
• Existing rafters underlined with rigid woodfibre board and opening trimmed out for rooflight over stairs Existing rafters underlined with rigid woodfibre board and opening trimmed out for rooflight over stairs
• Airtight membrane taped around original roof trusses in loft space housing MVHR Airtight membrane taped around original roof trusses in loft space housing MVHR
• Airtight taping and service void around flat roof light in rear extension Airtight taping and service void around flat roof light in rear extension
• Insulation upstand below service void prior to screed being poured Insulation upstand below service void prior to screed being poured
• I-stud construction to rear extension lined with Smartply I-stud construction to rear extension lined with Smartply
• PIR plinth insulation and stop bead up to DPM level ready for external woodfibre insulation to be installed above PIR plinth insulation and stop bead up to DPM level ready for external woodfibre insulation to be installed above
• Vent penetration through external wood fibre insulation Vent penetration through external wood fibre insulation
• New window installed in existing cavity wall, protruding from brickwork to link into external insulation layer New window installed in existing cavity wall, protruding from brickwork to link into external insulation layer
• External woodfibre insulation installed on the rear extension around the picture window External woodfibre insulation installed on the rear extension around the picture window
• Render beads and base coat applied to woodfibre insulation around window opening Render beads and base coat applied to woodfibre insulation around window opening
• Plywood stop plate installed at end of woodfire insulation/ thermal line Plywood stop plate installed at end of woodfire insulation/ thermal line
• Woodfibre sheathing board installed over the insulated doubled-up rafters Woodfibre sheathing board installed over the insulated doubled-up rafters
• Roof membrane taped and installed over exposed rafter feet to be lapped into gutter Roof membrane taped and installed over exposed rafter feet to be lapped into gutter
• Aluminium frame installed on front roof pitch for seamless solar pv array Aluminium frame installed on front roof pitch for seamless solar pv array
• Tiles on rear roof pitch Tiles on rear roof pitch
•  

Construction Photos: Jigsaw Design and Construction Ltd

Once work resumed, McNeill found more remedial work was necessary. “We ended up removing every partition upstairs. It was clinker block, built off floorboards with no support,” she said.

It only takes a fleeting comparison of the before and after photos of the house’s exterior and interior to get a sense of how extensive the remodelling and renovation works to the building were. The budget - £378,000, excluding VAT, site purchase and fees – is not insubstantial. But readers should pause for thought before drawing conclusions from this project about the cost of deep retrofit. The house was gutted from top to bottom, and completely remodelled.

The pertinent question would therefore be: what would the marginal extra cost be for the energy efficiency measures – or more precisely, the marginal extra cost for the extra energy efficiency measures beyond those required under building regulations for renovations of this nature?

Such a question may be beyond the scope of this case study – in truth it may require a significant article or academic paper in its own right. But the question of energy use and costs does bear asking, in spite of the energy crisis making price estimate hard to gauge.

We’re in a conservation area and the south-west pitch directly faces the road and no one else had solar installed

“Financial break-even was never the goal – simply energy and ideally CO2 net zero,” says Jonathan Dixon.

The use profile is key to understand. Sharing the house with their two cats, Jonathan and Emma Dixon both work from home.

“I have a lot of energy use for my work – several computers for software development projects,” says Jonathan Dixon. “Our overall usage is well outside the PHPP model for normal users.”

Aside from the energy saving elements that characterise passive and Enerphit projects – airtightness, high levels of continuous insulation, triple glazing and heat recovery ventilation – the project benefits from a number of other elements to reduce exposure of a household and office with high energy utility needs, to volatile energy prices.

The mammoth PV array has recently been paired with a 10 kW battery, and the house’s heating and hot water is generated by a Mitsubishi air-to-water heat pump, with a Showersave wastewater heat recovery system stopping precious energy from literally going down the drain. The net effect? Ultra-low energy use at the heat pump of 1,247 kWh per year. Even with the substantial contribution the PV array would be making to help meet that demand, particularly for hot water, the cost would be £424 per annum based on the UK’s current energy price cap.

But this doesn’t tell the full story. Taking account of the building’s extraordinary energy efficiency, export tariffs for selling spare electricity back to the grid, and the Dixons’ efforts to reduce the house’s baseload, Jonathan makes a startling claim in an energy crisis. “I might well make an operating profit on energy generation in the next year, but I’ll need to wait a year to really tell,” he says.

From London dirt and damp to fresh air

Retrofit is something I’ve always wanted to do, and it’s really not taught in architecture school

Although the decision to install heat recovery ventilation is effectively a precondition for passive houses in UK climate zones, Dixon was never going to object, based on his previous experience of older British houses.

“The 1930s house we previously owned in London was always damp and we were always opening and closing windows. With this, we wanted to keep the noise out, keep the dirt out and have better air quality. There’s so little dust in the house it’s something we just never think about and obviously there’s no damp. It doesn’t build up from even internal sources.

“One thing it doesn’t do is completely solve overheating risk. And that’s the one thing we didn’t model on a room-by-room basis” he said, though the issue was modelled in PHPP on an overall building basis.

Both client and architect agree that the final result was worth the effort, however. For example, Dixon says the performance of the building exceeds any expectations people typically have of retrofits. “We’re twice as airtight as we need to be for a retro. It’s as good as a new build. That required a constant attention to detail,” he said.

For McNeill, the house, and indeed the experience of the project, serve not only as proof that deep retrofits could transform the country’s housing stock, but also as a means for her to explain how retrofits should be approached and even taught.

“Retrofit is something I’ve always wanted to do, and it’s really not taught in architecture school. Explaining it to other people really helped me understand what I was doing,” she said.

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