Visionary vernacular

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House type: 107 m2 bungalow
Method: Wood fibre insulated timber frame infrared heating
Location: Dumfries and Galloway
Standard: AECB CarbonLite Building Standard
Heating & hot water cost: £90/month*
* Estimated space heating and hot water cost, based on Ofgem’s energy price guarantee for October 2022.
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It’s been shown clearly over the last few years that passive house design lends itself relatively easily to a variety of building forms thanks to the versatility of PHPP software.

But every so often, a combination of tight budgets and less than ideal form factors present a challenge that gives even those architects passionate about the standard a reason to rethink their approaches, without jettisoning the efficiency, sustainability, and attention to detail that fully certified passive houses demand.

Beechtrees, a lovely, modest larch-clad, zinc-roofed house in Scotland built by seasoned Dundee-based passive house designer Kirsty Maguire, is one such case.

It’s built to the AECB CarbonLite Building Standard, which is based on passive house and uses the same software (PHPP), but with less onerous targets for energy performance.

Located in a rural area in the middle of Dumfries and Galloway, a large region in the southwest corner of the country, it’s an unusual and innovative build that doesn’t try to ape a traditional farmhouse, but still strongly reflects the local vernacular of metal roofs and timber cladding of the neighbouring farm buildings.

The inspiration Kirsty Maguire took from around the site is infectious: the link building between the living and bedroom wings has the same colouring as “a beautifully weathered barn with a rust red roof,” and a similar nearby shed.

Built for two retirees, Tony Francis and Zoe Roberts, as their forever home, it’s also single-storey, meaning it will be easier for the couple to navigate in future years. That comes at an energy cost as it increases the building’s surface area to volume ratio or ‘form factor’. Future-proofing the house to allow for the occupants’ changing needs in this case means more walls, roof and ground floor per square metre, and therefore more surfaces through which heat is lost. The form factor is also not helped by the H-shaped floor plan, designed to give the couple two courtyards.

Built for two retirees, Tony Francis and Zoe Roberts, as their forever home, it’s also single-storey, meaning it will be easier for the couple to navigate in future years. That comes at an energy cost as it increases the building’s surface area to volume ratio or ‘form factor’. Future-proofing the house to allow for the occupants’ changing needs in this case means more walls, roof and ground floor per square metre, and therefore more surfaces through which heat is lost. The form factor is also not helped by the H-shaped floor plan, designed to give the couple two courtyards.

It’s designed essentially so that the external spaces are conceived as external rooms, as the clients spend a lot of time outdoors,” said Kirsty Maguire.

The courtyards each exploit the light at different times of day, a bit like a sundial. According to Maguire, the east courtyard, opening from the kitchen, is perfect for sitting outside with a morning coffee. As the sun moves around, it hits the south side with the pond and vegetable garden, bathing the outdoor dining table in sunlight from late morning until evening. It has a generous living space with a cathedral ceiling in the main part of the building. The bedroom wing is similar but more private and set back, and there’s a welcoming entrance space linking the two.

Tony and Zoe were adamant the house shouldn’t be too big for their needs or have any wasted space, and no clutter. “We have no space that’s underutilised, and no junk”.

What the whole space gains inside and outside as a result of this clever design is clearly worth the concession – if you can call it that – of building to the AECB CarbonLite Building Standard instead of passive house.

A quick look at the building fabric specs shows that Beechtrees has exactly what you’d expect to see in a passive house: the U-values for walls, roof, floor and windows, the thermal bridging and the airtightness, and the Zehnder heat recovery ventilation system. It also has infrared heating for the little warmth that’s required and an air source heat pump – using the ultra-low global warming potential (GWP) refrigerant R290 – for hot water.

Named after the mature deciduous trees from the site, Beechtrees is all about timber. As many trees were retained as much as possible, with the felled trees being turned into cills and shelving by a local craftsman. The building’s superstructure is timber frame, with Steico wood fibre insulation in walls and roof, and triple glazed timber windows. The project’s broader sustainability efforts extended into other materials too. Reclaimed materials are used as well, such as paviours, and the drystone dyke was rebuilt by Tony using stone from site. It’s reasonable to argue that if the budget was more flexible, a greater investment in wall insulation, for instance, could have brought the overall energy performance closer to the passive house standard, but other choices took priority, according to Maguire.

“That relationship to the [external] spaces on the site was really key to the layout of the building,” she said. “As a result, we’ve got these two wings and a link, which does make the form factor higher, and therefore there’s a point in time where you can keep putting more and more insulation in but there’s a balance [to be struck]. And that’s where we used PHPP – to start to identify where that balance is, and where you can still get good performance.”

The wall insulation in Beechtrees is far from skimpy, with 240 mm of Steico Flex wood fibre insulation fitted between the timber frame, with a further 100 mm of Steico Special Dry wood fibre boards wrapping the frame externally, delivering a U-value of 0.127. Nonetheless the design allows for more: an extra layer of insulation could be installed very easily by simply removing the timber cladding at some point later, giving this dwelling a highly satisfying degree of cheap and easy future-proofing.

Besides the lower energy targets, the key difference between the AECB and the passive house standard is around certification.

We needed a mud room for the dogs – a place where three filthy Labradors can go and shake.

In a nutshell, certification for a passive house can only be awarded by an independent certifier, while the AECB relies on self-certification by the designer or consultant.

According to the AECB website: “The AECB self-certification process is designed to make explicit the project’s claim to be a low energy design and to provide the consumer with a degree of protection under trading standards – without the AECB having to get involved in quality control and legal matters. “This approach puts the responsibility for performance claims clearly with the person signing the certificate and a duty of care on the client to ensure that the consultant is competent and suitably insured.”

Kirsty Maguire herself is a big advocate of passive house certification, and recently undertook passive house certifier training, becoming the only Scottish-based consultant to have this. Having designed several certified passive house projects over the last decade, Kirsty says with Beechtrees she made sure the quality assurance process was the same, along with the details and the general approach to the project.

Which was surely music to the ears of Tony and Zoe, who were introduced to the passive house concept by their daughter, and were inspired enough to make enquiries that quickly led them to Maguire’s door and the start of an in-depth but by all accounts very amicable and free-flowing discussion about what they wanted. Their brief was for an environmentally sustainable and warm home that maximised the access to the outdoor space around the site so that they could enjoy the garden and walk their dogs and entertain family, and also recycle as many of its elements as possible.

“There were quite a lot of things we put on the brief,” says Zoe, “such as being able to have the doors open when we’re cooking, and positioning the house so we could make the most of the private bit of garden… we needed a mud room for the dogs – a place where three filthy Labradors can go and shake.”

They were also adamant that the house shouldn’t be too big for their needs or have any wasted space. “I think we were very keen to have no clutter. We were ruthless before we moved in about throwing things out that hadn’t been used. And thankfully we have no space that’s underutilised, but neither do we have any junk.”

This article was originally published in issue 44 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

Kirsty’s experience and a highly communicative approach to working with Zoe and Tony, allied with the enthusiasm and commitment of a local contractor undertaking one of his first passive house-informed projects, meant that Beechtrees succeeded – even exceeded – expectations in terms of meeting their quite specific demands.

“Kirsty is so kind of efficient and pedantic, in the nicest possible way, about detail that she and her assistant really pushed us hard to make decisions before it even got out of the ground, such as things about light switches, outside lights, positioning of inside lights, colour of the kitchen, colour of the tiles,” said Zoe, who even admits to getting a “wee bit stressy” about it as the process went on, “but oh my goodness, did we realise the value of that during the build”.

They’re also full of praise for Kirsty’s choice of contractor, David Broatch of Broatch Construction, a firm based just 20 miles away, even though he had limited previous experience of working on passive houses. “They had never built a house like this before, and... Kirsty must have had some intuition about them,” said Tony, who was sceptical that they would take on the job with the limited budget they had, “but she was right. They really wanted to get it right. And they concentrated so hard on doing that.”

It was clearly a transformative experience for Broatch, because he has gone on to completely change his business focus as a result of this project, heading into passive house in a big way. The firm has recently completed building a home for the daughter of wellknown passive house architect Jonathan Hines, managing director of Architype, on the strength of Kirsty’s recommendation.

Adding to the highly positive build experience was the efficient sourcing of materials just before the chaos of the Covid pandemic, so that the only supply issue was bathroom tiles, which Tony and Zoe sourced elsewhere in order to keep to the schedule. Only the windows and doors were a little late, but this added just three weeks to the projected 16-week build time.

The choice of timber frame construction was a simple decision given timber frame’s dominant position north of the border, so expertise and skills were easy to find.

“We’ve got obviously great joiners across everywhere who are familiar with it so it’s a sort of standard way of doing things up here for buildings at this scale." Kirsty also describes Beechtrees as a bit of a dream project. “We had great clients, really fantastic to work with, really good at making decisions, and great ideas. We also had really good colleagues in the design team so we could work together quickly in a straightforward way.

“And then working with the contractor was fantastic,” she added. “They would come to us with the potential of how to make things even better than we had so we can learn from them as well and keep that discussion flowing. The work that they did was in accordance with drawings and beautifully done, and it was on time, on budget during Covid. I’m not quite sure what more we could have asked for really.”

It’s rare that a client is less than delighted with any project worthy of being featured in this magazine, but Tony and Zoe exude a particular pride in what their two-bed home represents in terms of its modesty, efficiency, sustainability and, above all, a dwelling that’s tailor-made for them right down to the last detail and with no attic or any kind of wasted space. And of course, a very comfortable home.

• The house is named after the mature deciduous trees from the site The house is named after the mature deciduous trees from the site
• Isoquick insulated foundation system and timber frame Isoquick insulated foundation system and timber frame
• 400 mm Steico Flex wood fibre insulation in the bedroom wing 400 mm Steico Flex wood fibre insulation in the bedroom wing
• Airtightness taping at window Airtightness taping at window
• Heat recovery ventilation ducts in void inside airtight layer Heat recovery ventilation ducts in void inside airtight layer
• Threshold detail Threshold detail
• Salvaged tiles Salvaged tiles
• Salvaged tiles Salvaged tiles
• Greencoat PLX standing seam roofing system Greencoat PLX standing seam roofing system
•  

Having lived all their lives in old houses with nooks and crannies, big sandstone walls that Tony describes as “just big sponges full of water”, they now live in an “almost perfectly controlled environment”.

“We’ve got the air circulation system in the heat exchanger, which brings in fresh air, collects the warmth from the house and pumps out the stale air. We’ve got fresh air to breathe; even if we don’t open a single door or window, we’ve got lovely fresh air, of the right temperature.”

They’re particularly pleased with the work of a local furniture maker, Philip Wilson, who cut down the beech tree that obviously inspired the house’s nomenclature and then created beautifully rustic shelves and windowsills with the wood, all finished with a very distinctive blue resin. “It’s terribly, terribly original and just so beautiful to look at.”

Viewed purely from an energy performance perspective, Beechtrees could have pushed the envelope a little more with still more insulation or a more compact form. But energy performance and thermal comfort are not the only requirements to consider, and this thoughtful, skilfully designed house makes a strong case for allowing a little leeway, and is tailored to meet the needs of its owners throughout their later years.