Scottish isle eco cottages need no central heating

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Building type: Two 89 sqm affordable timber frame cottages
Completed: June 2017
Location: Isle of Mull, Scotland
Budget: £417,000
EPC: B83
Standard: AECB Silver Standard
£20
monthly heating estimate

Located by the sea in a designated national scenic area on Mull, the community-led social housing project at Ulva Ferry consists of two semi-detached three-bedroom houses. Wrapped in larch cladding and sheets of corrugated steel, they are very much in line with the local aesthetic — and house bright, airy, open-plan spaces.

The project came about when Ulva School was threatened with closure, due to local population decline brought about by the area’s lack of affordable housing. This prompted the Ulva School Community Association to kick-start the affordable housing project, supported by the Mull and Iona Community Trust (MICT).

“Right from the outset, community members wanted energy efficient houses, because there’s a real problem with fuel poverty locally,” says Helen MacDonald, MICT’s local development officer. “There’s a lot of people living in very old, very poorly insulated houses, so they’re spending a lot of money on heating…I’ve heard of people spending £80-£100 a week.”

She explains that the emphasis of the project was on achieving an affordable and energy efficient building, and not necessarily on reaching a particular energy accreditation. However, as the chosen architecture firm Thorne Wyness has significant expertise in passive house design, it made sense to use the standard as a guide.

Architect Jenny Wyness chose cross-laminated timber – a smart engineered timber product which turns dimensional lumber into strong, rigid, dimensionally stable structural timber panels – for the building’s structure, alongside sustainably-sourced Scottish larch for the cladding, and corrugated galvanised steel roof sheeting, which is “a cheap, traditional roofing material in the area.”

The design makes a feature of this stripped-back but beautifully detailed palette of materials. There’s heat recovery ventilation and solar thermal collectors too, while space heating is primarily provided by Dimplex 1kW electric heaters in the main living spaces — there was no need for conventional central heating in these small, highly energy efficient homes.

Pipe runs were further reduced by closely grouping wet services, reducing heat loss. The houses are situated on quite an exposed site, which caused the team some challenges during construction. “We had concerns about manoeuvring the roofing panels into place in such a windy location. The wind also led to problems with handling and fixing the Kingspan insulation,” Wyness says. “We also had problems when applying the Extoseal tape, because of the cold weather.”

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Despite this, the building achieved an airtightness of 0.39 air changes per hour, well within the passive house standard of 0.6. But the project fell short of reaching the standard in other areas. “In general, we found passive house accreditation hard for a single-storey building,” says Wyness — this is because single-storey buildings have a greater surfaceto- volume ratio from which heat can escape, making it harder to reach the standard.

She believes the project’s tight budget also had a significant impact, explaining: “If we had upped our wall insulation by another 40mm, we may have reached passive house classification... But the budget wouldn’t allow it and we couldn’t justify it.” However, the two homes were certified to the less onerous AECB Silver Standard, which is based on passive house principles of fabricfirst design and high quality ventilation, and is designed using the Passive House Planning Package software. The homes also happen to meet the Passive House Institute’s standard for buildings which, for one reason or another, can’t quite make passive: the PHI low energy building standard.

Since June 2017, the units have been home to two families with six children between them. And accreditations notwithstanding, it is clear that these two new homes at Ulva Ferry are warm, comfortable and extremely energy efficient — quite a world away from the draughty and poorly insulated Victorian- era farmhouses and cottages that some local families are used to inhabiting.

“The families are absolutely delighted with their new homes and have commented on how warm and comfortable the houses are,” says Helen MacDonald. “Having been in private rental housing beforehand, they couldn’t believe how low the heating costs would be.” The only minor issue experienced so far has been some summer overheating. “One of the families sleeps with the windows open at night, and have been opening windows in the evening due to a build-up of heat,” says Jenny Wyness.

We’re delighted with the project,” Helen MacDonald summarises. “The houses are beautiful, I think they sit really nicely in their setting. And they’re working well, we’ve had no problems since the occupants moved in.” All in all, the social housing in Ulva Ferry certainly proves that beauty, energy efficiency and affordability can all be achieved in unison, even in a landscape as harsh as Mull’s.

1 Wire mesh in place to reinforce the concrete structural slab, below this is a DPM and 250mm Filcor 45 under-slab insulation; 2 the houses were built with cross laminated timber panels which provide strength, reduced construction time, and can help in minimising thermal bridges; 3 windows are Rationel triple glazed units with Extoseal Encors tape for sealing to the CLT panels

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