30 Jun 2017

A stirling example of Passivhaus best practice

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A traditional Scottish village in Stirlingshire may not seem like the obvious location for a contemporary Passivhaus project. For chartered Architect, Mhairi Grant and her Architectural Designer partner, Martin McCrae, however, the development of their new home, ‘Ostro’, provided the perfect opportunity to realise their ambitions for moving out of the city, creating more space for living and working, and pursuing their passion for Passivhaus principles.

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Mhairi and Martin had always run their architectural practice, Paper Igloo, from their Glasgow flat, working independently on projects but using their office space side by side to input into each other’s schemes.

“Home working is a formula that’s always worked for us,” says Mhairi, “but we knew at some point we’d move out of the city and that we’d want to design and build a home for ourselves.

“I’d always been interested in environmental innovation in design and it’s always been a cornerstone of our practice but the real catalyst came when I went on a one-day Passivhaus training course. I was genuinely inspired, not only to become an accredited Passivhaus designer, but also to embark on a project that would become a place where we could live and work outside the city and create a showpiece for our business, demonstrating what can be achieved with a Passivhaus and fabric-first approach.”

The couple began designing Ostro – named using an old Italian word meaning ‘south’ to reflect the south-facing position of their new home – around four years ago. Mhairi, with her Passivhaus credentials, took on the role of principal architect, while Martin effectively became the informed client, challenging the design and making suggestions as most self-builders would with their architect.

While the couple are architectural professionals, this scheme has been a self-build project in the very truest sense, with both Mhairi and Martin getting their hands dirty on site and being personally responsible for physically building their vision.

“The reason it’s taken four years from design to completion is that we’ve been fitting the build around working on client projects. Ostro may have been our first Passivhaus scheme on site but, since we broke ground in March 2014, we’ve completed on a private home designed to Passivhaus principles for a client, got a development of three Passivhaus houses on site and a further low-energy house has just started.

“While doing the work ourselves has helped us reduce the cost, there’s also been a huge professional value in researching the materials and physically using them ourselves because it means we understand the products we’ve chosen in terms of buildability as well as their environmental benefits and sustainability credentials.

Inspiration and support

“I knew I wanted to design an airtight building that was vapour-open and breathable, in line with Passivhaus principles,” Mhairi continues. “With the help of Google, I came across a supplier called Ecological Building Systems. They’re based in the UK and Ireland but offer a variety of environmental building products from across Europe, including Gutex wood fibre insulation from Germany and pro clima Airtight and Windtight membranes, tapes and seals.

“As I was researching their products online, I discovered that they also offered CPD (continuous professional development) presentations and signed up for one on ‘Airtightness & Moisture Management Using Intelligent Vapour Checks.’”

After attending their first CPD with Ecological Building Systems, the couple was invited to attend a ‘Better Building - Fabric First’ workshop by Ecological Building Systems in Ireland.

Explains Niall Crosson from Ecological Building Systems: “Fabric-first principles are all about creating a healthier indoor environment by selecting vapour-open products to create the ‘fabric’ of a breathable home, ensuring that heat is retained while allowing moisture to escape.

“In this way, energy consumption and costs are reduced, comfort is increased and the building is more sustainable because there is no risk of damp or interstitial condensation.”

Inspired by the workshop and its synergy with her Passivhaus goals, Mhairi returned to Scotland to work on the design and specification for Ostro and continued to work with Ecological Building Systems, enlisting the company’s support for some of the technical aspects of the specification process.

“Because Niall is a certified Passivhaus consultant and a certified Low Energy Buildings trainer, he really understood what we were trying to achieve and the need to achieve it as cost-effectively as possible,” Mhairi adds. “It was great to be able to draw on that knowledge both during the design process and throughout the build process.”

Passivhaus principles

From the outside, Ostro looks like a wooden box, with a square external footprint of 11.5 x 11.5m and a height of 7.5m. The square walls and flat roof have been softened by Mhairi’s choice of Siberian larch as the cladding material and a sedum green roof, which has been planted with species native to the local area.

“The house is completely different to anything anywhere near it,” says Mhairi. “We’re surrounded by typical Scottish villas and more modern but still traditionally-designed homes, so the house makes quite a statement.

“We didn’t design it as a cube to be controversial or futuristic though, we simply followed through the Passivhaus principles of reducing the potential for heat loss through unnecessary surface area or junctions, creating a more energy-efficient building through both simple form and high-performing fabric.”

The first step in the design process, aligned to Passivhaus best practice, was to maximise the natural aspect of the site. The house is on an exposed plot where it is not protected by any natural features and its Stirlingshire location is not known for its sunny climate. However, by ensuring that the house accesses maximum solar gain by orienting it to the south and west, the couple have been able to enhance the natural light in their living space and reduce the amount of heating required for the property.

What this means in practice is that there are fewer and smaller windows on the north-and east-facing elevations of the house, while a double-height living/dining area in the south-west corner of the building is framed by a 3.7m-tall triple-glazed window. This, like all the property’s other windows, was custom-built specifically for Ostro. This feature window maximises the sun’s heat and light from the south-facing aspect and draws light deep into the open-plan ground floor.

The ground floor accommodation is dominated by the large L-shaped kitchen/dining/living space that wraps around the south and west of the building, with a bathroom, guest bedroom, utility room and entrance hall taking up the remainder of the downstairs space.

Upstairs, there is a landing that looks down into the double-height dining area, along with two en-suite bedrooms and an office that has been given an airy, open-plan aspect thanks to a half-height wall that makes it part of the spacious double-height accommodation.

“Working from home has always been part of the vision,” Mhairi adds, “so finding a way to create a pleasant office environment that benefits from the most light-filled and spacious elements of the building without sacrificing living space was critical. This solution is both connected to the light-filled room and separate from our living area, so it’s an ideal approach.”

High-performance envelope

To create the indoor space, the house has been constructed using an insulated raft foundation; a certified Passivhaus component specified in line with Passivhaus standards that involves a concrete slab poured within an insulated formwork. Mhairi and Martin have chosen to use this concrete as the finish for the ground floor.

Mhairi explains: “We have tried to ensure that each element of the building fabric works as hard as possible, so we’ve exploited any structural element that can also be used as a finish or any material that can perform a dual purpose. It not only reduces costs, but also reduces the amount of materials we use, which answers our environmental aims and creates an honesty that is visible throughout the building.”

The building envelope is a twin-wall timber frame, with the two frames connected by ply gussets to create a thermal break; in effect, a timber cavity wall and 500mm of flexible wood fibre insulation was installed within the wall build-up.

On the external face of the timber envelope, the couple used 100mm-thick Gutex Multitherm wood fibre insulation from Ecological Building Systems, a vapour-open insulation made from untreated Black Forest spruce and fir timber with tongue and groove assembly. It combines breathability with excellent thermal performance and helps to eliminate thermal bridging.

On the internal face of the external timber wall, Mhairi and Martin elected to use DURÉLIS VapourBlock from Ecological Building Systems, a 12mm-thick high-density chipboard with a factory-fitted transparent airtightness and vapour control layer.

Mhairi continues: “Using DURÉLIS VapourBlock meant that we could achieve structural racking, airtightness and vapour control in a single, breathable product – rather than fitting both OSB and a membrane. When you’re doing the work yourself, achieving two things with a single job is a big help.”

The couple simply taped the joints with pro clima TESCON VANA between 2400 x 1200mm sections of DURÉLIS VapourBlock and installed battens onto the board before plasterboarding to form the service cavity.

The ground floor ceiling/first floor roof has been formed using engineered joists and the first floor also features a prefabricated board, composed of mineral wood which has been pre-fitted beneath two layers of acoustic board. This helps to manage the noise levels from the double-height space and the reclaimed parquet flooring that has been restored for the upstairs hallways and en-suites.

The roof build-up has, once again, focused on achieving both breathability and thermal performance. pro clima Intello Plus, an intelligent airtight vapour control membrane from Ecological Building Systems, was installed within the internal roof build-up to provide high diffusion tightness in winter and maximum diffusion openness in summer, protecting the building’s structure against condensation and mould growth. A fully insulated green roof system was then installed to complete the building envelope.

Low energy, low cost

Four years after first designing the house and more than three years after starting work on site, Mhairi and Martin have finally realised their goal of moving their home and their business into Ostro.

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