Passivhaus site visit: ‘We think this is the future’

The UK has finally woken up to Passivhaus. Seventeen years after the German low-energy standard was introduced, buildings are springing up here that adopt the principle of a well-sealed, highly insulated and carefully constructed envelope.

One such project now under construction, and attracting an army of followers on Building’s website, is Geoff and Kate Tunstall’s home in West Yorkshire. This is a twist on the approach as it is the first of the 10,000-plus buildings certified by the Passivhaus Institute to be built using cavity-wall construction.

Most Passivhaus buildings in Germany and Austria use timber-frame construction or solid masonry walls with external insulation and render. “It would have been easier for us to do that as it removes a lot of difficulties with air-tightness and thermal bridging,”

says Bill Butcher, director of the Green Building Company, which is constructing the house.

The planners in West Yorkshire wouldn’t allow render on the exterior, however.

“We had to come up with a regional response to meeting the standards and we’re used to building cavity walls, so why not take this approach? The thing about Passivhaus is you can achieve it in any way you want, you aren’t restricted to certain solutions.”

There are good reasons for not using cavity walls, though. Blockwork is not air-tight and the design relies on the wet plaster applied on the inner wall to create the air-tight barrier. But Butcher insists this solution could be rolled out on a large scale. “It does demand greater attention to detail and you need buy-in from the client right down to the workers on site, but it’s a construction method housebuilders are familiar with.”

On larger projects, he suggests employing an air-tightness foreman, to ensure the air-tight seal is achieved.

On top of the air-tight envelope, super-insulation and high-specification glazing, the magic figure for any Passivhaus building is 15kW/m2/year for space heating. To hit this difficult target, the original L-shaped footprint for Denby Dale had to be dropped. “There was simply too much glazing and external envelope in comparison to the floor area to get it to work,” says Butcher.

“So we had to persuade the clients to go for a rectangular design.”

The big draw of Passivhaus, say its supporters, is that it doesn’t rely on the new technologies needed to meet the Code for Sustainable Homes. The Denby Dale house is expected to consume just 13kW/m2/year, but achieving this requires careful planning and construction. Here, we offer an insight into how this is being done.

The clients’ story

Kate and Geoff Tunstall

Kate and Geoff Tunstall had never heard of Passivhaus when they began thinking about downsizing from their 250-year-old Yorkshire terrace and came up with the idea of building a new house in their back garden. “We were looking to the future and wanted something low-maintenance,” says Geoff.

“The idea was to build a stealth house, something that looked ordinary but was high performance in terms of sustainability.”

Geoff, a retired art and design teacher, and Kate, a textile designer, had not been involved in a building project before, but they researched their ideas before looking around for someone to build it. The initial design was for a dormer bungalow with ground source heat pumps, solar thermal panels and photovoltaics. When the Tunstalls approached the Green Building Company, however, they were introduced to the idea of building a highly insulated, air-tight house that would require less energy to run in the first place.

The couple initially had concerns about the Passivhaus approach. Would it mean small windows and would the mechanical ventilation system be too noisy? In January 2007, they travelled to Austria to visit some Passivhaus properties.

“These weren’t large expensive houses, just ordinary homes,” says Kate. “It was well below zero outside but inside they were comfortable even with the minimal heating they had, so we knew they performed.

“We were also able to talk to the occupants about what it was like to live in one and dispel some of the myths, such as not being able to open the windows. It alleviated our fears and we came back totally enthused.”

Energy consultant Peter Warm and Huddersfield architect Derrie O’Sullivan were appointed. A key part of any Passivhaus project is the planning package used to model the design. “We have had to compromise in certain areas but one of the things we wanted was reasonable-sized windows on the north side of the house overlooking the garden,” says Geoff. “We were able to model this and show that the design still worked. For us it is our home, we live in it, so our criteria are sometimes different to those of the designers.”

The budget was tight and it was debatable whether it could be built with the £140,000 available. Compromises were made, particularly concerning materials. “In Austria the government offers grants to people so they can build to Passivhaus standards. That’s what we need in the UK,” says Kate.

The house is due for completion next June. “We want as many people as possible to learn from what we have done here,“ says Geoff. “We think this is the future.”

Tackling thermal bridging

One of the topics that has received most comments on Building’s website is the issue of thermal bridging, and the designer has come up with some novel details to address this. The major thermal bridge is the junction between the ground floor and walls. To tackle this, 300mm of polystyrene block insulation is used in the wall cavities between the foundation level and the ground-floor slab while the blockwork beneath the floor slab is built from lightweight insulated blocks. Instead of the usual stainless steel wall ties, teplo ties made of resin and basalt are being used. These are good insulators and don’t act as a thermal bridge that could create cold spots on the internal wall. The thresholds for the doors and windows are also weak points. It was decided, however, to carry the concrete floor slab through the wall cavity at ground-floor level. Although this sacrifices some insulation, it was felt worthwhile to provide structural stability for the doors and large south-facing windows. The windows are also positioned halfway within the insulation layer, to again minimise thermal bridging.