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Wake Green Road

A 1920s house to be refurbished to Passivhaus standards without altering the appearance of the facade? A tough challenge, but one that architect Helen Seymour-Smith relished. External wall insulation from Sto formed a key part of the project and some careful detailing was required to make sure the planners were kept happy.

When the Isaac family purchased a ramshackle 1920s house on the outskirts of Birmingham they wanted to transform it into a beacon of energy efficiency. Nick Isaac's parents had built their own, highly insulated home which he says was "always warm in winter and cool in summer". So when the family set about refurbishing Wake Green Road one of the key challenges was to significantly improve the building's thermal performance.

The family set about researching super-insulation systems. "If you Google 'super insulation' you come across Passivhaus pretty quickly," Nick says. Passivhaus is a fabric-first approach to constructing low-energy buildings, which was developed in Germany in the late 1990s and is rapidly finding favour with energy conscious home owners in the UK. Aiming for a Passivhaus standard refurbishment would ensure the family home had both low running costs and low carbon emissions. "It ticked all the boxes," he says.

Finding the Right Solution

Having decided on a Passivhaus solution the Isaacs then needed to find an architect with experience of Passivhaus design who was capable of making the system work for their mock-Tudor family home. Their research put them in touch with Seymour-Smith Architects, a practice specialising in houses built to Passivhaus standard and with experience of responding to challenging planning situations. "We liked what Helen Seymour-Smith had done previously, she has the right experience and we got on well with her," Nick Isaac says.

"The brief was to deliver a building that would perform to Passivhaus standards in energy terms, while in space terms the clients wanted to reorganise and rearrange the house inside and to add a small rear extension," says Helen Seymour-Smith.

The architect's response was to re-orientate the back of the property so that it faces closer to due south. This enabled space to be added for a kitchen extension while increasing the amount of south-facing glazing to allow heat and light from the low-angled winter sun to flood the building. To prevent overheating in summer, the design also includes an overhanging roof to shade the glazing and give the rear of the building a more contemporary twist. "Our design opened up the back of the building," Seymour-Smith explains.

The Passivhaus refurbishment also had to satisfy the planners. They were keen that the front of the house retained its brick-base and half-timbered upper floor appearance. "The building was one of the original homes in the road; later additions to the neighbourhood were designed in a style that acknowledged this building so the planners were keen not to lose the original architectural reference," Seymour-Smith says.

The StoTherm Classic external wall insulation system was used to add 250mm of insulation to three of the building's four elevations. The advantage of the Sto external wall insulation system is that it enables acrylic brick slips and acrylic half timbers to be glued to the insulated wall to recreate the building's original appearance. "It looks really quite convincing," says Seymour-Smith. The building's fourth elevation formed the boundary with a neighbouring property, so external insulation was not an option. On this elevation, 100 mm of breathable wood fibre insulation was applied internally instead.

Comparing Internal & External Insulation

Having both internal and external insulation systems on the same project enabled a simple comparison to be made of two systems. The external insulation solution means that, within reason, there is no limit to the amount of insulation that can be added and there is no risk of condensation forming within the wall. It also keeps the thermal mass of the structure exposed inside the building to help regulate internal temperatures. "It's all good," says Seymour-Smith.

The internal solution was much less satisfactory because the thickness of insulation is limited by the need to prevent interstitial condensation. "The resulting U-value we were able to achieve was not a patch on the externally insulated solution," Seymour-Smith explains.

Achieving Airtightness

The design aimed to meet the Passivhaus EnerPHit standard for refurbished buildings, which meant that in addition to high levels of fabric insulation it also had to be airtight. It is much more of a challenge making a refurbished building airtight than it is a new building because many of the air gaps in the fabric elements are already in place. "The airtightness, and particularly the continuity of insulation and the elimination of thermal bridges was certainly helped using external insulation rather than internal, which needed to a lot of complicated detailing," says Seymour-Smith

Nick Isaac says that achieving airtightness was a "major, major effort that was by far the most difficult element of the refurbishment". After plaster had been applied to the walls, and with the airtightness membrane in place at ceiling level, the building was pressure tested by installing a fan in the front door opening to pressurise and depressurise the house. The tests showed the house leaked too much air to meet the strict Passivhaus criteria.

As a consequence Nick Isaac, the architect and the contractor went through the refurbished house pressure testing room by room and to improve the airtightness. "There was a two-week period where we pressure tested room by room; we taped over any leaking joints and silicone back-filled a lot of the electric points which were found to be prone to leakage," explains Nick.

Once the building was airtight, the remaining internal finishes could be applied.

Further Energy Saving Measures

In addition to the external wall insulation other energy saving additions included:

  • Replacing the rotted suspended timber ground floor with a highly insulated concrete system that incorporated an underfloor heating system.
  • Installation of a mechanical ventilation system with heat recovery to supply fresh air to the living spaces, warmed by heat from air extracted from the bathrooms and kitchen
  • Triple glazed windows
  • Three solar thermal panels installed on the building's south-facing roof at the rear of the property.

"The addition of solar panels means that for six months of the year we have virtually no gas costs" Nick says.

The family moved into the house on Christmas Eve 2011. The external wall insulation has been a big success in helping maintain a comfortable living environment. "A lot of people that come to visit are surprised by how warm the house is in winter and how cool it is in summer," says Nicks Isaacs.