Kingspan Thermawall TW55 for UK’s first certified passivhaus retrofit

With the help of super-insulation from Kingspan, a Victorian three-storey terraced house in a Holland Park conservation area in London has become the first UK retrofit certified to ‘Passivhaus’ standards.

Widespread use of Passivhaus, the rigorous German building standard, has made it the fastest growing energy performance benchmark across the world; and its principles can be simply expressed as: “build a house that has excellent building fabric thermal performance and exceptional air-tightness, and install mechanical ventilation with heat recovery”. 

The design of the house is the result of a collaboration between Paul Davis + Partners and Princedale Ecohouse Ltd (formerly Ryder Strategies) who was given the job of designing the technical features for the house at 100 Princedale Road, Holland Park, owned by social landlord Octavia Housing.  Because the property lies within a conservation area, external insulation measures were not an option, but internal space at the small, 1850’s terrace was also at a premium, as the company’s Director, Philip Proffit, explained:

“I specified Kingspan Thermawall TW55, as PIR has a very low thermal conductivity and where internal insulation is concerned, we had to minimise the loss of floor area. The challenge was achieving air-tightness to meet Passivhaus criteria in the whole house; and novel detailing was designed for this.”

A continuous layer of OSB was used to create an air-barrier at external and party walls, ceilings and basement floor. Doors and windows are connected to the OSB with a tape-jointed breathable membrane, in order to maintain air tightness.  This attention to detail achieved an airtightness of 0.5m3/hr/m2 at 50 Pa.   

Two layers of Kingspan Thermawall TW55 insulation were installed, one on either side of the OSB air-barrier.  Thicknesses of 150mm and 50mm of TW55 were used on the external walls.  Together with the 12mm OSB this achieved a U-value of 0.1 W/m2.K. 

The ceiling on the top floor had 150 mm Kingspan Thermawall TW55 between the joists.  The joists were underscored with OSB followed by 50mm Kingspan Thermawall then 12mm plasterboard.  The floor over the cellar was formed by two layers of OSB with 150mm Kingspan Thermawall TW55 between them.

With a thermal conductivity as low as 0.022 W/m.K, Kingspan Thermawall TW55 allows relatively thin layers of insulation to deliver very low U-values. Its closed cell structure renders it highly resistant to moisture ingress and delivers reliable thermal performance for the lifetime of the building. 

The Octavia Housing property retrofit, designed by Paul Davis + Partners, has many other energy saving features, including the UK’s first triple glazed Victorian-style sash windows; ESE solar thermal panels; and a cellar MVHR heat exchanger system.  Kingspan Thermawall insulation was also used within the structure of the external doors, which had to be specially designed in order to meet Passivhaus criteria and maintain the ‘London Door’ aesthetics.  

The house now requires no gas boiler, radiators or conventional heating system, yet stays at a comfortable temperature with healthy air flow all year round; and, incredibly, it can now be kept warm on a winter evening with just a couple of tealights.

These and other innovative design features have allowed 100 Princedale Road to achieve an 83% reduction in CO2 emissions and a 94% cut in energy use; and the building’s heating burden has plummeted to 15Wh of energy per m2 per year (the UK average is 130kWh per m2 per year) - saving the tenants around £910 annually on fuel bills and meeting the demanding Passivhaus low energy standard.

Octavia Housing aims to progressively reduce the environmental impact of its 4,000 strong housing stock and has recently been given permission for a large mixed tenure development in the capital, providing 30 affordable low energy homes.

For further information please contact:

Tel: +44 (0) 1544 387 384
Fax: +44 (0) 1544 387 484

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