Imitating the historic barn designs true to local style, work has started on 'Carrowbreck Meadow'; a 14 home development in Norwich and the first to achieve the air quality and comfort standards of Passivhaus certification. However, the development's unconventional structure proved a challenge for a roofing solution that could meet the required energy calculations.
Designed by Barron and Smith Architects for NPS Group on behalf of Broadland Growth, the properties will be located in a peaceful woodland setting, positioned to make the most of solar energy gain throughout the seasons. Building and construction company RG Carter will be using a refined material pallet of white render, black stained timber and either slate or plain red roof tiles to bring the designs to life.
In order to complete the development, which is set to meet Passivhaus and the Housing Standards Review Space standards, RG Carter has turned to Metsä Wood to propose a commercially and structurally viable roofing solution. One of the key challenges was that the external walls of the properties would not be able to take much horizontal loading – something that Ewa Ostrowska, a senior engineer at Metsä Wood, explains was overcome using their BIM compatible software, Finnframe, including the system details approved by the Passivhaus Institute in Germany:
"Due to unconventional wall types, this project was challenging when it came to the structural analysis. Using Finnframe, we were able to support the Barron and Smith Architects with the U values and timber fraction calculations so the project could achieve its Passivhaus certification."
The most viable solution was to design the roof so that most of the horizontal load was transferred through Kerto S purlins and the ceiling. A Finnjoist I-Beam roof, made from the only fully engineered timber I-joist to be manufactured in the UK, designed at a depth of 400m, achieved a low U value of 0.101 W/m2K with timber fraction within the roof of only 7.2%.
Each home on the Carrowbreck Meadow development has been designed to achieve a thermal bridge and draught free building envelope, which exceeds building regulation requirements for airtightness five times over. Healthy filtered fresh air will be provided to the homes 24 hours a day, utilising a heat recovery system capable of achieving over 90% efficiencies, and an equivalent power requirement of 20p per day or the same as a low energy light bulb.
William Bishop of RG Carter commented: "We are very pleased with Metsä Wood's roof system and we believe it is a good solution for use in conjunction with the clay porotherm blocks which were used."
Luke Roberts, General Manager at Donaldson Timber Engineering, the distributor and partner of Metsä Wood that delivered the roof material, said: "Metsä Wood developed a solution for RG Carter that not only addressed all the issues relating to passive house construction within the roof space, but was also more cost effective than any closed panel option on the market."