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Case study: Hamson Barron Smith’s Passivhaus homes in Norwich

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Carrowbreck Meadow, a development of 14 Passivhaus homes, is the largest of its kind in Norwich  

DATASPECIFICATIONDETAIL

Carrowbreck Meadow is a development of 14 detached and semi-detached Passivhaus homes, the largest of its kind in Greater Norwich. The scheme is tenure-blind and sets new benchmarks for sustainable development, not only meeting the requirements of full Passivhaus certification but also providing 43 per cent of the site as affordable homes, an over-provision of 13 per cent. The development has been designed by certified Passivhaus architects at Hamson Barron Smith and constructed by RG Carter for Broadland Growth, a company set up by Broadland District Council and the NPS Group.

Sarah Lewis, associate architect – sustainabilityHamson Barron Smith

Project data

Start on site November 2015
Completion October 2016
Gross internal floor area 1,548.3m2 (net internal floor area for the development)
Form of contract Two-stage Design and Build
Construction cost £2.8 million (total contract value including all landscaping, site works etc)
Construction cost per square metre £1,400 semi-detached units; £1,700 for homes with the roof terraces (cost of the homes per m2, including all abnormals to account for specific site ground conditions etc)
Architect Hamson Barron Smith
Client Broadland Growth
Structural engineer Robson Liddle
M&E consultant NPS 
Quantity surveyor NPS
Landscape architect Hamson Barron Smith
Planning NPS
Aboriculture AT Coombes
Passivhaus consultant Hamson Barron Smith
Passivhaus certifier WARM
Project manager NPS
CDM co-ordinator NPS
Approved building inspector CNC
Main contractor RG Carter
CAD software used AutoCAD
Annual CO2 emissions 10.75kg/m2 (calculated by Standard Assessment Procedure)

Specification

External load-bearing walls Porotherm T9 hollow clay blocks
External wall insulation, all rendered walls Baumit OpenTherm 032G
Internal parge coat Baumit KP36W (airtight layer)
Below-slab insulation Dow FloorMate 500-A 
Thermal break in parapets and below ground floor doors Foamglas Perinsul HL 
Roof structural components Metsä Wood Kerto LVL and FinnJoist
Roof insulation Warmcel
Windows and external doors Internorm KF410
External blinds Trojan external venetian blinds, supplied by Hallmark
MVHR Design and supply by Greenwood, combination of PAUL (Novus 300) units on the three-bedroom-plus homes and Zehnder (CA200) on the two-bedroom homes
Airtightness products ProClima, with technical support by Ecological Building Systems
Monitoring system Tensor BRE monitor with access to web-based BRE monitor dashboard
Thermal bridge free wall ties (to fix parapet walls back to gable ends) Teplo ties
MVHR duct insulation to outside ducts and soil vent pipes Armaflex

Roof ridge

Carrowbreck detail section 6

Carrowbreck detail section 6

Hamson Barron Smith worked closely with structural engineers at Robson Liddle and with the designers at Metsä Wood to develop a roof design that could be constructed on site to meet the demands of the project programme and budgets. Metsä Wood uses 100 per cent traceable wood from sustainable northern forests, which fits with the project’s overall sustainable and ethical ambitions. The design was developed using Metsä Wood’s Kerto LVL product, which is a strong and dimensionally stable timber beam. Combined with Metsä Wood’s Finnjoist I-Beam, manufactured from high-quality oriented strand board (OSB3) web, and flanges made from Kerto laminated veneer lumber, the system allowed the roofs to create the depth required for the Passivhaus U-value. 

The roof was filled with a blown recycled newspaper product by Warmcel. One of the challenges of blown insulation can be ensuring the full extent of all voids are filled. Thermography was used on the first roof to identify whether there were any areas missed, and did not find any.

The airtightness layer was installed as an OSB3 board with taped joints. RG Carter assigned a site-based airtightness champion, who, along with the site foreman, conducted air pressure tests on all of the homes. These considerably exceeded even the Passivhaus requirements.

Sarah Lewis, associate architect – sustainability, Hamson Barron Smith

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