Sharnbrook Passivhaus

Passivhaus, Sharnbrook, Bedford

This project consisted of 13 new dwellings on the outskirts of Sharnbrook near Bedford.

The client brief was to achieve Passivhaus Certification for each dwelling. The development consisted of 11 x houses and 2 x flats.

Planning & Design stage

Planning permission was already obtained for the development and the contractor was appointed. Low Carbon Box were appointed from design development stage.

As part of the design development stage, to allow an easier Passivhaus certification process, some windows were reduced in size to reduce heat loss and reduce risk of summertime overheating. Steps within buildings were also removed to reduce heat loss in thermal bridging and air tightness.

The outline specification to achieve Passivhaus includes the following:

  • 142mm SIPs (Structural Insulated Panels) with another layer of 110mm Kingspan K12 cavity insulation to achieve 0.10 W.m2.K U value
  • 500mm Knauf loft roll 40 in the ceiling to achieve 0.08 W/m2.K
  • 225mm PIR insulation in the Ground floor to achieve a U value of 0.077W/m2.K U value
  • Triple Glazed windows using Green Building Store Ultra Window with 1.05 W/m2.K U value
  • External door and frame achieved 0.90 W/m2.K U value
  • Air test results below 0.649 ACH
  • Mechanical Ventilation with heat recovery (MVHR) using Zehnder Comfoair 200 designed, supplied and commissioned by Green Building Store
  • Gas fired combi boilers serving 1-2 radiators and a towel rail per house.

Using the above key specifications, the project team implemented the specification. Low Carbon Box carried out the design stage SAP calculations and design stage Passivhaus design review based on the updated working drawings for issue to Building Control for Part L1A 2013.

As-Built Stage

Low Carbon Box monitored changes throughout the project and upon completion, were instructed to carry out the air tightness test.

The target was 0.649 ACH. Low Carbon Box completed the air tightness test and the target was met. Our air tightness tester also talked the client through areas to improve to help optimize the VCHR performance.

The as-built specification was updated with any changes and Low Carbon Box lodged and issued the air tightness test and final As-Built SAP calculation and Energy Performance Certificate (EPC)

All the houses were inspected on a regular basis and several air tightness tests were undertaken at different stages of the project to check they were on track to achieve Passivhaus Certification standards.

Low Carbon Box collated all of the evidence as the project progressed and submitted this to the Passivhaus Certifiers, in this case, Warm.

Passivhaus certification was achieved.


  • Key performance data from the project is outlined below:
  • Between 6-16% reduction in annual carbon emissions
  • Between 26-40% reduction in Fabric Energy Efficiency standard
  • Air tightness test = <0.649 ACH
  • EPC rating of 84 ‘B’

Low Carbon Box Services Provided

  • Eco Build
  • SAP Calculations & EPC
  • Air Tightness Testing
  • Passivhaus
  • Water Calculations

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