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New Forest EnerPHit

A New Forest home has been transformed to meet the EnerPHit (Passivhaus retrofit) standard by Ruth Butler Architects, with Julian Sutherland at Cundall as Passivhaus Designer. The building contractors on the project were Tuakana Construction.

21° was involved in the project in a number of ways, including supply of ULTRA triple glazed timber windows & doors, Lacuna fold-aside doors, airtightness products, and construction training delivered to the project contractors.


The original house was a detached 4 bedroomed house, built in the 1930s with a standard brick cavity, rendered with a tile roof.  The owners were motivated by both internal comfort and high indoor air quality when considering the refurbishment.

The retrofit involved a new ‘thermal jacket’ for the house, achieved through an external wall insulation (insulated render) around the outside and raised flooring to add floor insulation. Triple glazed timber windows and doors were installed as well as a mechanical ventilation heat recovery unit (MVHR). All the original heating system was stripped out, leaving one single electric radiator to heat the house. Direct electric heating was used for the hot water and solar PV panels on the roof help power the heating and hot water.

The design also included remodelling of the interior to contemporary standards, an open plan living space with new timber floors throughout.

The house has received EnerPHit certification, which is a version of the Passivhaus standard for retrofit projects. EnerPHit requires either a maximum heating demand of 25 kWh/(m²a) or alternatively the consistent use of Passive House components in accordance with the requirements for PHI certification of components. Retrofitting a house to the EnerPHit standard can result in energy savings of 75-90 %. 

Ruth modelled the embodied carbon of the EnerPHit project, using PHRibbon software. The modelling clearly demonstrated how retrofits have much less embodied energy in construction than newbuild projects. 

Windows & doors

The house had recently had the majority of its windows replaced with 21° triple glazed windows and doors but these needed to be re-positioned to accommodate the external wall insulation. This was reasonably easy to accomplish as straps had been used to fit the windows, so the windows were moved 200mm out into the insulation zone. To aid this process Green Building Store provided new deeper extension sills.

21° also provided some new ULTRA triple glazed timber windows and entrance doors, as well as two Lacuna triple glazed timber fold-aside doors as part of the retrofit project.  The inward-opening tilt and turn windows and doors were finished in white inside and out. One of the windows at the project was fitted with an electronic opener.

The clients were very happy with their recently-installed Green Building Store triple glazed windows and doors and were keen to use Green Building Store again for the remaining windows needed for the retrofit. The Green Building Store ULTRA windows and doors and the Lacuna fold-aside doors have worked really well in the project. They are very elegant and deliver the high thermal and airtightness performance needed to achieve the EnerPHit standard.

Ruth Butler

Airtightness products

21° supplied a Wellhofer airtight loft hatch to the project, to help keep that potential weak spot in the loft as airtight as possible.  21° also supplied the Pro Clima airtightness products for the project.

Consultancy & training

21° provided training to the building contractor on construction skills for low energy retrofits, including practical airtightness training. 21° Director Bill Butcher delivered half a day’s training to the Tuakana Construction building team, as well as some local Building Control officers. The training involved hands-on airtightness work that the team would need to use on the project. The project achieved an airtightness of 0.5 ach/ hour @ 50Pa, testimony to the high build quality of the project.

The building team were Passivhaus novices so setting up contractor training was very important. The success of the project was in many ways down to the quality of the training that Bill provided. The building team really related to Bill as a fellow builder. It meant that they all started off on the project with the same language and understanding.

Ruth Butler

Project details


Ruth Butler Architects

Passivhaus Designer

Julian Sutherland at Cundall

Building Contractor

Tuakana Construction


The New Forest EnerPHit project was shortlisted in the Small Projects category of the UK Passivhaus Awards 2021

Life changing homes