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Reform Cottage EnerPHit, Herefordshire

Retrofit Consultant Gervase Mangwana has undertaken a retrofit to the EnerPHit (Passivhaus retrofit) standard on his own home in Herefordshire. Gervase designed and project-managed the build working closely with a local project team, including Dai Rees, Tim Straker and Dave Hurds on the building team and Andy Simmonds, Nick Parsons and Tim Martel supporting with advice and PHPP modelling respectively.  At the outset, it was not clear if the project would achieve the EnerPHit standard but in the end was able to meet the EnerPHit criteria, achieving an airtightness level of 0.31 ach @ 50 Pa.

21° provided the ULTRA triple glazed timber windows and doors and designed, supplied and commissioned the MVHR (mechanical ventilation with heat recovery) system for the project. 21° also supplied the Pro Clima airtightness tapes and membranes for the retrofit.


The house was in a poor state of repair and Gervase consulted Hereford architect Andy Simmonds for a second opinion to whether it would be suitable for a deep retrofit project. The house was originally a 1838 stone cottage that had been modified a number of times over the years, most recently in 2000 when a brick and block cavity extension was added almost doubling the TFA.


A number of IWI (internal wall insulation) and EWI (external wall insulation) strategies were explored for the retrofit, including whether wood fibre and Diathonite IWI would be suitable if applied to the solid stone walls, with careful consideration of the options to manage moisture risk.

One significant challenge for the project was the fact that the original cottage and a Victorian extension had no foundations at all. The buildings were just set on clay at a depth of only 150mm below ground.  A structural engineer, with experience of similar buildings, suggested a triangular ‘earth footing’ option at the base of the walls. However, this would have created thermal bridging problems.

In the end, it was decided that the only solution was to underpin the walls. Although it had an additional financial impact on the project, underpinning the walls had the major benefit of addressing the moisture issues of the walls.  By underpinning the walls it was possible to add a mechanical damp proof course, addressing potential moisture issues from rising damp. This meant that a full 200mm EPS EWI solution for the building could be considered, which also meant that the project had more of a chance of achieving the EnerPHit standard.  Passivhaus certification to the EnerPHit standard was undertaken by WARM low energy building practice.

Wall/ roof junction

In addition to the 200mm of EPS EWI, the cavity wall extension (which had also been filled with mineral wool) was topped off with polystyrene beads and a detail that built it up so the wall insulation could meet the roof insulation and ensure continuity of insulation.   The roof was stripped and had to have rafter extensions to accommodate the detail.  The roof was designed as a ‘cold roof’ and was insulated with blown Warmcel cellulose insulation.

Key data
  • Airtightness: 0.31 ach @ 50Pa
  • Space heating need: 26.5 kWh/m2/ year
  • Treated floor area: 168 m2
  • Wall U value: 0.109 – 0.150 W/m2K
  • Roof U value: 0.091 W/m2K
  • Floor U value: 0.107 W/m2K
  • Windows installed U value: 0.94 – 0.83 W/m2K
Building team

The building team on the project included Dai Rees, Tim Straker (who part project managed the retrofit) and Dave Hurds with PHPP modelling from Tim Martel.  An additional connection with 21°was through Tim Straker whose nephew Isaac Straker also worked as part of the building team on the project, later moving to become one of the Green Building Company building team.

The building team was really amazing. I was incredibly lucky to have moved to a new area and to have pulled together such a good building team. On the whole it was an incredibly smooth process, compared to many.


Triple glazed timber windows & doors

21°’s ULTRA triple glazed timber windows and doors, with a whole window U value of 0.75 W/m2K, were chosen from the project The ULTRA inward opening tilt and turn window, alongside three fully-glazed doors. Some of the windows featured additional glazing bars. The colours for the window and door paint finishes were RAL 7037 externally and RAL 9016 internally.  Dai Rees installed the GBS windows and doors, having installed them previously elsewhere.

I was aware of 21°s window ranges through my work at Carbon Coop, so I had already worked with the 21° team. I looked at a few alternative companies but 21°'s quoting turn-around time reassured me by being more timely and efficient. I’ve always found Paul and Luke to be incredibly helpful. The windows are lovely. I’m really pleased with them. It was really positive working with 21° throughout the retrofit. 21°’s aftersales team has also been great and has popped in twice to address some minor snagging issues.


MVHR heat recovery ventilation

21°designed, supplied and commissioned the MVHR system for the project. A Zehnder Comfoair Q350 with a Comfosense C67 controller and rigid steel ducting was specified. Dai Rees installed the MVHR system for the project.

I love the MVHR system – it is the most amazing thing. I love it because you just don’t need to worry about it, apart from changing the filters, it really just gets on with it. Dai did a great job installing it as well.


Heating, hot water and energy generation

The house has a solar PV system installed on the roof and for 5 or 6 months of the year there is virtually no imported electricity for the house. The PV system now helps charge two electric cars.

The heating system is a novel design, using direct electric immersions to heat a thermal store which supplies to underfloor heating downstairs. The upstairs is unheated. Hot water also comes from the thermal store via a plate heat exchanger.

The system was designed to help with ‘grid smoothing’, which involves storing PV and low-demand grid electricity to use in peak times. The heating system has been largely successful but has been complex to manage and it is planned that a heat pump will soon be installed.

Heat meters show that heating demand has been much lower than the PHPP prediction though the property is not heated to 20ºC everywhere. The annual electricity bill, which covers all energy use, is roughly £400.

Project details

Design, Project management and air testing

Waxwing Energy

Building team

Dai Rees

Tom Straker

Dave Hurds

Warmcel installer

Gordon Lewis

Solar panel and battery system

Solar Kinetics

Heating and plumbing

Robert R McGowan

PHPP Modelling

Tim Martel 


WARM low energy building practice





Case study


Life changing homes