Green Building Store is now 21°

Whole house retrofit, Cambridgeshire

A hands-on DIY deep retrofit has been undertaken by clients looking to upgrade their 1968s F-rated cavity wall ‘chalet’ bungalow to high levels of thermal performance.

The clients Rachel and Jake have a construction background and have been undertaking all the retrofit work themselves alongside family and friends and with design input from architectural technologist Simon Ward. The retrofit has just been featured in Episode 3 of ITV’s series ‘Save Money: My Beautiful Green Home’ aired on Monday 1 November 2021.

21°’s PERFORMANCE triple glazed timber windows and doors were chosen for the project and Green Building Store also designed, supplied and installed the retrofit’s MVHR system.


The retrofit work started in October 2020 and required the complete gutting of the building. The bungalow’s flat roof dormer was replaced with one large sliding window and the bungalow was extended to a sloping roof, insulated with 175mm of PIR insulation.  The ground floor at the front of the house was dug out to a depth of 600mm, the old hardcore removed and replaced with crushed stone, concrete and 100mm insulation.  The floor at the rear of the house had already been dug out and, for structural reasons, was replaced with block and beam flooring.

The house’s cavity wall insulation was replaced and airtightness membrane and measures were installed throughout the house. The project is heated with a 5kW air source heat pump with underfloor heating manifold, tank and buffer tanks. There is a 5.1kW solar photovoltaic system on the roof, with an energy storage battery in the attic.

The layout of the house has been turned upside down, with the bedrooms moved downstairs and the upstairs an open plan living area with a large south-facing window.

The retrofit has been inspired by the Passivhaus low energy building methodology. We wanted to create a comfortable, sustainable home and were keen to retrofit as near as possible to Passivhaus standards, as we plan to live here in the long term.

Rachel & Jake

Triple glazed timber windows and doors

21°’s PERFORMANCE triple glazed timber windows and doors were chosen for the project.

Rachel and Jake chose inward opening windows, a Lift & Slide door, a Fold-aside door, and two entrance doors (styles PA2 and GR1) from the PERFORMANCE triple glazed timber range. The windows and doors were finished in high performance timber finishes in the colours RAL 7015 on the outside and RAL 7035 on the inside.

Costs for the windows on the project were kept down by having more fixed non-opening windows (where permitted) due to the fact there would be an MVHR system for ventilation.  

We got 5 different quotes for windows, which all gave different information. It was extremely hard to know if you are comparing apples with apples or apples with pears. So I decided to compare them across various areas on a spreadsheet: airtightness, U-value, water tightness, acoustic rating, security rating In the end, we chose 21° because they specialise in the energy efficiency of buildings and are extremely knowledgeable. 21° also provided a lot of support on ways to fit and position the windows to get the best performance out of them. We also realised that fitting a first floor window that weighed 650kg wasn’t a simple task and required specialist lifting equipment. It was important for us to find a window company that had an in-house fitting team that was prepared to install the large sliding door on the first floor. The windows and doors are great and we’ve been really pleased with them. They tick all the sustainability boxes. 21°’s customer service and aftersales support has been excellent. The installation team were friendly and efficient and did a great job installing the large sliding door on the first floor, using specialist lifting gear.

Rachel & Jake

MVHR (mechanical ventilation with heat recovery)

21° designed, supplied and commissioned the MVHR system for the retrofit. A UBIFLUX VIGOR 325 MVHR unit with spiral wound steel ducting was specified.

As a retrofit project it can sometimes be hard to fit the ducting into an existing space, so ducting was brought along the eaves on one wall, and some located in the attic. Rigid steel ducting was used throughout the project unless space was an issue when a small amount of rigid plastic ducting was used. The MVHR ducting had to be brought through the bathroom which meant that the shower area had to lose some height (100mm) to accommodate it. A small section of rectangular plastic duct above the shower was designed and specified as there was insufficient head height for the standard 100mm steel duct.

Rachel and Jake self-installed all the MVHR ducting for the project, working from Green Building Store’s installation drawings.

The MVHR has only recently been commissioned and the commissioning process was filmed for the ‘Save Money: My Beautiful Green Home’ TV series, although ended up on the ‘cutting room floor’.

t was helpful to get MVHR and windows from the same supplier. The MVHR Design team was really helpful at helping us squeeze the MVHR ducting into the tight space and were great with coming up with solutions. Steve from 21° also did a great job commissioning the MVHR system. The MVHR is working really quietly and well and quietly. We hope that a co-benefit will be that the MVHR will help filter pollen and pollutants as three of the four of us suffer from asthma and hay fever.

Rachel & Jake

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