'Millhouse'
D&J Project Management Ltd. won the 2025 FMB Sustainable Building Award (Central Region) for this Warm Core, Deep Retrofit Project
'Millhouse' Innovative Design by Property Revolutions Ltd.
The 2025 FMB Awards Judges comments:
"This Cambridgeshire home, built in the 1920s and extended in the 1970s and 1980s, underwent a major retrofit to improve energy efficiency while maintaining its traditional charm. The "Warm Core" strategy focused on upgrading the central part of the house with advanced energy-saving measures like solar PV with battery backup, underfloor heating, an air-source heat pump, and airtight construction. Challenges included tight deadlines, unforeseen roof defects, and extreme weather, but the project was completed on time. The approach emphasises sustainability with the use of local materials, waste recycling, and renewable energy solutions."
When Nicole de Sousa of FMB visited 'Millhouse' in November, 2023, she noted the “green skills in action”. This collaborative project aimed to produce a 70% reduction of carbon emissions overall, without undertaking a 'whole-house' retrofit.
2023 Article: Revolutionising retrofit with innovative solutions
For 'Millhouse', PRL's design team led by Carl Dodd, and Dave Clay from D&J Project Management joined forces.
Dave is a dedicated specialist in sustainable retrofit projects - and this collaboration marked the seventh year of David and Carl working together. Their projects always showcase innovative approaches and strong client relationships.
The ‘Warm Core’: Designed to Create Thermal Comfort
Saving money and creating warmth was key
"The Brief was to create a comfortable, cosy home that would retain its look without adding any unnecessary, expensive features. So, with that in mind, The PRL Team and Dave Clay, focused on making sure that the oldest and coldest part of the home would be the newly warm, central core of family life.
They also updated all of the buildings systems to modern, 21st Century standards, giving the owners a clear road map to net zero carbon by 2050.
The house consists of three wings: the original 1920s house, plus two wings that were added in the 1970s and ‘80s. The wings are both of cavity wall construction and have previously been insulated, but the original home is of solid wall construction with some single glazing and had been losing the most energy."
A uniquely sustainable approach
"The PRL road map is always to have a ‘fabric first’ approach and extend the ability to generate power and resource on site.
All PRL retrofits follow PAS 2035 methodology and RIBA stages, in a Trademarked Project Management system called “PRL 1-2-3”.
After carrying out a detailed survey and with the clear briefing from the client, we produced a Stage 1/ Whole House Report; this meant that we could develop a full set of detailed designs and calculations to deliver our target of a 70% reduction in carbon emissions and also create an almost self-sustaining solution for this large house.
We planned to carry out only minimal interventions in newer wings of the home - to emphasise the principle of spending money where it will have the most effect."
A testing time
"Waiting for the planning application and building regs approvals took us right up to the point where the works were due to start, and we needed to do a value engineering exercise - as the quotations were above the agreed budget."
Creating the Warm Core
"Once we were good to go, we honed in on the original part of the house, and created a warm core by making it airtight; a crucial step towards reducing energy usage.
We installed an insulated floor, as well as external and internal insulation. Additional insulation in the loft was added to create a new thermal envelope for the warm core.
We then stripped out the ground floor timber and other features that would not be practical to retain, and inserted a highly insulated new floor.
Next, we created an airtight box in the warm core using a proprietary spray-on vapour control layer called ‘Passive Purple’, from Intelligent Membranes."
(c) FMB 2023
‘Passive Purple’: innovative airtightness
"Passive Purple forms an airtight liquid vapour control membrane and air barrier, that can be easily applied by spray or brush. It fills gaps up to 10mm in the structure and was expertly applied by our specialist team. It's a smart choice and far easier than other membranes to install, plus, its purple colour makes it easy to visually check the seal.
(PRL always works to our motto: ‘Greener, Cleaner, Leaner’ – we pride ourselves on being ‘Smart People, using Smart Tools’!)"
(c) FMB 2023
Upgrading the windows
"All windows have been upgraded throughout, including adding secondary glazing to older double-glazed windows. We were able to use ‘Passive Purple’ here as well.
The design also employed aerogel insulation boards around steels, lintels and window reveals to tackle typical thermal bridges where condensation and mould could form.
Aerogel is a material known for its extremely low density and exceptional insulating properties. Despite being incredibly thin, aerogel is among the most effective of thermal insulators, thanks to its nano-porous structure that significantly hinders heat conduction.
Enlightened Windows supplied the timber frame windows and doors, for both Millhouse and Heath Lodge."
Client-led design
"The internal features of the house were kept more or less in the same location with only small amounts of remodelling.
We thought about how the rooms and spaces would best be used by the owners, and our re-design resulted into a better family home, complete with a chic, luxurious bathroom, by Victorian Plumbing."
"We also added a very modern take on a traditional country kitchen."
"As well as the more obvious measures, such as new LED lighting, the kitchen - supplied by Howdens - includes an efficient electric induction hob and oven. The client has also bought modern appliances to embrace other low impact living choices, as per our suggestion."
Next steps to completion
"The decision was taken to upgrade internally, whilst waiting for the weather to improve in time for the external rendering. It was extremely difficult to get this timing right as we had exceptionally inclement weather during this project.
Next, we swapped out an old oil boiler for a 16KW air-to-water heat pump. Then, we constructed an 8kw ground mounted PV solar array, with 10kw of battery storage in a purpose-built plant room. The array is capable of being extended by another 8 kilowatts, and if the clients choose to do this, they would become virtually self-sufficient for power, including powering an EV.
As part of the ‘fabric first’ approach, mechanical ventilation with heat recovery (MVHR) was installed. The owners now have a smart meter and a smart electric tariff to minimise energy costs and avoid expensive electric tariffs at peak load times. During the summer months the solar energy can be diverted to heat up the hot water, too."
"The remaining radiators in the other wings were upgraded and new highly insulated pipe work was installed throughout.
Modern Wi-Fi TRVs were routed to the central hub and room sensors in 6 different zones – they give much better control of temperature and they sensibly allow heating only when rooms are being used. As a good old-fashioned measure, heavy thermally lined curtains were made for the inter-connecting doors between cores and wings.
The project took six-months from start to finish and the results speak for themselves. The overall energy efficiency for the whole property was improved by 3 EPC grades, taking the house from a low EPC of G to a very respectable C; this is a very significant achievement."
Life in ‘Millhouse’ now
"We created a highly adaptable and year-round usable family home. The client has commented that the whole house feels more comfortable, there is a perceptively noticeable improvement in the air quality, and the comfort levels throughout the whole house are immense.
Although each of our projects are unique, we work to a template for other large, hard to treat homes, to avoid them becoming stranded assets in the future.
We expect an honest appraisal from our clients after they have lived in their improved homes for at least six months. We also monitor and verify the home’s performance to ensure that it delivers exactly what we promised. Our ‘Millhouse’ client is extremely pleased with the results!"
We believe in the power of team work
"We believe that our projects only get created to such high standards by having a truly collaborative team approach, which includes client contributions and amazing builders and suppliers. We excel in bringing all of these elements together to create fantastic projects that deliver in terms of quality and effectiveness.
For ‘Millhouse’ we had tremendous support from the supply chain, including LCS (they installed the solar PV and battery technology).
Dave's supply chain also included the essential electrical and mechanical contractors TH Electrical Ltd. of Essex, who installed the ASHP and MVHR, delivering warmth and comfort to this classic 7-bedroom home.
The process was well managed and really enjoyable, as the entire team worked so well together."