Choppards Mill: Designing the Difference

Where comfort and energy efficiency thrive in every intricate detail of a radical retrofit project 

Achieving the perfect balance between design, comfort, and energy efficiency is an art that promises not only a more environmentally responsible home but also a more pleasant living experience. 

As one walks around our radical retrofit project at Choppards Mill, one can see how high performance sustainable design doesn’t mean you must compromise on beauty. Our work at Choppards perfectly demonstrates how retrofitting a property allows the owners to infuse high performance energy efficient elements into the existing architecture, creating a harmonious and visually appealing result. By restoring original features lost to previously inefficient design, a deep retrofit project can allow you to create a beautiful, comfortable and energy efficient building. 

At a time when there is increasing environmental consciousness and rising energy costs, retrofitting homes for improved energy efficiency has become a priority for many homeowners. The desire to reduce carbon footprints and lower utility bills has driven a surge in home improvement projects focused on integrating sustainable solutions. However, in the quest for energy efficiency, paying attention to the essential elements of design and comfort should be paramount to creating the best building possible. 

What our building experience has taught us about low energy design 

At Green Building Store today, we focus on how our products, like our high performance windows and doors or our MVHR design service, can positively impact the self build and new build markets. But for nearly three decades, our in-house construction team has advanced our industry’s knowledge of ultra efficient low energy and Passivhaus building techniques through its new build and deep retrofit projects.  

Our first-hand practical experience in understanding Passivhaus principles and low energy building has given us the pragmatism needed for the detailed-focused design we promote—a pragmatism which balances aesthetics, comfort and energy efficiency.  

The importance of a whole house approach, attention to detail, knowledge of materials, a strategy for correct sequencing a build, and many other crucial factors all feed into a building’s performance in terms of its U-values, insulation, airtightness, mitigating moisture risk, and ventilation. Choppards Mill is the perfect example of what can be achieved when our pragmatic design is blended with the quest for high performance energy efficiency. This blog discusses in detail what we did for Chris and Sian Salt at their home.  

Choppards Mill – The Background 

Our first blog on the project at Choppards Mill told how the owners’ desire to install a water source heat pump to heat their house, using a pond on the mill’s grounds, led the project.  

They had approached Bill Meadowcroft at Green Source Heat, who deduced that the building needed a radical retrofit for the heat pump to work successfully and be a plausible solution to heat the house. He recommended Green Building Store, and we came on board to help improve the thermal performance of the building. Our aim was simple: to create a strategy and complete the work to ensure that the house functioned at the optimum temperature for comfort of 21°C inside, whether it was – 5°C or 35°C outside.  

Paramount to our thinking, and based on our years of experience, was the quest to create the perfect balance between comfort, aesthetics and energy efficiency. A home that is energy efficient but ugly is unlikely to win over its residents. So, when retrofitting, it is essential to consider factors like thermal comfort, indoor air quality, acoustics, and appearance to create the most pleasant living experience we can. 

The three-storey sandstone and gritstone building had many tales to tell about how it had evolved as a building over the years. Several extensions and evidence of several changes provided proof that it had been a residential property for many years despite starting life as a cloth mill in the 1800s. 

Local architect and pioneer Arthur Quarmby, who built the Underhill Eco-home in Holme, even played a role in the building’s development in the 1970s. But as Green Building Store’s founding director, Bill Butcher, joked to the home’s owners, “Insulation wasn’t really on Arthur’s radar.” As such, we found a building that required significant work to transform from a cold and draughty building into a much more comfortable home, all whilst heated with a highly efficient water source heat pump. 

At the start of the project, we used SAP modelling for three scenarios for the home. SAP modelling allowed for quicker assessment of the renovations and alterations planned to improve the fabric and make it adequate for the heat pump to heat the house successfully. You can read more about the SAP modelling and the scenarios we looked at in the original blog on Choppards written here. 

We could use projected U values for our proposed procedures thanks to the SAP modelling. Through the project, we have altered about 75 per cent of the home and perhaps as much as 90 per cent of the walls. Of course, each build and retrofit has its own story to tell. Decisions and choices made by the client during the project also influence its narrative. Whilst we adapted as the project evolved, there were, however, several key factors we considered. These included: 

Factoring in bad form:  

Due to the evolution of the building in terms of refurbishments and additions. The house had developed a poor form factor. The form factor of a building, also known as its shape factor or compactness, plays a significant role in determining its energy performance.  

The form factor is the ratio between a building’s useable floor space and the building envelope.  Buildings with high form factors have more exterior surface area relative to their floor area. In comparison, buildings with low form factors are more compact and have less surface area in relation to their floor area.  

Regarding Choppards’ form factor, the extensions to the kitchen and garages had negatively affected the ratios between the floor area and the external envelope. We, therefore, needed to consider the whole envelope of the house in terms of limiting the impact of its form.  

Minimising Thermal Bridging: 

Minimising thermal bridging in a building can significantly increase its comfort by improving the overall thermal performance of the building envelope. Thermal bridging occurs when there is a break in the insulation layer of a building, allowing heat to flow more easily between the interior and exterior environments.  

At Choppards, we did whatever we could to benefit its airtightness strategy and to minimise thermal bridging within the house. We ensured that the roofs achieved good U values and airtightness, as we always do. 

Thermal bridging can attract condensation and cause heat loss from the building to be increased, which is why you want to minimise it. To stop thermal bridging, you need to insulate it, applying a good airtightness strategy to inhibit condensation forming behind the insulation. We know from experience that having a continuity of insulation is more important than having lots of insulation in certain parts. 200mm of insulation done well with a good airtightness strategy is better than 400mm of insulation done poorly with no consideration for thermal bridges or air movement around it.  

Not over engineering: 

Essential to any retrofit and based on the experience we have gained over the years, we always recommend not over engineering the solutions for low energy design. You want to avoid planning or doing any work that could provide a solution in one part of the house that causes a problem elsewhere. Neither do you want to provide solutions that would inhibit or limit the opportunity to improve the house’s performance in the future. There is always the risk when retrofitting a property of  ‘sterilizing’ a home by providing an inappropriate low energy strategy. It is better to do sections of a retrofit well than to spread resources thinly and to do things poorly. When retrofitting a home today, you should consider what the building needs to be by the targets of 2050.  

The Importance of Detailing 

Detailing is crucial in any self build or retrofit project because it involves the careful design and implementation of specific measures and components that will significantly impact the overall performance of the building.  

Paying attention to the details in the plan will ensure that the retrofit achieves its intended energy-saving goals, enhances building comfort, and avoids potential issues that could arise from incomplete or poorly executed work.  

In our experience, seamless integration is essential regarding the detail. Proper detailing ensures that new energy-efficient components, such as insulation, windows, doors, and ventilation systems, are seamlessly integrated into the existing building structure to prevent thermal bridging, air leakage, and other energy losses that can occur when components are not correctly connected. 

Attention to detail goes a long way to ensure an effective thermal barrier. Detailing plays a crucial role in creating a continuous thermal barrier that minimises heat transfer between the interior and exterior of the building. Without proper detailing, thermal bridging and condensation mould growth can occur, leading to localised areas of heat loss or gain and reduced overall energy performance. 

By addressing potential areas of moisture ingress and ensuring proper ventilation, you reduce the risk of building damage and indoor air quality problems. Good detailing also minimises air infiltration. Sealing gaps, cracks, and joints prevent uncontrolled air exchange between the interior and exterior. 

Detailing in any retrofitting project involves meticulous planning, precise execution, and holistic consideration of various factors. Choppards was no different. By focusing on the details, we knew we would enhance the retrofit’s overall effectiveness, improve building performance, and create a more comfortable, energy-efficient, and sustainable environment for Chris and Sian. The right decisions need to be made regarding the house’s materials. For example, several house walls were up against the earth and built into banking. These had to be considered differently from those walls that weren’t.  

Choppards Mill: A tour of comfort in design  

With the above principles in mind, the work we completed at Choppards is now visible for anyone to see if they can explore the beautiful home that Chris and Sian have created. Whilst we can’t offer you, as the reader of this blog, an in-person tour of the house. We can do the next best thing and take you on a pictorial tour to show you the transformations that have taken place and some of the things we did to help improve the house’s comfort, health, and efficiency. 

On arrival at Choppards Mill, the property’s uniqueness is instantly apparent. The house’s West façade shows the property’s quirks and unusual form. Choppards appears from this orientation to be a two-storey property, but this point of view hides the secrets that unfold as you journey into the house.  

Before we started work, there were two garages on the east side of the house. The one visible on the right of the photo remains a garage and still exists outside the house’s envelope.  

The garage on the left, however, was not initially considered within the envelope. However, Chris and Sian wanted this space to be a second study during the retrofit, so it was brought inside the envelope.  

A timber-insulated floor was added to the bedroom above the garage on the right. An engineered timber floating floor on a concrete slab with insulation beneath was added to the garage floor on the left. Above the newly created study is the bathroom.   

At Green Building Store, we pride ourselves on the bespoke solutions we can provide our clients. Whether it’s our MVHR design service or our windows and doors, we know that all properties and all clients are unique. Choppards Mill was no different. From the image above of the West façade, one can see an example of the bespoke features we incorporated for Chris and Sian.   

The arched frame window added to the left garage, and replacing where the garage doors were, has a tilt and turn inward opening window in the middle lower section. Looking closely at the image, you will notice that the keystone in the arch above the door is slightly lower. A feature very typical for arched stonework. As a result, we had to have the frame made with a notch cut out to allow for that feature. To do this, our windows team conducted a site survey of the opening and drew this up in CAD. This allowed us to perfectly design the notch in the frame to allow it to fit around the keystone. Doing it this way allows the modification to be done before the frame is painted and, therefore, guarantees that the timber is fully protected whilst creating a neat finish on site. 

Also visible from the West view are the other windows from our Performance range, which we installed, including the fixed circular window above the door. Key features of the Performance window range included:  

  • Whole window Uw value 0.85 W/m2K (calculated for a standard window to BS EN14351)  
  • FSC-certified timber frames 
  • High security, Secured by Design option 
  • Ten-year warranty 
  • Class 4 airtightness (the highest rating to BS EN 12207) with robust and durable dual compression seals  

The windows and front doors visible are RAL 7021 externally.  

The Glassroom

Entrance into Choppards Mill is through a bespoke door design from Green Building Store. Again, what’s behind the door will surprise and reflect how different the property is. As one enters into a glazed atrium area, which was once actually an open area between two separate buildings, one glimpses the quirks of the owner’s interest in stone masonry with capstones and cornerstones from other buildings, including a methodist church incorporated into the walls of the garage which now acts as the walls of the atrium area. The atrium area’s glass roof was constructed after the Green Building Store’s work on the main house and exists outside the envelope.

The Stairwell 

Through the atrium, one enters the main body of the house through another Green Building Store door. As you do, one is instantly hit with one of the first dramatic changes we made to the house. To the right, one can see an ash wooden staircase, which descends to the lower rooms of the house. 

We created this entire space, replacing a bedroom and wall cupboard which existed initially. The walls are diathonite and wood fibre. The original double glazed windows were kept as they worked within the SAP modelling – a perfect example of having a pragmatic approach and not over-engineering where it wasn’t needed.  

The Kitchen 

The house’s stunning kitchen and dining area are on the left as you enter the building. As well as improving the house’s thermal performance, we simultaneously restored and exposed many original features that had been lost and hidden.

In the Kitchen, the wooden ceiling beams were initially hidden behind a suspended ceiling, but this was removed. At the same time, the timber floor, which was initially draughty and inadequate structurally, was replaced with a new suspended timber floor. 

The roof over the kitchen had previously been renovated, so we didn’t need to reroof this section of the house. We had to maintain a ventilation gap under the slater’s membrane due to the traditional quality of the membrane. The owners wanted to expose the beams in the kitchen, which allowed us to include 50mm of high performing mineral wool between the rafters to maintain ventilation. We used 100mm x 50mm timber framing with 0.32 mineral wool. An Intello vapour control layer was taped to the walls and the exposed timbers. A U 65mm Polyurethane backed plasterboard with skimmed plaster finished the new ceiling.  

For the floor, we installed a brand new suspended 250mm I-joist timber floor with a windtight vapour open membrane underneath. A full fill mineral wool insulation was added. This was covered with a tongue and groove chipboard with glued joints. The heating screed with the incorporated underfloor heating coils was added on top of the chipboard, and then it was finished with ceramic tiles.  

Underfloor heating was chosen to take advantage of the heat pump and avoid needing large radiators. Where radiators have been used in the house, like in the living area, we used electric fan-assisted radiators, which dissipate heat quickly.  

We even had the space in the floor to dispose of and bury an old oil-fueled AGA oven – a surprise for future archaeologists – to avoid the removal costs of the appliance.  

On the north side of the kitchen, the windows were replaced with triple glazing but double glazed windows were also kept on the opposite side.  

Walking through the kitchen, one then reaches a bathroom and the master bedroom. 

The Bathroom

The stunning bathroom with unique visual features, like the standing bath and sink with face sculpture, now also has the exposed beams of the original building visible. The house does not have any MVHR, so we needed to incorporate and develop a bathroom ventilation strategy for the home.  

In this room, we installed The Greenwood Airvac CV2GIP Unity, a white round IPX5 mains voltage continuous running de-centralised MEV fan with humidistat and adjustable overrun timer suitable for wall or ceiling mounting. 

This near-silent fan is continuously running, which delivers a total solution for all wet rooms, including the kitchen, bathroom, utility and WCs. With an ultra-low energy DC motor and mixed flow impellor, Unity CV2GIP assists in delivering Guaranteed Installed Performance (GIP) – a fundamental part of building regulation compliance. 

With outstanding energy performance and built-in SMART technology, it meets and exceeds the current Building Regulations in new and refurbished housing projects. Its SMART Technology is a fusion of sensors, controls and innovative design that makes ventilation work ‘smarter’ for a building. It has an integral timer, and the humidistat features an innovative, logical approach to fan timer operation. In contrast to traditional timers, it looks at the duration of occupancy to define running time rather than have a fixed running period. This eliminates nuisance noise, especially at night, and unnecessary energy wastage and heat loss typically associated with traditional timers. 

 The Master Bedroom

The master bedroom exists in one of the extended sections of the house – a 1970s cavity wall construction of the house compared to the Victorian 450mm rubble-filled stone walls that exist elsewhere as such moisture risk in this room was less. In this room, we added Polyurethane laminated plasterboard to the existing cavity wall block construction with a full fill adhesive.  

In the master bedroom, one aesthetic feature added was two Juliet balconies for the windows. These are two tilt-and-turn floating mullions, 78 profile windows from our Performance range. With a tilt-and-turn floating mullion window, one of the sashes tilts inwards to allow some ventilation, and then both sashes can turn and open inwards to create a whole opening to access the Juliet balcony. We dropped the windows and sills to create these full-height windows.  

The Living Room

Back through the kitchen and up a small flight of stairs, you find one of the living spaces of Choppards Mill. It is a dramatic and visually appealing area where the original beams of the ceiling have once again been exposed after being hidden behind a suspended ceiling.  

We made significant changes to the room’s ceiling to create this visual space. We needed to make structural changes to create one pitch to the roof and remove a section of the roof in an extended part with a much lower flat roof. Stud work was added parallel to the rafters to allow 400 mm of mineral wool to be installed. An Intello membrane was used. A U Value of 0.12 was achieved for this space.  

For the walls, we added 50mm of diathonite. The original double glazing was satisfactory enough to keep on the South side of the house.  

Alterations were, however, made to the arrangement of the door and window on the North side and a triple-glazed door and window were added to create a better use of space and flow into the room. The fan-assisted radiators are visible in the photos of the room.  

The decision to sand and keep the original floorboards was taken during the build and again reflects how such pragmatic decisions can be taken into account when a retrofit is modelled and planned well.  

The Study Basement

The basement of the home, which now has a second living area, is where we made the most significant alterations. Initially, this space was three rooms. We removed an intermediate wall between the three rooms. The intermediate wall removed was the original gable end of the building and had a chimney. As a result, extensive steelwork was required to structurally secure the original gable end and remove the chimney structure. Diathonite and wood fibre walls were added to the room.

There was also a stone porch, which had been added to the South side of the house. The porch didn’t add any additional function to the home so the decision was taken to remove it. The stone and slate from this porch were repurposed and used elsewhere in the project. 

A new opening was created for the Lacuna outward opening fold aside door on the East end of the house, which now opens onto the beautiful gardens of the house. Lacuna bifold doors are very different to typical bifold doors; they are made from FSC-certified thermally modified beech, which has excellent dimensional stability and will not absorb water; they are supplied all over the world and can withstand the most extreme climates of both hot and cold temperatures. Like all GBS products, Lacuna doors have been tested to class 4 airtightness and PAS24 security, which is entirely something for a bifold door. Even with their superior airtightness and security classifications, their frame profiles remain slimmer than most on the market, creating a great aesthetic that allows plenty of natural light to enter the house. 

ADP Architects of Holmfirth did the original concept drawings for the house. Still, the works done in this space by Green Building Store demonstrate perfectly how, whilst the concept drawing took it to a point, our structural engineering, design, SAP modelling and thermal performance strategy achieved the final outcome.  

This room also has underfloor heating with a ground slab, 150mm of Polyurethane insulation, a heating screed and a tile finish. 

One of the noticeable things about this space is how quiet it is, which is due in no small part to the quality of the work we did in this room in terms of insulation and ensuring the room’s envelope was as airtight as possible.  

The outward opening floating mullion casement window with antique pear drop handles is visible in the photos above and shows the RAL 9003 colour featured on several of the windows internally.  

Outside the house 

From outside the building, the three storeys and the layout of the windows are more visible. We had to factor all of these into our low energy design for Choppards.  


In the house’s gardens, at an elevation higher than the building itself, one can see the pond, the source of the heating for this incredible building. Our first blog on Choppards goes into more detail about the heat pump and how it functions for the home.  

In conclusion, the project at Choppards demonstrates perfectly how retrofitting a home for energy efficiency is not just about reducing energy consumption and saving money.  

Our work on Choppards also shows the vast extent of our offering for windows and doors. In this project, we have supplied both traditional windows with glazing bars to one part of the house, alongside traditional-looking door sets based on the character and age of the property. At the same time, on other parts of the property, we have supplied contemporary tilt-and-turn windows and the Lacuna bifold. Not all companies can offer this wide range of products and bespoke detailing. 

At Choppards, we have created a sustainable living space that harmonizes with the environment and enriches the lives of its occupants with healthy and comfortable living spaces. And by striking the right balance between design, comfort, and energy efficiency, homeowners can embark on a transformative journey towards a healthier, more comfortable, and happier home. 

10th October 2023

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