AXIS Architecture has designed a selfbuild low energy home, built on a plot of land in Yorkshire purchased with planning permission. The architects worked closely with the clients and the local planning authority to ensure the subsequent slightly adapted design would continue to satisfy the ‘very special circumstances’ rule, for any new projects on greenbelt land. At the planning stage, a commitment was made to making the property an exemplar of sustainable construction, through the use of low energy technologies and a highly energy efficient building fabric.
The clients had a vision for the house to be as energy efficient and sustainable as practically possible and were interested in the Passivhaus low energy building standard. While developing the construction design, AXIS Architecture undertook a Design Review with Green Building Store’s consultancy service. Owing to the unusual form and generous footprint, which called for a steel frame as the structural solution, the clients were advised by Green Building Store that it would be unrealistic to expect full Passivhaus certification to be achievable. This was partly because of the high area of external envelope relative to the internal building area, but also because of the large expanses of glazing designed to make the most of views of the site and surrounding area and to make the house as naturally light as possible. However, the team were able to work on the detailing of all the junctions within the project to maximise airtightness and minimise thermal bridging.
In addition to Green Building Store’s consultancy and training support, Green Building Store provided ULTRA triple glazed timber windows and doors, Pro Clima airtightness products, MVHR mechanical ventilation with heat recovery design, supply and commissioning for the project.
Set into the hill-side, the house features dry stone wall cladding and the wildflower blanket green roof to blend in and harmonise with its rural setting. This was achieved through careful design of the curved plan form of the house, with a gently sloping green roof, in order to visually blend the house into the rural surroundings by mimicking the site topography.
Design & Planning Challenges
The site was purchased with planning permission for a single storey dwelling but additional living space was needed, so Axis Architecture were faced with the challenge of rethinking the internal layout and utilising the roofspace to fit in a further two bedrooms, bathroom and second living room, all without raising the roofline or increasing the massing, in order to satisfy the local planning authority that the amended design would have no greater impact on visual amenity than the original planning approved design.
A commitment was made to design and construct the building envelope to high levels of energy efficiency, requiring particular attention to levels of insulation, thermal bridging and airtightness. Green Building Store’s consultancy service reviewed the various external envelope details and guided the architects in correctly specifying products and materials to reduce the impact as far as possible. The project aimed for a maximum U-value of 0.15 W/m2K for the external walls, roof and floor.
- Roof:, 170mm rigid Kingspan insulation, as part of the green roof construction.
- Floor: 150mm Kingspan insulation, below the screed, which encapsulates the underfloor heating coils.
- External walls: Constructed using thermally-broken timber Structural Insulated Panels (SIPs) to the inner leaf, with a total 100mm thickness of Ecotherm rigid insulation and 100mm of Knauf Earthwool.
Triple glazed timber windows and doors
Green Building Store’s ULTRA triple glazed timber windows (both inward and outward opening) and doors were specified for the project, including a series of lift and slide doors and tilt and slide doors. The ULTRA range offers a whole window U value of 0.75 W/m2K. The solid frame windows and doors were made from FSC® certified redwood, finished in a paint colour of RAL 7022. Two FSC® certified oak entrance doors were also specified and finished with a clear finish.
Green Building Store worked with the architects and builder to develop detailing around two of the sliding doors. This involved insulating around the steel rectangular post and then making a bespoke timber cover piece. The team also designed in a Compacfoam block beneath the windows that went alongside the sliding doors, so that they could be made the same height to work with different floor levels.
The clients commented: “We spent a great deal of time researching similar products from several other companies and the quality and design of Green Building Store’s range stood out against the rest.We were able to look at working examples of the windows and doors in their showroom and the performance, clearly visible attention to detail plus the level of customer service meant there was no decision to be made. The very large sliding doors, despite their size and weight move easily and silently, they provide excellent sound-proofing and certainly add to the overall cosy feel of the house.”
Minimising thermal bridging
In order to ensure minimal heat loss through the fabric of the building at junctions in the external envelope, including wall to floor, wall to roof, wall to window, the designing out of cold bridging was carefully reviewed which led to the specification of innovative new insulation products.
- Floor to wall junction
Marmox Thermoblock is a block of load-bearing insulation material which eliminates the cold bridge at the wall/floor junction. Thermoblock was used in the 65 x 140mm format below DPC level at the base of all external walls and internal loadbearing walls and changes in level. It was consistently positioned at the level of the floor insulation to eliminate cold bridging .
- Door thresholds – Compacfoam
Compacfoam is a high strength rigid insulation used for minimising thermal bridging, with a compressive strength and density similar to timber. It is a lightweight thermoplastic foam, based on polystyrene, that offering excellent thermal insulation. Compacfoam was used below the threshold / cill level of all the external patio doors and full height windows, to minimise cold bridging (and heat loss) through the external wall at ground floor level.
- Roof overhang
Given that the steel frame was already designed, Green Building Store’s consultancy service advised on ways to reduce thermal bridging at the roof overhang, separating the steel supports from the eaves using Sh’o’ck connectors.
The project called for a continuous airtightness barrier to the external walls, roof, floor, windows and doors. Green Building Store’s consultancy service provided advice on how best to achieve this, alongside onsite construction toolbox talks. The entire internal face of the external walls, and roof, was boarded with 18mm OSB 3 board, to provide a continuous air and vapour barrier.
As all internal plasterboard finishes were fitted separately in front of the OSB board, leaving a clear service cavity for electrics and plumbing pipework, this meant that penetrations through the airtightness barrier could be kept to an absolute minimum.
Pro Clima airtightness tapes, supplied by Green Building Store, were carefully applied around window and door openings by the building team, so as to avoid leakage around the perimeters of the frames. Taping also occurred between internal walls and the polythene vapour control layer beneath the floor screed, to seal this junction. The airtightness detailing, resulted in an airtightness result of 2.34 air changes per hour, at least three times better than Building Regulations.
Mechanical Ventilation with Heat Recovery (MVHR)
Mechanical Ventilation with Heat Recovery (MVHR) allows for sufficient and comfortable fresh air ventilation to all areas of the house, whilst minimising the loss of indoor heat energy. Green Building Store carefully designed the MVHR system to physically fit inside the building, and modelled the system using 3D software to check performance of the system in terms of noise, pressure, balance and airflows.
Ductwork routes were carefully considered at an early stage and AXIS Architecture specified open-web engineered joists (Easi-joists) to allow concealed ducting to be threaded through the ceiling void under first floor, and within a ceiling bulkhead running along the front curved wall of the open-plan living room. Jameson Consulting Engineers specified circular cut-outs to primary beams in a number of locations where ductwork could not be routed around the steel frame.
Owing to the large size of the house, Green Building Store’s MVHR design team chose to use two PAUL Novus 300 MVHR units, with a heat recovery rate of 93%, to supply the 490m2 project. The two MVHR units were linked so that they were working on the same ducting system rather than having two completely separate systems, reducing the overall amount of ductwork that was needed in the building. However, as a result, much of the ductwork needed to be wider as it was carrying a higher volume of air.
The MVHR system includes the extracts to the kitchen, 4 ensuite bathrooms, and 2 WC rooms within the house. As the ground floor consists of a large open-plan kitchen, dining and living space, it was important to consider the air flow in that space in order to control the spread of cooking smells and air borne grease. The MVHR Design team ensured that the kitchen extract was large enough to drag the supply air from the open plan space rather than the opposite way around. The commissioned extract air flow rate from that kitchen space is 95m3/h on fan speed 2 (standard operation) and 127m3/h on fan speed 3 (boost rate). This is much higher than the building regulations requirement of 47m3/h on fan speed 3, which would not have been sufficient for this space. There are two Touchscreen control panels on the wall in the Kitchen from where the occupants can boost the system if necessary
Commenting on the MVHR system at the house, the clients said:
“This was an area which, whilst we understood the principles behind the technology, was something neither we or anyone we know had any experience of. Essentially we were in the hands of the Green Building Store team to design a system which would add something very significant to both comfort and efficiency of our home.
The installation of the system was completed by our building team but Green Building Store was constantly on hand to provide telephone support and visits if required. The system was fitted quite easily and we have not been disappointed with its functionality and the fresh air quality it provides. Our house doesn’t suffer from misted bathroom mirrors, cooking smells (despite its very open plan design), hot and cold spots or damp areas. Dust production is significantly less and we simply enjoy a very ambient atmosphere which I guess we now take very much for granted!”
Green Building Store
The clients added: “From start to finish the entire Green Building Store team was fantastic. At every stage the advice and guidance was invaluable and I always found a patient, listening ear no matter how many questions I had! Each member of the Green Building Store team share a passion and knowledge for their field. I felt very much that they were all with us on our home building journey and have been genuinely interested in seeing it reach completion.
We had a vision for what we wanted to create but lacked an in-depth knowledge of the intricacies and principles that would allow this to come to fruition. We were educated, guided and supported and couldn’t have asked for anything more. Any follow up advice, purchases, small repairs and additions since completion have been swiftly dealt with in the usual friendly way we have come to expect from Green Building Store.
We are delighted our house is now complete and has become a great family home. It’s proving to be all the things we were promised and more. Green Building Store played a very significant role throughout and their input at all stages has been invaluable, something for which we are very grateful. We couldn’t recommend them highly enough.”
Case study: 2018
Axis Architecture www.axis-architecture.co.uk Tel: 0114 249 0944