Low energy + low carbon timber framed extension

• No gypsum plaster – hemp straw boards are used instead of plasterboard – discovered by the Clients, not me.
• Vertical larch timber cladding to the external walls.
• Kitchen worktops that use recycled wood chips, called Foresso – also discovered by the Clients, not me.

Trying to eliminate concrete foundations and brickwork was difficult

We spent a lot of time investigating several ways of doing concrete free foundations, and while we couldn’t fully eliminate concrete or bricks because of the tricky ground conditions, we did reduce the concrete needed by about 7 tonnes. So, a low concrete foundation system was used. Preliminary calculations have demonstrated that we’ve saved about 1050kg of CO². Watch this space, I’m intending to complete a proper Life Cycle Analysis / LCA soon. You can read all about the journey of finding the best foundation solution by clicking here.

The family said they really valued and enjoyed an existing magnolia tree in their garden and wanted to somehow incorporate this into their project. We positioned the rooms and layout around the magnolia tree and added in a corner window seat near the tree so it can be enjoyed all year round.

The Clients were uncertain that the layout I described that incorporated the existing magnolia tree was possible so two concept options were prepared. The Clients were surprised and impressed that the layout I described was possible and created a lovely set of family friendly spaces, so this was chosen and is now being constructed.

A large overhanging roof canopy is also used to increase shading and reduce overheating in the warmer summer months.

This project also has some other really interesting design features, such as using an angled side wall to reduce the impact on the neighbours, internal walls at 45-degree angles to create better flow and improved layouts and ceiling height sliding doors to create a ‘broken plan’ layout.

Bio-based materials with minimal processing

Using natural organic or bio-based materials with minimal processing, also provides other benefits to the Clients such as a healthier home and increases occupant wellbeing. These materials promote a healthier internal environment as they emit less harmful Volatile Organic Compounds / VOC’s into the air.

The thermal performance of the extension is approximately 10% better than the minimum Building Regulations, quite thick floors, walls and roofs. So, better material choices and better thermal performance mean a building with reduced environmental impact all round.

A tried and tested vapour diffusive build up is used to all parts, that means the construction can breathe, it absorbs and lets out certain amounts of moisture, it’s not a sealed build up, which goes against some popular thinking in the building world. There is no vapour control layer. An airtight membrane is going to be used to reduce ventilation heat loss, but there is no vapour control layer.

To achieve the desired room layout, while working with the existing building, the structure is relatively complicated. I helped check and co-ordinate the structure to ensure everything fitted together just right without expensive clashes discovered on site.

Initially, we experimented with a central post and roof timbers positioned in a fanned, circular arrangement. While this looked really cool it was eventually discarded due to the high cost and complexity. We’ve used regular rectangular bays with a three small, thermally broken, steel frames to help support the overhanging roof timbers. You can read more about that journey by clicking here.

Fire safe timber cladding

Some of the extension side walls are quite close to the neighbours boundary and there are limitations, under the Building Regulations, on how combustible timber cladding can be used. As a result of this we sourced timber cladding from a company called Russwood who supply timber cladding that has undergone specialist fire safety treatment so the wood doesn’t burn. This meant the Client’s requirements of timber cladding could continue to be used. You can find out more about Russwood’s fire retardant timber cladding by clicking here.

As this is a self-build project I helped further by providing ongoing aftercare support to the Clients to ensure the design intent was delivered and the principles of building were clearly understood.

Once the extension project is completed there is also the possibility of looking at the rest of the existing house and how that can upgraded to improve the energy efficiency and create a heathier and more comfortable environment.

This has been an absolutely brilliant project to work on, challenging in lots of ways, but amazing to work with such forward thinking and open minded Clients. I’ve learnt a lot and can’t wait to see the project completed and being used in the near future.