Our clients aim was to build a spacious new family room, that was warm, bright and with good connection to the garden, and as natural and sustainable as possible.
From early on our clients had in mind to clad the extension in timer. Veering towards a timber-frame construction was therefore a natural process since it worked best with a timber-clad finish and also with the shape and design of the extension. It also provided the opportunity to use natural materials. It worked very well with the fact that the main contractor was a carpenter by trade.
The timber framed walls which are formed of sturdy 150mm deep studs had the advantage of forming a deep enough space to use a batt type insulation rather than having to rely on the use of costly phenolic Kingspan type insulation. Adding an insulated installation void on the inside of the walls achieved a better insulating value than required by building regulations.
The obvious choice of insulating material in this case was sheep’s wool. There is a variety of natural batt insulations available on the market such as hemp or wood fibre but most of these are imported from the continent. Sheep’s wool is a fairly local material from the UK and does help keeping local farming traditions alive by providing an additional income for farmers. Sheep’s wool is also usually cheaper than other natural insulation material. Together with the contractor we compared the cost of sheep’s wool with the cost of conventional rock or glass wool type insulation, and it turned out that the sheep’s wool was only £150 more expensive for all the walls. We also found that using Kingspan to achieve the same insulation value would have been £310 more expensive than the sheep’s wool insulation. The contractor was very glad to be able to work with sheep’s wool which feels like installing a sweater into the walls rather than the often very itchy rock or glass wool or the rigid Kingspan which requires great care to fit tight and creates very fine dust when cutting.
In general, using the sheep’s wool insulation provided the opportunity to form breathable, moisture permeable walls to help internal moisture control and air quality by using internal lime plasters on wood wool boards. However, in this case, due to budget and practical constraints, a conventional plasterboard lining was used.
For the cladding our clients first wanted to use recycled timber boards. But it turned out to be very difficult to source affordable boards for this use. So, the choice was to use as local a timber source as possible. Being in East Sussex with plenty of sweet chestnut coppices the choice was simple. The boards where ordered from Inwood who are a sawmill specialised in machining local sweet chestnut. The boards where machined to three different widths with a half lap detail. This provides a crisp appearance for the vertical cladding and the randomly laid different widths avoid the strict rhythm of equal cladding and create a more uniform appearance to the walls.
The house has a north facing garden. Therefore, it was unavoidable that the new window and doors to the extension do not have direct sunlight during most of the day. They therefore do not benefit from any direct solar gain. In this case, we chose the use of good quality triple glazed and well-sealed units - provided by the Green Building Store. They are made in the Baltics but taking in account the energy saving they provide during their lifetime and the low embodied energy in the timber frames we think that the energy used during transport will easily be offset.
The undulating profile of the roof enabled the use of zinc as a covering, rather than having to rely on a felt or plastic membrane, as would have been required for a flat roof.