Our clients purchased a ‘back land’ site in West Sussex which had already a planning application for a four bedroom house in place. They asked us to rework the design of the existing application to give the house a more contemporary appearance with a generous open feel and a good flow from inside to outside. An important part of the brief was also to make the house as energy efficient as possible. Being restricted by the tight site and the foot print and massing, established by the previous planning application, we decided together with our clients to use a contemporary timber frame building method also used for Passive house construction. This provided a very high level of insulation while keeping the structure at a reasonable thickness and also uses predominantly natural hygroscopic materials which was an important factor. In order to maximise the internal space a slightly thinner wall-build-up than required to achieve a Passive house standard was used but still achieving an outstanding levels of thermal performance.
Our clients moved into their house beginning of December. They told us that after living in the house for 2 months - then well into winter - that they still didn’t have to switch on their heating system.
After researching various timber frame systems and getting an understanding of associated costs we decided with our clients to use a timber I-joist system. This was supplied and fitted by Touchwood Homes who are specialised Passive house timber framers.
The system provides a sturdy timber-frame with studs and rafters at 400mm centres rather than 600mm used by other methods.
This is covered on the outside by a continuous layer of wood-fibre board which is well sealed at the joints and at all corner and also taped to windows etc. in order to form an airtight layer to the outside of the timber-frame. Having the air tightness layer on the outside instead as usual on the inside this enabled safe runs of services within the external walls without penetrating the air tightness layer and made the use of special air tightness membranes and extra installation voids to the inside obsolete. The wood fibre board is hygroscopic and lets any moisture that might build up within the wall and roof structures, permeate to the outside and therefore avoid the risk of interstitial condensation.
The walls and roofs are filled with cellulose insulation which is made from recycled newspapers. Being a natural fibre material it has the ability to permeate moisture and therefore forms a hygroscopic build-up together with the external wood fibre board. The flocked cellulose is blown under pressure into the wall and roof cavities. All spaces are tightly filled with insulation. This provides a good amount of density within the structure and avoids any gaps which could cause air movement within the structure and thermal bridges leading to condensation. This would not be achievable to this point with batt or board insulation fitted into the 250mm deep wall and 300mm thick roof structures. Due to its density the blown cellulose also works as good sound insulator and gives the house a fairly sturdy feel. Therefore our client decided to also use the blown cellulose insulation for all the internal walls and the intermediate floor.
The house is brick clad at ground floor level and timber clad to the first floor with a minimal flush detail between the two. The build-up for the timber cladding is thinner than the build-up for the brick cladding. In order to utilise this to gain extra internal space for the first floor the timber frame had to be designed with a slight overhang all around between the ground and first floor. This was carefully detailed together with Touchwood Homes in order to achieve continuous insulation and air tightness layers.
The ground conditions prevented us from using an all around continuous insulated concrete raft usually used in low energy construction to avoid thermal bridging. Instead we had to use a suspended ground floor on strip foundations to be able to deal with the possible heave of the clay soil. The thresholds between internal and external levels were to be near flush. Therefore masonry plinth wall was required to lift the timber frame adequately up from ground level. The external brick skin to the ground floor also added another level of complexity in relation to the plinth wall design. We developed a whole series of plinth wall details tailored to the various conditions along the perimeter of the house to provide thermal bridge free, airtight and water tight connections between the ground floor construction, the timber framed walls and the floor to ceiling glazed elements.
The windows and large floor to ceiling sliding doors are triple glazed, well insulated passive house standard aluminium clad timber windows made by an Austrian manufacturer. They were fitted by a specialist window fitter who understood the importance of appropriate seals to achieve a high level of air tightness.
The specialist frameless ridge and eaves roof lights are also triple glazed and well sealed to the external wood fibre board.
As a pressure test showed a very high standard of air tightness was achieved which would comply with passive house standards.
A whole house ventilation system with heat recovery provides fresh air to the house with minimal heat loss and distributes the warm air throughout the house. We integrated this into the design at an early stage in order to find the best position for the unit and the best duct routing together with the ventilation engineer. We designed the required ventilation openings between the rooms and the hall integrated into the shadow gap detail of the minimal oak door linings. This not only works very well aesthetically but also provides an extra level of sound proofing for the ventilation gap.
The floor finish to the entire ground floor area is a polished screed. This lent itself perfectly to install a wet under floor heating system which provides efficient low temperature warmth to the ground floor, if required, and provides also a good element of thermal mass to the house.
Although our clients where advised that extra heating to the first floor might not be necessary they still wanted to install under floor heating below the engineered oak floors as a back-up. As mentioned 2 months into winter, after our clients moved into the house in beginning of December, they neither had to use the heating to the first floor nor to the ground floor. Just using the house, cooking in it etc produced enough warmth to the house up to then which was retained in the house through the heat recovery ventilation system.
Knowing that the heat demand for the house would be fairly minimal we have assessed various options for heating and hot water, including renewable options. It was decided that the use a good quality condensing mains gas boiler as heat and hot water source would be the most practical and by far the most cost-effective option.
We looked into fitting solar hot water elements to the south-facing roof in order to produce part of the hot water, especially during the summer months. But having assessed our clients likely usage pattern of the house it was decided that it would be more beneficial to fit solar PV panels to the limited south facing roof area. These produces electricity all year round that can either be used directly within the house or that will be fed into the grid, contributing to the production of small scale renewable energy.
The site previously was part of a large back garden which is downhill from the front house and the mains drainage within the road. The soil on the site is predominantly clay and tests showed that there is minimal soakage capacity. It therefore was decided to install a system pumping the foul and surface water up underneath the drive to connect to the mains sewage. In order to keep the system under minimal use a large rainwater harvesting tank was installed which collects water now being used within the house to flush toilets and for washing clothes and to irrigate the garden.
While digging to install an additional attenuation tank to hold back possible storm water it was found that the soakage at that particular location was much better than the previous test showed. The tank was therefore formed with an open bottom to allow most water to soak away into the ground on site to minimise the amount of surface water fed into the mains drainage system.