Passivhaus standard does not require one to build using any particular construction method. All it’s really interested in is whether the performance criteria has been met. As a result, around the world there are Passivhaus buildings constructed using all manner of methods, systems and materials.
Potton are specialists in the provision of high-performance building envelopes using timber-based systems and accordingly a timber system was an obvious choice for the superstructure of the showhome. We chose the Kingspan TEK® Building System (more on this later), because we have used it to construct literally hundreds of low-energy buildings and understand, in some detail, its performance characteristics. The system is ideal for use in projects which require low U values, low and defined thermal bridging characteristics and excellent airtightness. It’s also certified by the Passivhaus Institute which, if nothing else, provides some comfort.
When developing the structure of a Passivhaus, the main challenges are firstly to ensure that structurally it performs like any other property (i.e. it meets Building Regulations), and secondly to ensure that the structural design is both able to achieve the thermal and airtightness performance targets and makes achieving them as simple as possible (primarily because they’re pretty onerous in the first place).
Like any property, work starts in the ground. Many or most Passivhaus buildings are constructed using a solid reinforced concrete raft that sits or floats on a bed of insulation. The Potton Passivhaus is no different in this regard: a 225mm thick reinforced concrete raft sits or ‘floats’ on a 250mm thick bed of insulation.
One of the challenges for our structural engineer was to ensure, for the simplicity of construction, that the raft was one thickness throughout and still able to resist the loads applied by the posts, frames and walls required. We are pleased with the end result because, at one stage of the design, it looked as though we might need to have a thickened band around the perimeter of the raft.
One of unusual things about the Potton Passivhaus is that it uses masonry cladding, in the form of Terca Megaline bricks from Wienerberger; the long, narrow format provides a contemporary appearance, but is also a nod to traditional brickwork elevations of the vernacular. Most Passivhaus builds exclusively use lightweight cladding systems which are carried by or fixed back to the building structure. To use the masonry cladding and not introduce any thermal bridging meant that we needed to construct a separate foundation just to carry the brickwork façade — this was a simple strip foundation which is completely isolated from the main insulated raft.
Having sorted out the foundations and ground floor construction we turned our attention to the superstructure. The Kingspan TEK® Building System consists of 142mm or 172mm thick SIPs (structural insulated panels) connected with a unique jointing system. The System is typically used for the external walls and roof structure of buildings. We have combined the SIPs external walls and roof with open metal web floor joists and timber frame internal load-bearing and non load-bearing walls.
Our walls are constructed using TEK 142mm panels with an external layer of Kingspan Kooltherm insulation to help achieve the required U value — there is also a 40mm service zone on the inside of the panel. The roof is constructed using TEK 172mm panels, this time with an internal layer of Kingspan Kooltherm insulation. We have used the thicker panels for the roof because they are capable of spanning further without requiring additional support. We have wrapped the entire structure externally using A Proctor Group’s Wraptite System; a self-adhesive, high-performance breathable membrane and air barrier combined in one.
The Kingspan TEK® Building System panels are manufactured using an almost unique continuous casting process whereby the two sheets of 15mm oriented strand board (OSB), that form the skins of the panel, are suspended apart from each other on a slow moving production line. The liquid mix for the rigid insulation core is introduced into the gap between the boards, filling the space between. The whole panel then goes through a press that controls the thickness of the finished product and therefore its density, thermal and structural properties. The large panels are then typically further processed, using a CNC router to create the shapes required by the design of the building.
As with all Potton projects we design and build using the Kingspan TEK® Building System, the construction and manufacturing drawings for our Passivhaus have been developed using specialist 3D CAD software. This starts with the structural concept, checking the design will work within a Passivhaus environment and then moving on to developing the detailed manufacturing design; these checks provide a useful opportunity to spot any last-minute niggles and ensure that we end up with a structure that is as easy to build as possible.
Self build Director Dr Paul Newman has worked within timber-based construction industries for over 18 years