When building a Passivhaus, there are various considerations that would not apply to a standard build. These are set out in the following step by steps. We start with the foundations but check back as more will be added over the duration of the build.
1. Digging the foundation trench
The Passivhaus floor slab is very different to ‘standard’ foundation details. In the case of the showhome, it’s effectively a ground-bearing reinforced raft sitting on 250mm of Kingspan Styrozone, which is also carried up the side of the concrete to eliminate cold bridging around the perimeter. This means that an independent foundation is required to support the facing brickwork.
Once the site has been prepared, the first step on this site is to excavate the foundation trench that supports the external brickwork. It is incredibly important that the position and depth of the foundation trench is exact, as the top of the concrete sets the starting position for the brickwork. The foundation is therefore set out by the setting out engineer with physical dimensions checked against boundary positions, for instance, just to make sure that there are no errors.
Once the foundation is excavated, the bottom of the dig is inspected by both the structural engineer and the building control inspector to make sure that it is of the required bearing capacity.
Top tip: Check the invert level of service entries to make sure that they don’t clash with the position of the foundation. It is quite common for services to clash with foundation concrete and arrangements may have to be made to ensure that services can properly enter the building.
2. The concrete pour
The pouring of concrete foundations tends to be relatively straightforward, provided the excavation is in the right position and there’s good access for the concrete mixer to tip the concrete into the trench. Before starting to pour the concrete, it’s important to make sure the ‘TOC’ (top of concrete) level can be accurately established. We decided to do this with a laser level but installing level pins is another good alternative.
Once the concrete has been poured and spread into place, a vibrating poker is used to help compact the concrete and remove any air that may be present within the mix. Once poured and levelled, the surface is finished with a tamp to provide a good base for the brickwork to be built off.
3. Building the corner blockwork
Once the concrete has cured sufficiently, the foundation blockwork is started, with the corners built first. The corner position of the walls are firstly marked out to check that they correctly sit on the foundation. It is much more efficient to build the corners first as this enables the straight walling to be ‘run in’ quickly using lines to set the position and height of the brickwork.
4. Building the blockwork walls
Once the corner blockwork has been constructed and checked for position and height, the straight block infills are then ‘run in’ using a line at each course. At the location of service entries, openings are created with concrete lintels supporting the blocks above. It’s important that services such as drainage have sufficient clearance to allow for settlement without displacing drains.
An important feature of building a Passivhaus home is maintaining optimum airtightness. This requires attention to detail at every stage — the foundation blockwork is no exception. The mortar in the substructure is therefore pointed to make sure that every bed and perp is full and that air cannot leak through the construction.
Once the blockwork has been constructed a final dimensional check is carried out by measuring the diagonal lengths between each corner. If the diagonals measure the same then the blockwork can be considered to be square. Also, check the level of the blockwork to the entire perimeter of the substructures to make sure that it is level and within tolerance. It’s much easier and cheaper to correct any errors at this stage than once the raft or the beam and block has been installed.
Top tip: Once the blockwork has been construction carry out a final dimensional check by measuring the diagonal lengths between each corner. If the diagonals measures the same then the blockwork can be considered to be square. Also check the level of the blockwork to the entire perimeter of the substructures to make sure that it is level and within tolerance. It is much easier and cheaper to correct any errors at this stage rather than after the raft or beam and block has been installed.
5. Sand blinding
Once the substructure blockwork has been constructed and the services have been installed and inspected, the ground below the raft is then prepared and levelled to receive the compacted stone and sand blinding. The purpose of the sand blinding is to provide a level surface to support the insulation and this must also be compacted to prevent any later settlement.
Top top: It is important that the compaction equipment used and the method adopted will meet the design provided by the structural engineer. Ask for a method statement from the groundworker and obtain approval from the structural engineer prior to starting this element of work.
6. Laying the insulation
Kingspan Styrozone has been specified here due to its excellent thermal properties and its load carrying capabilities. The insulation board is placed in three layers with tight, staggered joints to help spread the imposed load from the raft and reduce any air leakage and thermal bridging potential. The Styrozone is also installed up the side of the concrete to eliminate cold bridging around the perimeter.
7. Installing the steel reinforcing mesh
Once the Styrozone insulation has been laid, the preparation works for the concrete raft can be done. Firstly, a DPM (damp-proof membrane) is laid over the insulation. This membrane prevents damp rising into the concrete, as well as preventing grout loss during the pour and air leakage once the concrete has cured (concrete will leak air). Service penetrations through the membrane are also sealed using proprietary sealing tapes, again with the focus of preventing air leakage.
The steel mesh reinforcing provides the skeleton of the raft foundation and affords the structural strength required by the raft. The correct installation is therefore paramount and it’s a good idea to ask the structural engineer as well as the building control inspector to inspect the installation prior to the pouring of the concrete.
8. Tamping the concrete
Once checked, the concrete raft can then be poured. Access for concrete mixers is difficult on this site and therefore a concrete pump is used to help distribute and place the concrete.
The concrete is once again compacted using a vibrating poker and vibrating tamp which removes the air within the mix and ensures that the concrete spreads around the steel reinforcing properly.
The concrete slab can be finished in different ways, from a simple tamp finish to a power floated, polished finish. The Passivhaus will have a screed and therefore we have opted to leave the concrete slab with a simple tamp finish, taking care to ensure that service penetrations are properly filled around. (It’s a good idea to seal the top of any services with tape to prevent concrete spilling down pipes.)
Top tip: Seal the top of any services with tape to prevent concrete from spilling down drains and water pipes causing unwanted blockages.
9. The finished slab
Depending upon the time of year, the curing (drying out) of the raft may need to be controlled. In winter, frost could freeze the concrete damaging the surface, adversely affecting the concrete strength. In summer and in times of high winds, the concrete could dry out too quickly causing excessive shrinkage and cracking of the surface. Care must therefore be taken to protect the concrete and allow adequate curing before any following trades start to work on the surface.
Once the slab has sufficiently cured, the protection can be removed along with any temporary formwork. In this case, we’ve used blockwork to provide extra support to the turned-up insulation. (This was removed and will be replaced with brickwork at a later date.)
The Building Envelope
1. Levelling the Sole Plates
The first pieces of timber to get fitted on every project are never the most glamourous but arguably the most important. The sole plate is a single strip of solid timber that runs around the top of the foundations, fitted to the top of the ‘kicker brick’ course; the last element of the foundations. It’s essential that the sole plates are level and set out accurately in accordance with the setting out drawings. Mistakes incorporated at this stage of the build invariably cause much bigger problems further down the line and so the job has to be done correctly. If there are any errors with the foundation construction then these should be addressed before work proceeds.
2&3. Installing the ground floor panels
Once the sole plates are set then the ground floor internal and external Kingspan TEK® Building System wall panels can be delivered and erected. The panels are fabricated in elevation or part elevation form, depending upon access conditions on site, so that the number of joints requiring attention are minimised. The ground floor panels are nailed first to the sole plate and then fixed to each other. The erectors brace the panels to ensure that they are vertical. Panel junctions are sealed with an insulating foam and sealant to help ensure that they are airtight.
Corner junctions on the ground floor need particular attention as they must be set square. Internal wall panels, which can be load-bearing, non load-bearing or contribute to the overall building stability normally, get added into the structure as the build progresses.
4. Fitting the post and beam members
This Passivhaus is not simple: it has large open plan spaces on the ground floor, a central courtyard area and a spectacular feature window measuring approximately 8x3m. The courtyard is framed with four posts and the large openings are constructed using portal frames to provide stability to the structure. These large post and beam members are fitted as the ground floor build progresses and care must be taken again to ensure that they are accurately located and aligned.
Large members are typically fixed using resin anchors and shimmed (wedged using shims) into position with steel strips. It’s essential that once the resin fixings have set, and before the ground floor is screeded, the shims are grouted using a structural non-shrinking grout.
5. Constructing the first floor
We elected to construct our first floor using open metal web joists with Norbord Cabershield floor decking. We use open metal web joists (MiTek Posi-Joists) on all our projects — they’re great because they really do simplify the installation of the services, particularly the large ducts required by MVHR (mechanical ventilation with heat recovery) systems. They can also be manufactured to a variety of depths and widths so that a range of span and load conditions can be accommodated.
The Cabershield chipboard is a new product for us — a departure from traditional plywood. It has a permanent protective coating that protects the product from moisture during construction; we now know that it works very well. In this build the Posi-Joists are hung off of the external Kingspan TEK® Building System walls using metal hangers to help reduce thermal bridging. The Cabershield floor deck is then glued and screwed, using a special gap-filling adhesive to the top of the Posi-Joists. The adhesive helps prevent squeaks and creaks in service and contributes to the moisture protection performance of the system during construction.
6. Raising the roof
The Kingspan TEK® Building System 172mm panels used for the roof are capable of spanning over 3m without requiring any additional support. Where spans are longer than 3m then the panels are either sat on a load-bearing wall, supported by a purlin running underneath, or have rafters incorporated into them. While our overall build is somewhat complex, the roof itself is relatively straightforward so we were able to design and deliver some large-format panels to simplify the build and reduce the time spent working at height. Once the large panels are laid on to the supporting structure, they can be nailed together and fixed through on to their supports.
7. Sealing the structure
Being a Passivhaus, the building has to be very airtight. All of the panels are prefitted in the factory with a Wraptite System membrane, so once the superstructure is complete the final job is to seal up all of the panel junctions with a narrow sealing strip to provide continuity of performance. The next step is fitting the Vieo standing seam roof from Euroclad, and installing the rooflight and the Kloeber windows.