Part 4 - The Passivhaus Foundation

12 May 2016 Brent Ackerman, Delta Project Management

The Substructure: A step-by-step guide

Image of groundworks for a SIPs house
The foundations of a building are a critical part of any self-build project, but when building a Passivhaus, there are various factors to consider that would not apply to a standard build.
These factors are set out in the following steps, starting with the passive house foundation:

1. Digging the foundation trench

Digging a foundation trench on a self-build site

The Passivhaus floor slab is very different from ‘standard’ foundation details. For the showhome we’re building, it’s effectively a ground-bearing reinforced raft sitting on 250mm of Kingspan Styrozone. This is also carried up the side of the concrete to eliminate cold bridging around the perimeter, which means that an independent foundation is required to support the facing brickwork.

Once the site has been prepared, the first step is to excavate the foundation trench that supports the external brickwork. It is incredibly important that the position and depth of the foundation trench are 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 to ensure 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 Foundation Tip!

Check the invert level of service entries to make sure that they don’t clash with the position of the foundation. This clash is quite common and arrangements may have to be made to ensure that services can properly enter the building.

2. The Concrete Pour

Concrete pouring into foundations on a self-build plot
The pouring of concrete foundations tends to be relatively straightforward, as long as 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 to build the brickwork on.

3. Building the Corner Blockwork

Building the corner blockwork on a self-build site
Once the concrete has cured sufficiently, the passive house foundation blockwork begins, with the corners built first. It’s important to first mark out the corner position of the walls 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

Blockwork walls on self-build
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, which support 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 to maintain optimum airtightness. This requires attention to detail at every stage and the foundation blockwork is no exception. Subsequently, the mortar in the substructure is 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 length, then the blockwork is 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 rather than once the raft or beam and block has been installed.

Building a Passivhaus Show Home

A complete step by step guide, we take you through the story of building our Passivhaus show home. From the design process, constructing the superstructure, achieving airtightness right through to the finishing touches.

Download the guide here.

Building a Passivhaus Download

5. Sand Blinding

Sand blinding
Once the substructure blockwork is 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, which must be compacted to prevent any later settlement.

6. Laying the Insulation

Laying Kingspan Styrozone 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. Laying the Steel Reinforcing Mesh

Installation of steel reinforcing mesh on a self-build site
Once the Styrozone insulation has been laid, the preparation works for the concrete raft can be completed. Firstly, a damp-proof membrane (DPM) is laid over the insulation. This membrane prevents damp from 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 with a quality foundation and gives 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

Two men tamping concrete on a self-build site
Once checked, the concrete raft can then be poured. Access for concrete mixers is difficult on this site, so 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 properly spreads around the steel reinforcing. 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, so we have opted to leave the concrete slab with a simple tamp finish while taking care 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 the pipes.

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, which damages the surface and adversely affects the concrete strength.

In summer and in times of high winds, the concrete could dry out too quickly, which causes excessive shrinkage and cracking of the surface. Therefore, care must be taken to protect the concrete and to allow adequate curing before any following trades begin 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.

Read Part 5: Superstructure

Potton Elsworth Passivhaus Show House

The Elsworth is open to the public

We have five self-build homes for you to browse, but the Elsworth is the UK's first permanent show home built to Passivhaus standard, created using our Kingspan TEK Building System. Come and experience the Elsworth show house for yourself.

Editor’s note: This blog post was updated in May 2019 for accuracy.

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