A passive slab foundation is a raft that is fully insulated from contact with the ground below eliminating any thermal bridging. Insulation forms the shuttering for a concrete pour, and layering of the insulation allows a ring beam to be formed around the perimeter and thickening under load bearing internal walls. Reinforcing steel is added to the raft according to calculations from a structural engineer to ensure all loadings and ground conditions are accounted for. Due to the raft having a very large surface area compared to traditional strip foundations, the load at any given point under the house is much lower. The structural engineer needs to design the raft to suit ground conditions, which is why a full ground bearing test and soil analysis was undertaken at the start of the project.

Levelling the sand blinding

The stone sub base is laid and compacted to within a 20mm tolerance. A sand blinding layer is then laid and compacted to a 5mm tolerance as a base for the insulated slab.Laying drainage runs

Once the blinding is levelled and compacted the drainage runs and ducts for incoming services are dug and laid. All of the drainage runs are pressure tested by bunging the lower end of the pipe and filling with water, any leaks will show up with a drop in level. It is critical to plan in advance where all drains and ducts and services are going to run in the build as it is not possible to add them at a later date. It is far better to have services coming through the slab in a controlled manner than try and make holes through the walls at a later date as these are much harder to keep an air tight seal on. Drainage should not penetrate through external walls or roof for vented stacks as these will create a large thermal bridge, not only reducing efficiency, but increasing the chance of causing damp patches around the thermal bridge.

Perimeter insulation and drainage

Perimeter insulation set out

First layer of insulation down

Further insulation laid over damp proof membrane

Once the perimeter insulation was set out the remaining internal area was filled with a 100mm layer of insulation. A damp proof layer/Radon barrier was then laid over this and taped together to form a huge red paddling pool. A further 200mm of insulation was then laid on top of this, leaving a gap around the edge for the steel ring beam and also gaps where the structural internal walls would be located. This would allow a thickening in the concrete raft when it is finally poured.

Steel ring beam

Bottom layer of steel mesh over insulation

First layer of steel mesh laid over insulation

All had been going so well but a couple of ‘elastic tape measure’ moments by the ground workers had put a duct and soil stack in slightly the wrong place. Fortunately spotted before the concrete was poured and easily fixed, but a major disaster to have a duct coming up in the middle of the living room. A variation of the old measure twice cut once and just showed how important it is to check and re-check things that cannot be changed later.

Under floor heating pipes over bottom mesh

Laying the top mesh over the ufh pipes

I could have laid the under floor heating pipes myself as it is really not a tricky job, but decided as there was only one of me, and its a job made much easier by at least two I would leave it to the guys laying the slab. They made pretty short work of it and all eight runs were finished in half a day. I decided to relay the last couple of meters of all the pipes myself as they had not been aligned with the ports on the manifold very well. The connections to the manifold are really fussy about being square and straight, and the effort was worth it to enable neat connections. Whilst connecting the pipes to the manifold I managed to mislay an olive. After spending a morning visiting every plumbers merchant locally to find a replacement with no luck I decided to fabricate one so I could get the pressure test completed. It was a pretty straight forward job purging all the air out of the pipes, and then left the hose running to build up the pressure. I left the system at 4 bar for a few minutes with no loss of pressure, so all good. The system will be left at 2 bar during the concrete pour so any damage to the pipes can be  spotted straight away, although I think it is unlikely as the pipes are pretty tough.

Under floor heating manifoldPressure test

Slab ready for the concrete pour