Passivhaus diaries, part seven: The walls

I'm actually on holiday in my native Devon this week, but work on the cavity walls will hopefully happening on site in my absence – more details on this below. And while I'm away I'm also giving our clients Geoff and Kate Tunstall a chance to give their perspective at this stage in the build (see box).

On site, the wall is being built in dense concrete block to help increase thermal mass inside the house, with 300mm of Dri-Therm fibreglass insulation batts (three 100mm layers) in the cavity and 100mm of natural stone on the outside.

Below ground we've used polystyrene solid closed-cell insulation within the walls, which will provide stability if there is any ground movement. Also, because it is made using closed cell materials, it will not absorb any moisture if groundwater gets into the cavity. As soon as insulation gets soaked its effectiveness is reduced, increasing thermal conductivity.

Wall construction for the Passivhaus

Just above floor level we need to join the polystyrene foam with the fibreglass insulation. To do this, we have cut the polystyrene solid insulation at an angle to help control the flow of any moisture within the cavity wall. Because stone is porous there will inevitably be some water percolation coming down the inside of the stone outer leaf. We've put a plastic cavity tray on top of the angled foam, which acts as a damp-proof course and will also take any water that might be blown into the cavity.

The fibreglass batts are woven in such a way that they are layered so any water that does come in through wind-powered rain will not penetrate beyond a minimal surface layer. If we'd cut the polystyrene off square there might have been the potential for some pooling of water, which would have acted as a thermal bridge. The cavity tray itself is just a thin bit of plastic, which won't cause any significant thermal bridging problems.

Instead of stainless steel ties, we will be maintaining the cavity wall using Teplo ties – made of resin and basalt. Developed during the Russian space programme, these are extremely strong and a good insulator in their own right. The basalt ties also have BBA certification, so we have had no objections from our building control officer!

One disadvantage of the Teplo ties is that they are rigid – you can't use them if the coursing is slightly out, so we've had to be very careful in this build to keep courses of block in line with the courses of stone. Two courses of block on one side match three courses of stone very accurately, tied every second course of blocks with a tie. The fibreglass batts fit neatly as they are, very conveniently, two blocks high.

Our choice of wall tie has been carefully modelled in the Passivhaus Planning Package (PHPP) - the design tool for architects designing to the Passivhaus standards. At this level of performance, stainless steel ties would act as a thermal bridge and would create a cold spot on the plaster in the internal wall with the possibility of condensation forming on the internal plaster. The use of basalt wall ties therefore offer good building practice and give us a nil reading for heat transference in PHPP.