Passivhaus diaries, part five: Materials

In the next few days we will be pouring the concrete into the floor slab. A criticism that could be levelled against our build might be that we are using concrete floor slab and concrete blocks as opposed to other, more "natural" materials.

Maybe, in an ideal world, we would use "natural" materials such as straw bales, cob, Hempcrete or rammed earth to build the Denby Dale Passivhaus. Natural building materials tend to use less energy in manufacture and so create less carbon emissions during manufacture. Some materials, such as timber, can also sequester carbon dioxide, locking it up for the life time of the building. However, there are a number of reasons why we've had to use concrete at Denby Dale.

• Cost – in this particular build we're working to a tight budget and concrete block, fibreglass batts, poured concrete ground floor and a concrete tiled roof provide the cheapest, quickest and most accessible method available to us.
• Readily available and familiar – concrete products can be sourced from any builders' merchant and builders are more familiar with them. We wanted the Denby Dale project to be easily replicable at a low cost, as a solution for mass housing needs.
• Thermal store – we're trying to create as much mass as possible within the thermal envelope at Denby Dale - so the concrete will allow the house to act like a cave - helping to even out the temperature within the building.
• Embodied energy – in the life time of a Passivhaus building, the amount of embodied energy is dwarfed by the energy savings from reduced space heating need compared with a conventional building.
• Climate change is an issue that we need to deal with now and the construction industry is set up to use the materials available now. Tradesmen are trained in masonry cavity wall construction, concrete block and brick/stone. We're not all going to be able to use straw bales for all the 300,000 houses that we're meant to be building. Ultimately, it's about pragmatism.




Team members at the 'tool box talk' briefing: from right to left - Andy Nutall, Stevie Gruszka, Callum Weston, Jay Wilson and Jude Wilson (site foreman)

On site

We are carrying on with the foundations, and so far all has gone to plan. The weather's been good and cups of tea have been flowing. We've been working on more than just the house of course and we've been paying attention to retaining walls at the back, patio and garage. We're just about getting ready for putting the insulation in the floor - we're expecting it to arrive today - 225mm of Knauf polyfoam.

We're also working out the detailing around the edges of the concrete slab and how to cast it. Jude, our site foreman, has come up with a great idea - we're going to use the polystyrene insulation in the cavity as the formwork, whereas I had thought we were going to have to use some sort of timber work to contain the concrete slab, which would have added to the costs.

On Thursday last week we had what turned into a "toolbox talk" training session on site where we went through the next phase of the build. Questions from the team covered: wall ties, how the windows fit into the walls, right through to MVHR and why there are difficulties having wood burning stoves in Passivhaus buildings.