Bill Butcher
The project has reached concrete-pouring stage
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.
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Readers' comments (11)
Jenny J | 13 Jul 09 4:41 pm
Presumably that tea that's flowing is green tea...
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Mike Roberts | 14 Jul 09 9:27 am
Climate change is indeed an issue that we need to deal with now which is why we should be avoiding carbon emissions now. Buildings with large amounts of concrete are responsible for high levels of carbon emissions before they've even started in use. Concrete is clearly not a sustainable material, so surely we should not be planning on building '300,000 houses' with it. Surely if Bill Butcher is trying to innovate and provide an exemplar he would be better served doing so with a truly sustainable model. Hemcrete sequesters carbon, has excellent thermal properties, is affordable and does not require significant skill levels to use. It is also highly scaleable and doable now. See The Renewable House for evidence of this www.renewable-house.co.uk
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jim | 14 Jul 09 8:17 pm
What's the cost comparison of hempcrete to concrete block ?
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Chris Herring | 15 Jul 09 9:54 pm
We welcome the comments and debate regarding embodied energy of materials that the blog has raised. Hemcrete and similar natural building products offer significant benefits in terms of embodied energy savings and carbon sequestration. Ultimately issues I think once we are building effectively low energy buildings, we shall need to look at embodied energy and carbon sequestration issues too. But as Bill says, these issues are however dwarfed by the saving which can be made from reductions in operational energy of the sort achievable by building a Passivhaus. An additional consideration for the Denby Dale house was that we were not able to use a rendered finish which I understand a hemcrete construction would naturally lend itself to - the planning department insist on a natural Yorkshire stone outer face. This would I think lead to an excessively thick wall with Hemcrete construction. I am also interested to see that the renewable house exemplar Mike refers to has a wall U value of 0.19 W/m2K (too high for the Denby Dale house) and an air permeability of 2 m3/m2.h @ 50 Pa (Denby Dale will need to be below 1 m3/m2.h @ 50 Pa), although I understand that it is possible to achieve Passivhaus standard with Hemcrete. As Bill says above, our main aim in this build has been to build a Passivhaus at low cost using methods and approaches familiar to the majority of builders in the UK with materials available from the builders merchant down the road (literally in the case of the Denby Dale house). The urgency of the need to address climate change calls for the need to start building Passivhaus homes now and our approach reflects one way of doing this. 'There are many ways of crossing the river' and our approach is but one way, of the many, to achieve Passivhaus buildings in the UK. Our primary target is to pioneer Passivhaus approaches in the UK and we've had to work within our clients' budget guidelines. It would be very interesting to see details of the sort of wall build up we would need in the Denby Dale house with natural stone external face and a rough costing for using Hemcrete.
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Steve Simm | 16 Jul 09 2:42 pm
This is just a quick query on Chris Herring's last comment; in it Chris you've stated the Denby Dale construction will need to be under 1 m3/m2.h @50 Pa air permeability; is this calculated with respect to the volume of your house to achieve less than 0.6 air changes per hour or taken straight from the UK Passivhaus site please?
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Chris Herring | 16 Jul 09 4:39 pm
Steve, as we are intending to certify the building we shall of course need to achieve 0.6 ac ph @ 50 Pa, which I would expect to mean that the permeability is something below 1 m3/m2.h @ 50 Pa. Haven't actually calculated to see what permeability will actually need to be.
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John Quail | 17 Jul 09 2:11 pm
Is there any confirmation of the rumour that this project was offered to Grand Designs but turned down? The heart of this project in to work towards mass housing built to passive house standards. This is clearly far less important than vainglorious ego trips and a presenter who appears far more interested in how things look than in whether they help save the planet.
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Nick Grant | 21 Jul 09 8:16 am
I'm all for reducing embodied energy. Our own house which Chris and Bill know well, is made of local timber and straw. However I would always prioritise significantly reduced operational energy over embodied if I had to compromise. I am surprised that Chris says hemcrete can meet Passivhaus U values as I understood it struggled to meet Building Regs without arguing for a different sort of U value, rather like the multifoil people argue. How thick to reach < 0.15 U? Cost and practicality are also crucial. We have a polished concrete floor which is also the foundations (thermal bridge free). A Grand Designs Architect liked the look of it but was against concrete so tried to do in limecrete! Result our floor/foundations cost about £3k, limecrete one was (from memory) £15k over budget and had to be tiled.
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Bill Butcher | 21 Jul 09 8:58 am
John, I could'nt possibly comment.
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Chris Herring | 22 Jul 09 10:12 pm
From Hemcrete site it looks as though about 500 mm will get to around 0.1 W/m2K, depending on mix...which is the same thickness as Denby Dale Passivhaus. Hemcrete website indicates that it is possible to get an air permeability of around 2.5 m3/m2.h @ 50 Pa, so there may be a bigger issue here. However one of my points was that DD house had to be stone outer leaf, so Hemcrete would not appear to have been a possible consideration anyway without unnecessary wall thickness
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