Sunday 16 March 2014

Straw Works, and the dangers of cement in building

On Wednesday I went to Todmorden for the day to learn first hand about some of the design details of straw bale buildings from local expert Barbara Jones of Straw Works.

It was a really good day and Barbara really showed her knowledge and experience - having worked as a carpenter and joiner since the early 80s and about 20 years of building with straw bales.

I've read her book of course but there were lots of little details that I appreciated, from the banal (but important for me) observation about 10mm graded gravel being excellent for drainage (in a capillary break) because of the gaps that are created between the individual stones, to the slip joint that Straw Works use to attach straw bale extensions to existing buildings.

Most of all I loved Barbara's little lecture about the dangers of cement in foundations (and buildings generally). It turns out that if you have a well made foundation - particularly one that's self-draining, breathable and flexible - then it doesn't have to be very big and it certainly doesn't need a damp proof course. Barbara stressed that having an impermeable plastic sheet (DPC) in the middle of your wall was a pretty bad idea, because any water in the wall is eventually going to drain down and collect on the plastic. In fact the main reason as far as she was concerned for having a DPC was to protect the house from inappropriate concrete foundations, as the concrete sucks in water and without the DPC it can then help to rot your walls or feed mould on your kitchen walls!

Lime is the saviour here, as it helps to manage moisture. I'm already a lime convert, but this added insight was very interesting. There are technical sheets available to download from the Straw Works website that include drawings of different cement- and DPC-free foundations, details for installing doors and windows, and roofing details.

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