' the Woodlouse: July 2013


Sunday, 28 July 2013

Strawclay - combining my two favourite materials

I'm still way behind with this blog.  I'll try and make a concerted effort to catch up in the next few weeks.  Life on and off site is as full as ever, but I have had a few days off to go to a friends wedding in Spain.  That was lovely!  As has been the fact that we've actually been having a summer.  After seemingly constant rain last year and working in a world of mud, that is particularly welcome.  I did spend the first week of sunny weather stuck in the dark, hot, sweaty loft running ventilation ducting, but I'm now back in the daylight and enjoying it.

Around the same time as the masonry stove was being built (see last blog: http://thewoodlouse.blogspot.co.uk/2013/06/lovely-cosy-heat.html) we were building the thick strawclay wall.  The hope is that this wall will provide reasonably good sound separation between Anna's studio, my office, and the corridor, so that we can both make noise (listening to music, playing music, running sewing machines, swearing at computers) without annoying each other.  I'm particularly hoping it allows me to play my small selection of musical instruments in the evening without keeping Anna awake (she tends to go to bed early, and that would often be the best time for me to play).  My mandolin, trumpet, whistles and vocal chords have been neglected and I look forward to rectifying that (some results of when they have been used are here: https://soundcloud.com/bicipital-groove).  I also hope that the thick strawbale walls, triple glaze windows and well-insulated roof will prevent anyone outside the building hearing us.

As you might expect, strawclay is a mix of straw and clay.  Clay slip is poured over a pile of loose straw, and this is mixed together with pitch-forks (I tried using the cement mixer but it wasn't much help).  When there is just enough clay in the mix to lightly coat the straw it's ready to use.  This is then pushed between boards fixed to the timber studwork of the wall, and lightly tamped down with a bit of 4x2" timber.  When the boards are full they are moved up and the process repeats.  The tricky bit is just below the ceiling, where it is impossible to fill from above.  Here the board on one side is fixed right at the top, and on the other side about 150mm below the ceiling.  Strawclay is pushed into the space as well as possible, then the board is raised and fixed at the top to compress the mix into the wall.

Some people mix Boric acid in some form into the mix to prevent mould and insect attack.  I didn't do this, but I can see that it could be a good plan.  Boric acid is fairly innocuous to humans, but is very effective preservative treatment (this site has more info on boron and it's safety: http://www.woodworm-info.co.uk/tech_info.htm. Disclaimer: it is a site connected to a boron preservative shop, so maybe not a neutral source of info).  A week or so after finishing the wall a few patches of Ink Cap mushrooms developed.  The mushrooms themselves aren't such a problem, but like icebergs it's the bit you can't see that is a potential problem.  The mycelium fibres that the mushrooms spring from will rot the wall (basically they'll digest it from the inside out) if given a chance.  They may have died off (or at least gone dormant) once the wall was dry, but given it's thickness (around 200mm) the wall has taken a long time to dry.  I was concerned that the mycelium could spread during the drying time and either cause a lot of damage, or re-activate one moisture from the clay-plaster reached them after plastering.  Ink Caps have been known to lift concrete so clearly clay plaster would be no match for them!  In the end I caved in and injected the wall with a boron paste, and sprayed the surface with liquid boron preservative.  I'm not wildly comfortable at having had to do this, but it does seem to have saved the wall.

For good sound separation I'm told a mix of dense and less dense materials are needed.  Apparently  the less dense materials absorb higher frequencies of sound, and the more dense ones absorb the lower frequencies.  Straw is low density and clay high density so in a way the strawclay is already a bit of both, but in reality it makes a relatively lightweight structure, as it is only loosely compacted.  Listening to the radio through the walls it does sound bassier as the higher frequencies are effectively absorbed by the strawclay.  To absorb those bassy frequencies we'll plaster both sides of the wall with a thick layer (about 30 - 40 mm) of dense clay plaster.  Together with the strawclay, and the fact that the studs are acoustically separated from surrounding structure by rubber foam (see http://thewoodlouse.blogspot.co.uk/2013/05/emerging-from-dark-winter.html), this should contain sound pretty well.

A bath full of clay.  No strawbale building site should be without one.

Clay is soaked with water (the clay is from the site - it was dug out to install the rainwater harvesting tank and soakaway)

Mixing clay slip in the mixer.  I later found it more effective to mix the slip in a gorilla tub with a drill mixer.  When mixed with sand the clay breaks down readily in a mixer, but on it's own like this there are always lumps remaining in the cement mixer.

John forking a mix.  Looks ready to use. (photo Sam Wilberforce - thanks Sam!)

Strawclay tamped down between shuttering

The shuttering can be removed immediately.  The strawclay holds it's shape well and be built up straight-away.

The alternating position of the studs on each side, with the mix woven between them, prevents the strawclay from falling out of the wall.  It also aids sound separation

Sam and John filling a section of wall.

It's messy.  But lovely.

Debs, Sam and John prepare another mix.

Thanks to Sam for this photo.

The awkward top bit.  Board on the far side is at full height.


Uh oh... Ink Cap mushroom.  I tried to make sure any mouldy straw didn't make it into the mix but some got through, clearly.  Given the near-impossibilty of keeping spare straw dry through all the rain last year, we not surprisingly had quite a lot of mouldy straw.

The wheat-grass has died-off now the wall is dry.