Sunday, 5 September 2010

A wall of straw and clay / Uma parede de palha e argil

The Alambique - timber frame and windows (from Jon and
Mary in Scotland!). Shuttering below to hold the straw-clay.
August. Summer flying by, progress on main house painstakingly slow. We need to move in, somewhere at least. Focus moves to the alambique, which we'd planned to move into while finishing the main house.

We've been working on the walls, floors, roof, plumbing, drainage, power supply since May. Now we needed a front wall. We have a few choices for wall material: bricks and mortar, earth bricks, timber frame with plasterboard and insulation slabs, and so on.

Nikita as Paulo's carpentry assistant, building the timber frame.
Note the stone wall built below the timber, to keep straw and wood
well away from any water.

We wanted a wall that would retain as much heat as possible inside, during the winter, would not cost too much, would be breathable (allow moisture built up inside to get out).  So we chose light earth building whereby straw and clay slip are rammed into formwork (shuttering).  Used for centuries in various parts of Europe presumably because it was cheap, used locally available materials and provided good insulation.

The clay takes loads of time to soften up, at least 24 hours. Then it needs whisked, then then the slip decanted into another container.

Paulo, surrounded by clay slip buckets, showing our Galician
friends the right mix of clay to straw, in Donald's old bath. 
Federico on clay stone crushing duty
What we didn't realise was the enormous amount of time the whole process would take. Making of clay slip alone is a fair bit of work, which needs constant attention. I found a source of fairly pure clay from a local brick factory in Galicia (10 mintues away). Cheap, certainly (25 euros a tonne).

Irena, Cesar and Jorge working the clay slip into
the straw. 

Then you need to have at least 2 people fluffing up the straw in a container of some sort. On bigger projects people have all sorts of equipment to do this, but these are not available here in Portugal, and if they were they would cost a fortune to rent.

Bugui, Galician yoga and meditation teacher, working with
the Zen of Straw
Cristina, recent Galician resident from Italy, adding her labours
to the wall compaction. 
Next, moving the straw-clay mix to the shuttering and tamping it down. Here, according to research by Salvatorre Gangitano (Dhanya) who kindly forwarded me his essay on light earth construction, says that if you press it in too hard you loose insulation, and have more of a thermal mass wall. If you leave it too loose, better insulation but the possibility of air gaps, and limited thermal mass.

Once the wall sections are done you can remove the shuttering right away. What you have is a damp walls of clay. Looks nice, it's straight and ready for plastering. But not for months! Here's another disadvantage: it takes up to three months to dry properly.  It's been 6 weeks since we built this wall and it's been really hot, so it is almost dry inside. But in colder climates this could be a real problem, mould can occur and delays in the building process can cost a lot.

Irena, great friend from Northern Galicia, and child (still at one with her)
one of the hardest workers on site. Impressive.

Paulo with first 3 straw bales, re-cut. Did we save time? Not sure,
because recutting them and notching the bales around the posts
was also a fair bit of work. But a definate benefit was the lack of
additional moisture. Bales are dry, ready to plaster right away. 
After a while we realised it would be quicker to fill the larger sections with dry straw bale, cutting the strings and retying them.

Me, driving ash stakes
through the bales to keep
them stacked in place.

We stacked bales all the way to the top then compressed them down with our van jack, then stufffed straw-clay in the remaining space.

OK. What about thermal performance? Dhanya's research found that the U-value of a 40 cm straw-clay wall can be around 0.239 W/m2K (this refers to the U-value, which is a measure of energy conduction, in watts, per square meter, conducted through a material, for each degree difference on each side).

How does this compare with other types of walls? Well, here's a selection, taken from the Whole House Book:

Wall type                           U-value
Solid brick  (225mm)         2.20
Timber frame with
50mm insulation                 0.96
Timber with 100mm
insulation                            0.35
Our straw-clay wall    
(maybe?)                            0.23
Superinsulated house
with 250mm insulation         0.14
Straw bale wall                   0.13

The latest UK building codes state that external walls must have a minimum U-value of 0.35 (W/m2K), which means straw clay/ light earth would be considered more than acceptable.

Overall, we are not that impressed so far with straw-clay as a system. It is so labour intensive that this can increase the cost of the job. And it takes a long time... Also, the problem of waiting for it to dry out enough before plastering  means that it could delay a construction.  A few mice have also moved in...

That said, now the wall is done and almost dry, I think it's going to perform well. But we'll have to see after plastering, and after a winter of rain and cold.

Petrus, building up stone wall in front of the alambique where
we plan to build a wooden structure for a veranda.  More on this later. 


Rupert Wolfe Murray said...

Fascinating. Inspiring.

Steven Gordon said...

Moona, looks great, kind of grown up camping! Will be interesting to read once you get the plaster on the straw. Will either be a good or damp winter, imagine trying it here in Scotland...
I've got SEA pictures, but nothing electronically, I'll try and get some scanned and forward to you. All is good, apart from an injured knee, and the fact I'm meant to go to Pakistan this week. Love to all and see you soon!