Les Piques Farm builds home extension with straw bale walls

July 12th, 2011 by Richard Lord

James and Claire Watts of Les Piques Farm decided on a straw bale extension to their home in St. Saviour after seeing the construction of a straw bale house on the Grand Design television programme.

Building with straw bales has many advantages. The building materials are produced locally and can be made to order.

Straw is highly insulating.  Straw bales of over 45 cm thickness have a U-value of 0.13 W/m²K, which is two to three times better (the lower the U-value, the higher the insulation value) than the building materials currently used on most properties.

Lime rendered straw bales store latent heat. They absorb heat from the sun and release it slowly so there is no sudden temperature changes in rooms.  Straw bales are also acoustically insulating.

Amazonails, the company that advised the Watts family, is pioneering the use of load-bearing straw bale walls as thermal and acoustic barriers for semi-detached houses.  Straw bale construction is being used near airports and motorways for this purpose.

Contrary to what might be expected, plastered straw bale walls are less of a fire risk than traditional timber-framed walls. Straw bale construction is also strong. The straw bale walls of the Watts family home extension is load-bearing.

The building technique originated on the plains of Nebraska, USA where wood was scarce.  Straw bale buildings exist today that were built over one hundred years ago.

Straw bale construction provides a healthy living environment.  Because the straw doesn’t contain pollen and is innocuous, it is a good choice for allergy sufferers who are increasingly prone to indoor air pollution.

Perhaps most importantly, straw bale construction is affordable not only because the construction material is inexpensive but because the thermal insulation value of straw bales reduced substantially the cost of heating the structure.  It is also a building technique that family and friends can participate in.  With a little expert help, it is the perfect material for a self-build project.

Les Piques Farm growing hydrid wheat in St. Saviour in 2011. The straw from this wheat could be the building blocks for a Guernsey home in 2012 (click image to expand - ©RLLord)

James Watts planted wheat which he harvested last year for storage in the barn across the courtyard from his family’s 1970s bungalow.  He received instructions on how to bale the straw in an extra tight fashion, suitable for use as a building material.  Each bale is about one metre long, 36.5 cm high and 45 cm wide and weighs about 24 kilograms.

The combine harvester cuts the standing plants two inches from the ground.  Inside the combine the grain is separated from the straw by threshing.  The straw comes out of the back of the combine and is laid on the ground.  The stalks are dry at the time of cutting so baling can commence within a few hours.

Usually the baler produces large, round bales but for producing bales for building construction, smaller rectangular bales have to be produced.  The rectangular bales have to be collected by hand and placed on a trailer for transport to a barn.

Each bale is held together tightly by two pieces of strong Nylon string.  The bale is composed of a series of wads of straw. If the Nylon string is cut, wads of straw can be removed to shorten the bale, and the bale can be tied-up again.

Architect Simon Cottell of BasMooArc measures one the straw bales that will become part of the wall of the extension (click image to expand - ©RLLord)

Simon Cottell of Bas Mooarc designed the extension to their home.  He attended a course in the UK on straw bale construction.  Rae Parkinson of Amazonails came to Guernsey to provide a course for the builders of the straw bale construction of the home extension.

James Watts carries one of the straw bales from the barn to the building site (click image to exand - ©RLLord)

The straw bales are fixed with hazel stakes, which were imported from the UK.

Claire Watts sharpens hazel stakes for use in holding the straw bales together (click image to expand - ©RLLord)

A cavity wall provides the foundation of the straw bale walls.  The external wall is concrete block.  The interior wall is made of a blown glass building block (Foamglas), which is made of selected 66 percent recycled glass.  It is light weight and has a high insulation value.  The cavity is filled with a granular material called Leca (Lightweight Expanded Clay Aggregate) that is commonly used in the bottom of flower pots for drainage.  The Leca is supplied by Ronez.

A cavity wall provides the foundation of the straw bale wall. (click image to expand - ©RLLord)

The walls of the house extension are six bales high.   Two hazel stakes rise out of the foundation to hold the straw bales in place.  When the wall has reached four bales high another two hazel stakes are driven vertically into the straw bales.  Two more rows of straw bales are added and then the wooden wall plate to cap the top of the straw bale wall.  Holes are cut in the wall plate so that two more hazel stakes can be driven down through each bale.  The top two hazel stakes are attached to noggins in the wall plate.

The walls of the extension are size courses high. Hazel stakes are used to hold the bales in place (click image to expand – ©RLLord)

Hazel is used to secure the bales in place because it is fibrous and it doesn’t break in half easily.  The hazel can be bent to form the shape of a staple, which is used to pin together the corner of two walls.

Architect Simon Cottell of Bas Mooarc cuts a notch in a straw bale to accommodate a fixing post attached to the granite wall (click image to expand - ©RLLord)

James Watts uses a two by four piece of wood to gauge the size of the notch for the four by four fixing post (click image to expand - ©RLLord)

Simon Cottell of Bas Mooarc and James Watts of Les Piques Farm place a straw bale in the wall and makes sure the fixing post fits in the notch (click image to expand - ©RLLord)

Rae Parkinson of Amazonails checks to see the straw bale fits next to the fixing post (click image to expand - ©RLLord)

Stuart Ogier cuts notches in the wall plate so that the hazel stakes can be driven through the bales to hold them in place (click image to expand - ©RLLord)

The straw bale extension showing the 4x4 fixing posts which will remain and the plywood corner guides which will not remain when the walls are covered in lime render (click image to expand - ©RLLord)

The wall plates lie in the yard ready for installation (click image to expand - ©RLLord)

The straw bales are staggered along each row. Any bales that have to be cut are rebundled with nylon twine. Surplus straw is used for animal bedding (click image to expand - ©RLLord)

The straw bales sit on a foundation made of an exterior wall of concrete blocks and an interior wall of blown glass blocks and a wood base plate. White strapes will hold the straw bales tightly together. (click image to expand - ©RLLord)

The cavity wall foundation exposed in a door way (click image to expand - ©RLLord)

The outer wall is made of concrete block.  The inner wall is composed of a cellular glass insulation block produced by Pittsburgh Corning Europe Inc. called foamglas T4+ This light black block is made of 66 percent selected recycled glass. The blocks cut easily with a saw.  The have a high insulation value and are non-toxic.

The light weight foam glass brick from Pittsburgh Corning Europe used for its high insulation value in the construction of the foundation and the in wall plate (click image to expand - ©RLLord)

The cavity is filled with a material called Leca (Lightweight Expanded Clay Aggregate) Insulation Fill, which was supplied by Ronez for this build.

Driving a hazel stake through the hole in the wall plate and into the straw bales below (click image to expand - ©RLLord)

Before the straw bales are strapped tightly together two hazel stakes are driven through the wall plate into each bale (click image to expand - ©RLLord)

Moving the noggin in the wall plate so that it can be secured to the hazel stake (click image to expand - ©RLLord)

The wall plate sits on top of the straw bale wall. The noggins are next to the hazel stakes (click image to expand - ©RLLord)

The wall plate is filled with foamglas. The hazel stake is secured to a noggin (click image to expand - ©RLLord)

Stuart Ogier ready to apply lime render to tie in the granite wall of the cottage with the straw bale wall of the extension (click image to expand - ©RLLord)

Stuart Ogier prepared the lime mortar by mixing two buckets of Natural Hydraulic Lime (NHL) 3.5 to five buckets of well-graded sharp sand provided by Annandale Guernsey.  (Hydraulic lime is graded by strength (feebly hydraulic (2), moderately hydraulic (3.5) and eminently hydraulic (5).   Water is added to the mixture to produce a putty-like consistency.  Stuart also added two handfuls of straw to the mixture.  He said the mixture should be sticky enough not to fall off the trowel when held upside down.  He applied the lime mortar to the granite wall in a process called harling to form a seal with the straw bales.  The lime render is flung onto the granite wall to get into all the nooks and crannies of the granite.

Stuart Ogier applying lime mortar to a granite wall to provide a seal for the straw bales (click image to expand - ©RLLord)

Stuart Ogier flings the sticky lime mortar onto the granite wall (click image to expand - ©RLLord)

The plastic strip provides a condensation point for any moisture that might be inside the wall.  It acts as a conduit for moisture to run down to the ground beneath.  The lime mortar is quick setting so it is applied just before the straw bale is put in place to join the granite wall.

Rae Parkinson of Amazonails retying a straw bale with Nylon string after removing some wads to shorten it (click image to expand - ©RLLord)

Stuart Ogier and Rae Parkinson of Amazonails place the straw bale in position next to the granite wall (click image to expand - ©RLLord)

Rae Parkinson of Amazonails pushes a straw bale into position (click image to expand - ©RLLord)

The straw bale walls at the end of the work day on 6 July 2011 (click image to expand - ©RLLord)

The image of one part of the straw bale extension showings the posts forming the window opening and the plywood corner guides, which will be removed before applying lime render to the interior and exterior of the load bearing walls.

Strapping and compressing the load bearing straw bale wall (click image to expand - ©RLLord)

On 8 July 2011 the load bearing walls with the wall plate sitting on top were strapped.  This operation is done with wide straps and ratchets.  The tightening is done slowly and evenly.  Once the compression has been completed, the thin white straps will hold the bales in place, and the wide straps can be removed.  The thin white straps are covered with hessian cloth before lime render is applied.

The walls of the extension showing the wall plate filled with Foamglas which gives a high insulation value (click image to expand - ©RLLord)

The whole straw bale construction is under cover to protect the straw from rain (click image to expand - ©RLLord)

Another part of the straw bale extension showing the incomplete wall plate that tops the straw bale walls (click image to expand - ©RLLord)

The north and western side of the 1970s bungalow has been encapsulated by the straw bale walls (click image to expand - ©RLLord)

Stuart Ogier of Limegreen Building and Rae Parkinson of Amazonails peer out of a window opening in the straw bale wall (click image to expand - ©RLLord)

A lime render will be applied in the interior and exterior surfaces of the straw bale walls.

This article is incomplete and work in progress….

1 Response to “Les Piques Farm builds home extension with straw bale walls”

  1. BAS Mooarc leads first straw bale construction in Guernsey at Les Piques Farm | Bas Mooarc

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