To start with, we need to establish the difference between hay and straw. Hay is cut when the plant is about to form seed, or has just formed seed and is at its heighest protein content. Straw is the stalk left after the grain has been harvested and has no protein content. It may provide a home for creatures if unprotected, but they would be unable to live on it.
Plastering, the oldest handcraft still in modern industry. This ancient art has never had any form of scientific research into why it works. It is a science of the behaviour of materials observed by plasterers that has been handed down from father to son, journey man to apprentice. This knowledge is jealously guarded and is not recorded in any written form. However it will form a part of the new standards for plastering over solid backgrounds.
There are two forms of lime used in building in Western Australia: hydrated and hot or lump. Hydrated lime is purchased bagged as a powder. Hot lime is available in two forms - in a putty form packaged in a 25 litre plastic bag, and as a dry form packaged in a wheat or super bag. At present hot lime either as putty or in a dry form is prefered for plastering strawbales.
Lime mixed with good quality sand and properly tempered provides a coating over walls and ceilings that gets harder with age. The process is that as the mortar loses its water content it then absorbs chemicals from the air. As this action takes place the lime hardens throughout the life of the building.
Powder types of lime contain a plactisicer to make them easier to work. Although it is suitable for brickwork and is used for floating and rendering onto solid bases, it has disadvantages on three coat work as used on strawbale construction. This is due to the added chemicals during manufacture.
Lime motar render is essential in allowing the passage of air and moisture to pass through the walls. When walls become saturated from leaky rooves and water-pipes, or from flooding, the stawbales have to lose the excess water very quickly. Lime mortar is the only cladding to provide this advantage.
There is a good example of this process at work on a strawbale house in Western Australia. In two instances where the strawbale walls were saturated with water, both sides had been floated with sand and lime mortar. The walls averaged 75% moisture but after a four week period in damp South West conditions, the moisture level returned to normal. If the walls had been rendered in cement or strong composition mortar the water would have been locked in for several years.
Lime mortar is applied to strawbales in three coats similarly to plastering onto any form of lathing. The first coat covers the straw ends and lath to form a base to which it is keyed. This coat is often called a "pricking up" coat or "scratch coat". There is a six week minimum between coats to allow mortar to carbonate.
The second coat is a strengthening or float coat. Its purpose is to shape and staighten the walls and ceilings.
The third is the finishing coat. It provides the desired surface finish or "fining up", a process of finishing the surface rendering with a teak float to provide a compact and water resistant surface.
FLEXIBILITY OF LIME MORTAR
Lime mortar will withstand the variation in daily temperature and movement of the structure. It is flexible enough not to form large cracks on the surface and is assisted by the consistent insulative properties of the straw on the internal surface of the render. It is not adversely affected by the variation of weather temperatures on the outside. During winter months the temperature can be down to zero and then up to 45 degrees C. plus in summer. Cement and strong composition mortar can crack, craze and creep in different weather conditions, not only on straw but on solid bases such as all types of brickwork.
HIGH BOND STRENGTH
The high bond strength of hot lime makes it an ideal material for the application and bonding to straw ends, lathing and previous coats of mortar, such as pricking up or scratch coats. Lime mortar flows into the pores of the straw stalks and previous coats of mortar. In return the straw acts as reinforcement to increase the tensile strength of the mortar. Cement mortars have compressive strength but it bridges the small pores of the straw stalks forming a weak bond.
Lime mortar properly prepared, then worked, compacted, scoured up with a teak float and then fined up becomes water resistant as it weathers and ages.
Cracks that occur due to structural movement seal through a process known as homogeneous healing. Cracks become rivers during storms or wet weather and the water carries a solution of lime that deposits along the cracks and eventually seals with a pure lime similar to the formation of a stalagmite/stalactite in a cave.
Cement mortars crack and craze because of their ridgity and brittle nature, as well as the daily expansion and contraction due to the variations in the weather and temperature. Cement mortars have the added disadvantage that there is no free lime solution to seal the cracks. If a crack did seal it would break again because of the continual movement of the mortar.
Lime mortar performs well in a fire and will often serve as a sacrificial coat to protect brick work, timber, steel and strawbales. It will also withstand the sudden temperature change when water is doused onto the hot surface. Lime that has experienced very hot fires never appears to have any spalls on the surface, but other masonry units will have surface damage.
There are examples of lime mortar plastering protecting straw bales in Western Australia. In the best example of fire resistance, half of a wall of a house had been plastered, the rest was still uncovered. When a fire broke out the unprotected bales received about 50% damage but those plastered with lime mortar remained unaffected.
There is a video by the English Heritage Architectural Conservation "Return to Windsor Castle" that shows lime plastering acting as a sacrificial coat that protected the brick work beneath.
Up until the mid 1960s the wood stove and copper were always built in with lime rich mortar because it was unaffected by intense fire and didnŐt expand or contract whether fired or cold. The lime mortar always outlasted the life of the wood stove or copper.
Cement and strong composition mortars in a fire situation expand, crack, spall and break the bond to the straw ends. Cement mortars usually break off in fairly large sheets ranging from one to six square metres. Cement render that remains in position crack, and these cracks can become chimneys and burn beneath the coatings.
Hot lime is well known as being the best and safe plastisicer. The addition of hot lime into cement and sand mixes makes mortars not only easier to work but flexible. These mixes that have the addition of hot lime are known as composition mortars (compo). The greater the hot lime content the greater the advantage of lime durability with no known disadvantages.
Liquid or patent type plastisicers (synthetic lime) have many disadvantages. Where these patent materials are used to the manufacturer's specification and used in ideal conditions, they do the job they were designed to do. But when they are overused compressive bond strength and mortar life is affected. The greater the addition of plastisicer the greater the deterioration of mortars, with up to 30 times above the recommended amount common.
THE DISADVANTAGE OF CEMENT AND STRONG COMPOSITION MORTARS
The following are the disadvantages of cement and strong composition mortars. (a) Poor bond stregth to mating surfaces. (b) Rigid and brittle render does not allow for structural movement. (c) Cracking and crazing. (d) Will not allow the wall to breath. (e) Expands and cotracts on a daily basis, breaking the bond to straw ends. (f) Cracks become rivers when shedding water. (g) Expands, cracks, spalls and explodes in fires. (h) Creep, with daily expansion and contraction. (i) Cracks become chimneys in the case of fire.
The above list will also apply to the covering of solid bases, lathing, two and three coat work and decorative work. There are very few plasterers who have the skill, knowledge and strength to work cement mortars without it being heavily loaded with plastisicer. This even applies to strong composition mortar gauged with some of the Australian powdered (hydrated) lime.
The same problems arise on what should be safe bases, such as clay internal bricks. It is doubtful that modern plasterers would be able to get cement mortar to bond to sand/cement scratch coats, or finishing coats to bond to render coats because of the overdosing of plastisicer and the inherent problems of cement mortar.
Lime mortar as a protective cladding over straw bales has the additional advantage that it is vermin resistant and a disinfectant. Vermin dislike boring holes through it because lime burns eyes and skin. Although vermin may take advantage of damaged areas made by plumbers or carpenters and make a home inside, they would not be able to live off the straw. Lime has been used as a disinfectant for centuries, usually as a whitewash.
There are least two high quality limes on the building trade market. One from Western Australia has the third highest lime content in the world. The other important one is from the Arab countries which has a higher lime content than that of the Perth metropolitan area.
Lime mortars have been used on almost all the strawbale houses and structures in Western Australia and have proved very successful. This mortar has performed as expected, and in accordance with the teachings of the journeyman Plasterers. Lime mortars have enough plasticity for the home plasterer to work and apply, especially after training at a strawbale workshop or TAFE.
The durability of lime is evident externally on almost all the heritage buildings, as demonstrated by the decorative fronts of the buildings in Fremantle, country towns, and the few left in Perth, as well as Government house and Treasury Building. These mouldings are still in good condition and most are aroud 100 years old.
ABOUT THE AUTHOR
This report was compiled by Francis E. Warren to assist those who have concerns about the Plastering of strawbale walls in load and non-load bearing structures, using the ancient and long lived art of Lime Mortar.
At present he holds the position of fulltime Lecturer in Plastering at West Coast College of TAFE, Balga campus. His teaching career commenced in 1964 lecturing on Architectural draughting, Architecture, Solid Plastering, Fiberous plastering, Architectural modelling, Granolithic and Concrete finishing.
*Certificate and a Diploma in teaching *Registered builder (by examination) *Five year apprenticeship with the P.W.D. (Architectural Division)
On completion of the apprenticeship he held the position of leading hand with the P.W.D.A.D. in conjunction with lecturing.
As a leading hand with the P.W.D.A.D. he was involved in a variety of work including new schools and hospitals, as well as the repair of Government, heritage and other buildings with original materials and finishes.