An Investigation into Straw as a Natural Green Building Material
written by: Willie Scott•edited by: Laurie Patsalides•updated: 1/25/2011
We previously looked at wood to be the best natural green building material, now we will examine the use of straw as a green building material.
Straw has great insulating properties, however the main problem will be resourcing rectangular bales, because, circular bales are favored by most farmers.
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Building with Straw Bales
In this second article in this series we shall look at straw as being the best natural green material for building.
Straw is the waste product from cereal crops such as wheat and barley. The farmers use it for spreading on the cowshed floor, plowing it back into the land or leave it to rot piled in the corner of the field.
This suits our plans to build a straw bale house as the bales will be cheap to purchase, although the transport cost could be high so it is better to purchase the bales as close to the building plot as possible.
Some years ago I was Project Engineer for a proposed straw burning heating system for a plant nursery here in the Highlands of Scotland. I had the opportunity to investigate the purchase and transport of the bales and designed the burner to accept the larger round bales, as these were the most popular available locally. I notice that there is still a prevalence of round bales in the fields, but we will go ahead on the premise of rectangular bales being available locally and that they are available use in building construction.
Remember these articles are not written as instructions on how to construct buildings from green materials. I have used general building techniques to illustrate how the two different materials are used in their construction.
We shall examine the suitability of straw bales as a green building material against the same criteria we used to access the suitability of wood logs in the preceding article as follows:
Insulation Qualities – Compressed straw bales have one of the best insulation qualities, just a bit better than wood logs, and well above the National Building Regs.
Aesthetic Qualities – Not as good looking as a wood log building, although if shaped nicely and rendered in a pastel shade, it beats the traditional brick building in looks.
Ease of Handling and Construction – Straw bales can be handled by sliding them over a plastic sheet laid on the ground. Lifting them can be awkward and a job for two people. Shaping with a chain saw is an art and can be dangerous. There is also an art to re threading and tying the holding cord or strings.
Durability in all Weathers – Provided a damp-roof membrane fitted between the bales and the foundations, the rising damp in the walls should not be a problem. The rendering will need water proofing and particular attention must be paid to roof and window sill overhangs that they do not allow constant dripping of water onto the walls.
This will be carried out in the summary at the end of the article, so now we go on to look at the construction techniques used to build dwellings from straw bales.
We covered the planning permission and choosing a suitable plot of land in the previous article on log buildings so we will start with the properties of straw bales.
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Straw is a waste product of farming and, as houses built from natural green materials are usually located in the countryside, they can be purchased locally.
When purchasing the bales, it is advisable to visit a couple of farms and inspect the bales, get there just after the harvest season in August or early September. Check to be sure that the bales are securely tied and more importantly, make sure they are recently baled and dry. Sometimes the farmer will store them undercover for you until you need them, or you may have to purchase them and transport them to your building site where it is imperative they are stored under cover until required.
Straw is an excellent insulator, with a U value above that required in the Building Regulations, thus preventing heat escape and the intrusion of sound, wind and cold.
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In this investigation of straw as a natural material to build a house with. We discuss the techniques of building a straw bail house. We look at properties of straw and compare it to logs and rate the use of building with straw.
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On page one of this article, we discussed the properties of straw. Let's look at some of the techniques of building a straw bail house.
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There are two trains of thoughts on foundations:
Concrete foundations used with a plastic membrane laid on top of them, prevents damp from rising up through the bales.
The bales are laid on top of the bare earth, anchored with steel wires into the ground.
I am in favor of the former as I firmly believe in a good foundation being fundamental to a safe and secure building no matter what the building material is, also steel upright rods can be set into the concrete to secure the first layer of bales. However the choice is with you-the person who is building the dwelling.
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Houses built from straw bales can be of two types:
Load bearing - in this type, the walls of straw bales are stacked on top of each other to support the weight of the roof.
Non-load bearing – a wooden frame is erected holding the stacked straw bales in the middle. The roof is then attached to the top of the frame.
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Walls, Roof & Floor Construction
We are building the non-load bearing type of walls, so the wooden frame will be fixed to the foundations. Remember to lay down a plastic membrane on top of the ground first. Once the frame is in position, the packing with straw bales on their side can commence.
On the top of the concrete foundations, steel rods will be incorporated into the foundation, allowing the bales to be skewered on them, and keeping them in position.
Start the row with a half bale, this can be cut from a full bale using a chain saw and retying with string. Fit a full bales next, right along to the end of the wall. This particular technique has been used in the building trade for years and is known as running-bond.
Lay the next row starting this time with a full bale, tamping down the bales and tying the rows of bales together with bamboo or wooden pins hammered through.
Bales can be sculptured with the chain saw to fit quite neatly round the door and window apertures. Any gaps can be filled with scrunched up straw and well tapped in, and then straw mixed with clay to seal the joints.
Finish of the walls by making sure the bales do not protrude beyond the wooden frame and nail chicken mesh onto the frame, inside and out assist the rendering which may be of cement base or lime base plaster, or ordinary clay, but there will be a weather coat required for whichever render is used.
Once this has dried, the doors and windows can be fitted. There are many recycling plants for building materials such as these items complete with frames and surrounds, so shop around rather than buy new – be environmentally friendly to your new natural green building.
This is similar to the wood log building
Once all the bales and wall frames have been secured with the wire mesh, the roof can be installed. Depending on available lifting, plant the gable ends. Rafters can be assembled on the ground, then fitted to the top of the frames and secured.
Roofing boards or sarking (again supplied rough-cut from the sawmill) are nailed to the rafters. Then, wood roof shingles are fitted over the boards using the same method as fitting roof slates.
Between the foundation, a simple damp-coat can be laid and the space above can be filled with aggregate and screened or more environmentally friendly filled with good quality garden soil, compacted. The final inch or so is then leveled with a mixture of clay and water and left to dry before final smoothing and sanding – the choice is yours.
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Summary and Verdict
Well folks, wood logs or straw bales - have you decided which natural green building material is the best? Let’s look again at the criteria and compare their qualities.
Insulation Qualities – both wood logs and compressed straw bales have excellent insulation qualities, but straw bales win this one.
Aesthetic Qualities – it is well known that folk love the aesthetic qualities of natural wood, whichever form it is in, therefore wood logs win this one.
Ease of Handling and Construction – both pretty easy to handle with an eye on lifting techniques, but in straw bale construction, the use of a chainsaw is dangerous. There is also an art to rendering the straw walls to achieve a good natural finish therefore wood logs win this one.
Durability in all Weathers – water proofing is the big problem here as the integrity of the building will be compromised if the bales or log are allowed to get damp and even more so for the bales. The structure and waterproofing of the logs and subsequent water repellent qualities means that wood logs win this one.
Although environmentally the straw is best, because it is a waste product and sustainable as it is available every year, and sustainable-grown trees for logs take many years to grow. Notwithstanding, on the criteria, wood is the best natural green building material. This is only my opinion – what do you think?
Here we look at different green construction materials and rate their durability, insulation capability, aesthetic qualities and more. We will take a closer look at using materials such as logs, straw and rammed earth, and adobe, as well as the use of recycled building materials.