Straw Bales Construction

There are many types of straw buildings that use a combination of ideas from the above techniques or use new ideas. It is still an experimental method, and being so simple, allows for invention during practice. Here the bales are used much more like conventional brick walls, with cement mortar holding them all together. The bales are stacked in vertical columns so the cement, in effect, forms posts between each stack. The whole building is cement rendered inside and out. It is rarely used now because of the knowledge of simpler methods.

Bales should be dry, well compacted with tight strings, be of uniform size, and contain virtually no seed heads. Bales must not be damp and must be protected from dampness during the building process. Moisture content should not exceed 15% (wet weight basis). Bales should be roughly twice as long as they are wide, and the larger the better. Strings must be very tight so that it is difficult to get fingers underneath.

Strawbale houses are simple, inexpensive, and are made from sustainable eco-friendly materials. Strawbales are placed around each other to form a wall. Then they are plastered using earthen plasters. The walls of strawbale houses are quite thick which is responsible for their excellent thermal insulation. The step-by-step guide to build your own strawbale house is discussed below. Following these steps carefully, you can make your own strawbale house with your own barehand with the help of a few friends or you can hire professionals to do it for you.

Clean the site. There should be no grass, plants on the earth. Also, when you make adobe bricks, don’t use the topsoil material as it contains a lot of impurities. The mixture should be free from glass plastic large stones, small pieces of wood, roots, etc. Also, compress the site if necessary to avoid settlement in the future. Compressing on loose soil is mandatory Once the site is cleaned the next step is to make a layout of your building. Layout simply means transferring the walls from plan to site. Take a limestone powder and place it where your walls are going to be on the ground.

All buildings need to have some sort of a foundation on which to build. This will always be a natural foundation, i.e., the earth beneath you, which can be bedrock, firm clay, compacted gravel, etc., and may also include an artificial foundation such as the widened base of a wall. As the foundation has to carry the weight of the walls above it, and other loadings such as floors, furniture, and roofs, it is important to know what type of earth (or subsoil) is found on your building site. Different types of the earth will carry different weights. Bedrock, for instance, will carry much greater weight than soft clay. When the foundation is built up above ground level, it not only provides drainage for the wall but also protects rising dampness. Like most natural earthen buildings, we can use rubble trench foundations or concrete foundations under the straw bale house. A stem wall made of stones of height 2ft or 3ft is required. In the case of straw bale houses, earthbags filled with sands can also be used in foundations.

This type of foundation consists of making a trench of 2ft by 2ft beneath the walls of your structures and then adding gravel in 3 layers of 8”. Each layer is compacted before placing of next layer. If water the slope of the trench is an important parameter in this foundation. The Foundation trench is made on such a slope that water should flow out of the building. When you had built a trench, take a bucket of water and test your trench.

Earthbag foundations offer many advantages over reinforced concrete foundations and work well with many types of sustainable buildings. In particular, they are low-cost, fast, and easy to build, require no cement, and require no forms or expensive equipment. Earthbags are simply polypropylene sandbags (rice or feed bags, for example) filled with soil, sand, or aggregate obtained from or near the building site. Unlike adobe or rammed earth, which require a rather specific mix, almost any type of soil or aggregate will work (expansive clay soils are not recommended for foundations). Aggregates are preferred for foundations because they will readily drain away any moisture and prevent wicking into the wall system. Designed to control flooding and resist explosives, earthbags are amazingly strong, durable, and versatile as long as they are protected from direct sunlight. Keep earthbags tarped until they are plastered.

Depending on the type of structure, the walls can be loadbearing or not. The load-carrying capacity of strawbales is comparable to cob walls. The walls of strawbale houses should not touch the ground level. Moisture can affect the strawbale very much. That’s because plastering of strawbale walls is very important. The plastering of strawbale walls should be done in two layers of 1.5” thickness. The walls of strawbale can be loadbearing or not, the details of both are discussed in later sections.

• Load Bearing Straw Bale walls:
  The straw bale walls are stacked up on top of the foundation to the roof height, some kind of a header (usually wood) is placed on top of the straw bales with attachments of one sort or another to the foundation, and then the roof is put on top of that header. One has to be careful to not put too many doors or windows into the wall, or there won’t be enough strength in the straw bales to hold up the roof. One needs to brace the walls as they go up, otherwise, there’ll be nothing preventing them from falling over until the top plate or header is put on. For larger loadbearing buildings, it can be helpful to use temporary corner braces, to provide a guide to keep corners vertical. Structural doorframes are fixed securely to the foundations or baseplate before the straw is laid. Window boxes are built into the walls as they go up and pinned through the base and sides with haze. The first course of bales must be placed slowly and carefully as these provide the template from which the walls will emerge. Bales go together like a giant. Work from fixed points into the center of each wall; place the corner bales first, and those beside any framing posts. Bales may need to be handpicked to ensure a snug, not over-tight fit. The straw bale walls are raised and exterior bamboo pins are installed. Temporary braces are used to plumb the walls. The bales are stacked on top of each other. Before putting it on walls, make sure to compact it. Bales are placed till roof height and are strapped with coppiced hazel. A header made up of wood is placed on top of the walls on which the roof is installed. The header will directly transfer a load of the roof to the bales wall.
• Non-Load Bearing Straw Bale walls:
  The frame is built, and with wood which will take the load of the roof. The strawbales will be fitted in-between the frame, so all they do is hold up their weight. The cost of this type of house is relatively high as compared to loadbearing. This type of structure allows us to use many doors and windows as compared to load-bearing walls in which we can have limited doors and windows.

Place the wooden door and window frames where you want to have the windows and doors and then build your strawbale walls around them. This is as simple as that. For placing the ventilator, always try to place the ventilator at the top of the wall. In this way, they will not need any support and no frame will be required. The roof will act as a lintel to them.

The design of the roof for a strawbale house is not unusual nor particularly different from that for any other building. The main consideration for loadbearing and compressed frame design is that the loading is spread as evenly as possible around the perimeter walls, and this must be remembered whilst the roof is being covered in. Truss rafters should be spread across the walls, not stored at one end of the building before fixing. As the roof is loaded up with slate, tile, etc., these should also be distributed evenly and not loaded in one spot, nor should half the roof be slated before the other half. Straw houses need a good hat to protect them from the weather. A large overhang is a feature of straw bale buildings, especially in rainy climates. Just as traditional thatched houses have a roof overhang of about 500mm (20″), so too do straw ones. This gives really good protection to the top of the walls against the rain.

Straw needs good ventilation around it to stay healthy. In practical terms, this means that anything used to weatherproof or decorate the straw must not block this breathable nature. The ideal finishes for straw are traditional lime-based plasters or natural clay plasters since these are also breathable materials, painted with lime-wash or breathable paints. The internal and external faces of the straw walls should be given a very short haircut – trimmed down to a neat finish. All the long, hairy, unkempt bits of straw should be removed. A long-lasting plaster ensures a long-lasting straw bale house. Two types of plaster can be used to plaster and finish the straw bale house. Lime-based plaster and clay-based plaster.

• Lime Plaster:
  Lime-based plasters are made by a mixture of lime(quicklime) and sand. They are mixed in the proportion of 1:3 (1 part lime and 3 parts sand). Lime-based plasters are the most used choice for an exterior plaster system on straw bale homes. The fantastic mixture of breathability and durability makes lime-based plaster a predictable, low-maintenance finish. A plaster pump to apply 2 to 3 coats of lime and sand plaster to the prepared straw walls. A final 3rd or 4th coat is often applied by hand after windows and doors have been installed. When using lime-based plaster on straw bale walls it is best to do all the coats out of lime.
• Clay Plaster:
  The systems used with Clay-based plasters are as varied and important as the systems used with lime-based plasters. The most important detail is not to use metal mesh or wire with clay-based plasters. The reasoning behind this is that the clay allows moisture to move through it so easily that the metal may rust and destroy the plaster. For this reason, fiberglass mesh, plastic mesh, or burlap is used in the place of metal. The two main clay plaster systems used on straw bale houses are sand-clay plaster and straw-clay plaster. When installing sand-clay plaster a plaster pump can be used to apply 2 to 3 coats of plaster. Then a fiberglass or plastic mesh is troweled into the still-wet plaster for reinforcement. A topcoat is normally done by hand after the doors and windows are installed. Firstly, the timbers and metal mesh with plastic or fiberglass and the mix is applied by hand and smoothed out with wooden floats. As with the sand-clay example, a topcoat is normally applied by hand and may or may not have straw in it.

Nobody wants to lay their feet on the uneven surface. The floor of the unfinished strawbale house is just as same as the floor outside the building. So, we need to provide some kind of finishing to give it a proper surface. The ground is compacted a little bit and then plaster is applied on the ground. This plaster is done with a mixture of fine clay and water. After plastering, the surface is coated with linseed oil.