How do I build with strawbales
There are a number of different methods of building with strawbales.
Chosing a building method will influence the building design, so take a look at the options here.
Nebraska (Loadbearing) • Lightweight compressive frame • Infill and timber frame • Hybrid design
Nebraska (also called Loadbearing)
This is the original method of building, pioneered by the Nebraskan settlers in the USA. In this method, the bales themselves take the weight of the roof; there is no other structural framework. They are placed together like giant building blocks, pinned to the foundations and to each other with coppiced hazel, with a timber wall or roof plate on top, which spreads the floor and roof loads across the width of the wall. The roof plate is fastened to the foundations and the bales with coppiced* hazel and strapping, and the roof is constructed in the usual manner on top of the roof plate.
Windows and doors are placed inside structural box frames, which are pinned into the bales as the walls go up, or they are fixed into 100 x 100mm uprights attached to the base plate and fed through slots in the wall plate. This is the simplest method and the most fun way of building; it requires little previous knowledge of wall construction and is very accessible. The majority of straw buildings in the UK are constructed using this method. Owner builders tend to prefer it because of its simplicity, ease of design, minimal use of timber, and the opportunity it affords for a modern day wall raising. The potential for empowerment through working together on a shared project is one of the main differences between this type of building and any other.
Advantages:
- A simple, straightforward and accessible building method
- Easy for non-professionals to design, following readily comprehensible basic principles.
- Designs from one room to two-storey homes can be created using a simple, step by step approach.
- Curves and circles are easy to achieve, for little extra cost.
- Ideal for self-builders because of its simplicity, accessibility, ease of design, and low cost.
- The straw is very forgiving. Total accuracy in plumb is not a design goal but wilder variations can be brought back into shape easily!
- Great versatility of design shape.
- It's fast!
Disadvantages:
- The straw must be kept dry throughout the whole building process until it is plastered, which can be very difficult on a large building, or one that is being constructed slowly.
- Openings for windows and doors should not exceed 50% of the wall surface area in any wall (but hybrid methods can be used in these cases).
The Nebraska style is the most common method of building to be found in Ireland and the UK. However, for larger buildings, or where it may be difficult to keep the straw dry for long periods, the following method is also appropriate.
Lightweight Compressive Frame
This design is a way to retain some of the benefits of the loadbearing style, yet enabling the roof to be constructed before the straw walls are built, thus giving protection against the weather throughout the wall-raising process. It uses a timber framework which is so lightweight that it could not stand up alone, and which requires temporary bracing and props to give it stability until the straw is in place. The straw is an essential part of the structural integrity of the building, more so than the timber, and it works together with the timber to carry the load of floors and roof. Timber posts are located at corners and either side of window and door openings, and are designed such that the timber wallplate at first floor and/or roof level can be slotted down into the posts, or screwed down with threaded rod, once the straw is in place, allowing for compression on the bales. Compression of the straw bale infill walls is essential for stability. To increase stability, the bales can be pinned externally, and the pins are secured onto the base and wall plate of the framework once all the settlement of the walls is complete. It is constructed in such a way that the wallplate and roof are held approximately 100mm above the finished straw wall height whilst the wall is being built. Once the bracing and props are removed, the roof is allowed to come down, which causes compression of the straw beneath it. The roof weight can be encouraged to compress the walls faster by strapping it down to the foundations and mechanically compressing the walls using ratchet straps.
Advantages:
- The roof can be constructed before the straw is placed, giving better weather protection.
- Framework and posts can be constructed off site.
- Provides good stability for window and door frames.
- Vastly reduces the amount of timber required compared to the more traditional post and beam method.
- With good compression, external pins can be avoided.
Disadvantages:
- It is more complicated and costly than the Nebraskan style to construct.
- Greater technical ability is required to make the structure stable whilst the straw is being placed.
Infill (also called Post and Beam or Timber Frame)
In this method, the weight of the roof is carried by a wood, steel, or concrete framework; the bales are simply infill insulation blocks between the posts. This has often been the preferred option for architects, as the structural concepts are not innovative and rely on an already established method of building, therefore the risk associated with an experimental technique is minimized. There is no need to satisfy oneself of the capacity of the bales to take the roof weight, since the framework does this. This method requires a high level of carpentry skill and uses substantially more timber than the two previous methods, so has significant cost and environmental implications.
Advantages:
- The roof can be constructed before the straw is placed, giving better weather protection.
- Framework and posts can be constructed off site.
- Provides good stability for window frames.
- In conjunction with a steel frame, can create large open spaces and multi-storey blocks such as warehouses, car parks or industrial units.
Disadvantages:
- It is more complicated and costly than the Nebraskan style to construct.
- It requires a high level of carpentry skill (or metalwork experience in the case of a steel frame) to construct the frames.
- It uses a large amount of timber.
Hybrid Methods
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. For instance, it's possible to build well-insulated loadbearing walls to protect your house on the cold North side and combine this with a framework method on the South, allowing for lots of windows to maximize solar gain.
This type of hybrid design was used for the 2-storey loadbearing house in Wales. Here we had a combination of rigid timber and glass framework next to flexible loadbearing straw, so to work these two dissimilar methods of construction together we designed the framework so that it could be lowered to suit the straw. First an estimate had to be made of how much the straw would settle, then the tenons of the posts were cut long to accommodate this settlement, and the top beam was held high by using timber folding wedges sitting on the shoulders of the wedges. (This could also have been done using acrow props to hold the beam). The beam ends sat on the straw and were connected into the timber wall plate on top of the straw walls. As the straw walls compressed under the weight of the floors and roof, the beam was lowered down onto the shoulders of the tenons by removing the folding wedges little by little. After all settlement had finished the beam was fixed permanently onto the posts, any gaps between it and the shoulders being filled with timber.
Other Aspects of Straw Bale Building
The methods described above are for a type of wall building system that is different to the methods we have become familiar with in the 20th century. All other aspects of the building remain the same, including plumbing, electrics, roofing etc. The main differences, as mentioned above, would be found in the design of foundations, type of wall building material, and type of render or plaster.
Straw, being a breathable material, functions best when used with other such materials. Therefore, it is common to design foundations without using cement, or where cement is used, to protect the straw from it by using a different material in between, usually timber plus a damp proof course, and to incorporate drainage into the foundation itself. In the same manner, cement renders and gypsum plasters would not be used, but instead, traditional lime, and/or natural clay renders and plasters would be applied. Most straw bale houses, of whatever type of construction, are rendered inside and out, so that when finished they can look very similar to a traditional style cottage, very beautiful and with deep walls-it is hard to tell that they are made of straw. Several coats of lime-wash are essential as a surface finish and weatherproofer, and this must be re-applied, as with all other painted houses, every few years.
Durability
Because of its simplicity, it is possible to build a wide range of different quality structures, from a straw bale shed to last 10 years, to a straw bale house to last upwards of 100. Straw bale building is still a relatively new concept, and as such some areas of design are still experimental. In the UK, the oldest buildings have stood for only 10 years, and some of the early ones were never intended to be more than experiments. However, there are now under construction homes for families, two-storey loadbearing houses, urban bungalows, classrooms and schools, centres for community groups as well as numerous owner-built houses, offices, studios and garages, animal shelters, food and machinery storage barns, etc.
No straw bale building in the UK or Ireland has ever been refused planning permission or building regulation approval on the grounds of it being made of straw, or on the question of durability.
Design Pointers
- Raise the first course of bales up from the ground by at least 225mm (9"), put a 450mm (18") overhang on the roof to protect the walls from rain, and you won't go far wrong. What cob builders call ‘a good hat and a good pair of boots'!
- One of the most important design features of a loadbearing straw bale house is to distribute the loads as evenly as possible around the whole building. Never use point loads.
- Loadbearing houses are subject to settlement as the straw compresses under the weight of the floors and roof. Allowance for this must be designed in by leaving settlement gaps above doors and windows.
- Avoid using metal in the walls if at all possible, since it is a cold material and may encourage warm, moisture-laden air from the inside of the house to condense on it.
- In particular do not wrap the walls in chicken wire or metal lathing if you are using lime or clay plasters, as it can affect their long-term durability.
- Where possible use like materials with like e.g. straw goes well with clay and lime because they are all permeable materials. It does not go well with cement, plastic or metal, all of which can induce damp into the walls.
- The key to durability with a straw bale house, as with any other, lies in good design and detailing, quality work, & maintenance when necessary throughout its life.