Loadbearing or infill?
Loadbearing, also called “Nebraska Style” because of the historical precedents, describes construction in which the weight of roofs and floors above the ground are supported, partly or entirely, by the strawbale walls. Loadbearing structures are easier to build because the skilled work of framing is replaced by the simpler task of stacking bales.
The easier and quicker system of building loadbearing walls will save costs by reducing timber (or other framing material) needs as well as reducing the need for skilled labour. However, the loadbearing system does have some disadvantages and faces more difficulties from building officials than other systems.
Firstly, the straw bale walls are raised before the roof is completed, leaving the bales unprotected from storm and rain. Loadbearing walls need time to settle under the weight of the roof before they can be rendered. It is best to wait between four and six weeks for the walls to settle; unfortunately this waiting period leaves the straw bales unprotected from rain and storm again and it is not always practical to have to wait before the project can progress. It is possible and recommended to pre-compress the walls however, it is still necessary and important to observe a settling period. The top or roof plate must be a very strong and continuous plate, especially over door and window openings, in order to be able to transfer roof loads to the bale walls without bending or buckling. Because of the loadbearing role of the bale walls, openings in these structures are commonly fewer and/or smaller than in infill structures. It is also more difficult to build very high (more than eight courses) or even two story structures with the loadbearing method and walls cannot be too long or too wide apart without a supporting cross wall structure. Often temporary bracing has to be installed to stabilise not yet rendered bale walls. Last, loadbearing wall systems are not always appropriate, especially in regions that experience heavy snow loads, seasonal rains and high winds.
Wall raising for a loadbearing structure with temporary corner braces in place
Today, most strawbale buildings in Australia are infill structures, mainly because building inspectors, architects and builders are still unfamiliar with the loadbearing concept and retain some resistance to it.
Non-loadbearing, also called post and beam or infill, describes construction in which no weight of roofs or floors above the ground are supported by the strawbale walls. Instead, strawbales are used as infill material between or around a structural frame, which can be timber, steel, concrete, masonry, LVL (structural plywood) or any combination of these. This construction method has become predominant, as most builders, architects, building inspectors, lenders and insurers are familiar and comfortable with its application. Again, this method has advantages and disadvantages over others.
Infill construction has usually a better chance of council approval. The design is very flexible and easily adaptable to conventional systems. Large openings for doors and windows to gain from passive solar energy do not pose a structural problem. There is even compression on all walls even the ones with little or no bales. It is easier to replace sections of a bale wall if moisture or other problems with the bales were experienced. Completion of the roof provides dry storage and protection for the bales while not rendered, as well as for the balers at wall raising time. Last, there is no settling period and work on the building can continue without delay.
Wall raising for a post and beam structure; steel posts are wrapped in Hessian
Disadvantages of the infill building method include the possible greater expenses for framing material and frame construction time, extra footings for posts and increased need of skilled labour.
The potential for volunteer labour and owner-builder involvement may be lower with this more complex construction system.
Combinations of the two building systems are usually called hybrids; they have the potential of taking advantage of the best aspects of both these systems however, it needs to be considered that due to the different structural systems compression of the walls will be different. As all systems have their appealing and flawed points it is left to you to make a decision of what system best meets your needs, budget and skills.