Hybrid Systems

Straw bale builders are not limited to an either-or choice between load-bearing and post and beam. Many successful hybrids have been constructed that take advantage of the best aspects of both; many others await creative invention. Rammed earth pillars, stone, lumber, concrete, and earth berms support walls — all can work well in conjunction with straw.

The most important consideration in any hybrid design is the calculation of the finished height of the load-bearing straw walls. Exact finished height is dependent on bale size (it is easier to cut lumber to meet even courses of bales than it is to cut bales to meet post heights!) and the amount of pre-compression applied. It isn't difficult to make a very close guess as to the fn-ished height of a load-bearing wall, but reaching an exact height will take careful precompres-sion and calculation of bale heights, as well as the thickness of curb rails and top plates.

Hybrids created from a central frame with load-bearing walls use a simple structure of lumber, rammed earth, stone, brick, block, or concrete with perimeter walls of precompressed straw bales. The central frame is erected first,followed by construction of the bale walls. Since the roof framing will extend down from the central frame to the bale walls, compression of the walls will only change the roof pitch minimally, and bales need not be the same height as the solid material. The bales that are used to infill the space between the roof plate and the angled roof framing are not load-bearing, since the roof framing is bearing the full loads.

Jack posts are adjustable steel posts. They can be used in load-bearing designs to provide support where load-bearing walls of a material other than straw will be employed. The threaded end of a jack post can be adjusted to meet the height of the compressed straw wall.

Earth berm homes — sometimes called earthships — are dug into hillsides.While straw bales wouldn't be appropriate to use against the earth berm due to moisture concerns, they could be used to build walls that are not buried, in either a load-bearing or post and beam design.

Mortared bale structures use straw bales just like concrete blocks. If you have plenty of experience with mortar and blocks,you may find this option suits your talents. Bales are set down on a thin bed of mortar, and more mortar is poured between the abutting ends. This technique can create a strong structure and does not require a framework or any pre-compression. It does require a lot of time and mortar. It also creates thermal breaks between each bale, causing the insulation rating of the entire wall to suffer because the spaces between the bales offer only the minimal R-value of concrete. Building with mortared bales eliminates one of the main advantages of straw walls — walls do not go up quickly and simply.

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