Traditional timber framing uses wood-to-wood joinery between timbers, typically including the use of wooden pegs to fasten joints. Post-and-beam systems will use metal bracketry and fasteners to connect members. These metal fasteners can be standard, off-the-shelf materials or can be custom-welded to achieve particular structural and/or aesthetic goals. Straw bale walls have been built in North America for over 100 years, but since the late 1990s this type of construction has moved from a fringe alternative to recognition within the International Residential Code (US) in 2015. The term plastered straw bale walls is a more accurate term for this category, as a wet-applied plaster adhered directly to the straw bales is a common feature among all types of straw bale walls. The plaster provides structural capacity, water protection, air sealing, and fire protection and is an integral element of this wall system. The wide variety of options in straw bale construction requires you to research the best straw bale wall system to meet all your criteria. Cost, complexity and ecological impacts can vary greatly based on framing design, plaster materials, and specific detailing. Though straw bale walls are often attractive to owner-builders who appreciate the seeming simplicity of stacking large bales to make a wall, projects will definitely benefit from having team members with straw bale construction experience; “best practices” are not as firmly established Afn^ffiAxe TpOU WDATI OK/ as for more conventional building types, so prior experience is a great advantage. There are three categories of straw bale walls: Infill straw bale Rectangular straw bales are stacked within or adjacent to a structural framing system. Frames styles include: conventional stud framing (see above) with studs spaced so one bale fits in each stud cavity; Larsen posts; post-and-beam of many variations; and traditional timber frames. The use of some type of framing system is the most common approach for straw bale walls. In some cases, the frame is designed to handle all the structural loads, but it can be much more economical and simple to design a frame that works in conjunction with the structural qualities of the plastered straw. Load-bearing or Nebraska-style Rectangular straw bales are stacked in courses, often in running bond, to form the wall. A wooden top plate or beam at the top of the wall transfers loads into the plaster skins and provides attachment for the roof. Window and door openings are created using wooden frames. The framing details can vary widely. Load-bearing straw bale walls incorporate some form of pre-compression/tie-down system to connect the foundation to the roof plate to prevent uplift and to allow builders to settle and level the bale walls prior to plastering. Straw bale SIPs (structural insulated panels) Prefabricated straw bale walls are beginning to emerge, and they capitalize on the low material costs of straw bale walls while greatly reducing the labor required for plastering, as this can be performed with the panel lying horizontal. Panels can be built on site and tipped up into place, or fabricated off site and delivered/installed via boom truck or crane. Embodied carbon: Sequesters more carbon than is produced. Energy efficiency: Insulation value exceeds code requirements. Airtightness strategy is important to meet high targets. Indoor environment quality: Plaster materials and finishes will determine overall IEQ. Moisture problems may lead to mold. Building code compliance: Building code reference in Appendix S of International Residential Code. (2015) Alternative compliance required in other jurisdictions. Material costs: Full-system costing must include framing, mesh (if required), plaster materials and labor, cladding (if required), and finishes. A subsoil mix composed of clay, silt, sand, and aggregate (and sometimes reinforcing fiber) is mixed to produce a dense and strong material. There are two basic types of earthen walls: Rammed earth A suitable soil mix is lightly moistened and compressed forcefully to produce a dense and strong material. Traditionally tamped manually, much modern rammed earth is tamped with pneumatic machinery. Some type of formwork is used to contain the earth mix while it is being tamped, and the mix has an initial strength equal to the compressive force used for compaction and develops additional strength as the binder dries or cures. Rammed earth walls can be made in two ways: Unstabilized Using only naturally occurring soil ingredients. A quality unstabilized mix can have high compressive strength, but will be susceptible to degradation from exposure to water. Stabilized Using natural soil ingredients and a hydraulic cement binder. The proportion of hydraulic cement can range widely, in some cases equaling the proportion found in conventional concrete. Stabilized mixes tend to have higher strength characteristics and will be more resistant to water damage. Rammed earth can be produced in several formats, all of which can be unstabilized or stabilized: Formed rammed earth Ingredients are placed into temporary wooden forms in relatively small (4-6 inch) lifts and tamped in consecutive layers, creating a monolithic wall. Rebar or other reinforcement materials are often added to improve tensile strength and performance in seismic conditions. Compressed earth blocks and masonry units Ingredients are placed into a block form and manually or hydraulically compressed. The individual blocks are mortared together to create a wall. Earthbag, or flexible form rammed earth Ingredients are placed into a polypropylene bag or tube, and the fabric acts as a flexible form while the mixture is being tamped, either manually or mechanically. The bag or tube typically remains in place after construction, though it is usually not necessary after compression and curing. The bag can help to provide stability for mixes that are less than ideal, and protect the mixture from erosion. Earthships, or rammed earth tires Ingredients are placed into a used car or truck tire, and the tire acts as a form while the mixture is being tamped, either manually or mechanically. The tire remains in place as a permanent form, with tamped earth filling both the sidewalls and the open center of each tire. The tires can help to provide stability for mixes that are less than ideal, and protect the mixture from erosion. The indentations between consecutive tires are filled with rammed earth, mortar, or other materials. ‘ Sun-dried earth A suitable subsoil is wetted and mixed, most often using a natural fiber like straw, into a plastic state and is formed into a wall and allowed to sun dry to a hard state that has structural capacity. There are two main forms of sun-dried earthen walls: Adobe block (mudbrick) A soil with clay content of25-40% and a good distribution of sand and silt is moistened and thoroughly mixed with chopped straw or other natural fiber. The resulting mix is placed into rectangular block forms, released from the forms, and allowed to fully dry. Once dried, adobe blocks are laid up in a running bond similar to other masonry techniques. Mortar can be clay based, rather than using more conventional lime or cement mixes. Window and door openings use wooden, concrete, or steel lintels. At the top of the wall, a wooden, concrete, or steel beam system is used to provide rigidity and a fastening point for the roof.
Cob A soil with clay content of 10-40% and a good distribution of sand and silt is moistened and thoroughly mixed with chopped straw or other natural fiber. The resulting mix is hand-formed into monolithic walls that can bear the weight of floors/roofs. The top of the wall typically incorporates a ring beam, usually made of wood or concrete. Window and door openings are typically created using wooden frames, often built identically to openings in frame walls. Cob can be used as an infill wall with light wood frames, post-and-beam, or timber frames.
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Cob walls are not typically made with any sort of formwork, though it is possible to use forms for a more uniform wall surface.
Cordwood walls are formed by placing lengths of dry, rot-resistant softwood transversely across the wall and embedding the wood in a mortar (clay-, lime-, or cement-based) matrix.
There are two ways of building with cordwood: Through-wall Mortar is used at the inner and outer edges of the wall and thermal insulation is placed in the voids between wood and mortar. In this method, the cordwood is an integral part of the thermal performance as it bridges from inside to out. Double wall Two separate walls of cord-wood are built with a continuous insulation layer between them. Load-bearing walls use columns of cordwood wood and mortar at corners to provide stability. These are built first, and have tie-pieces that key into the walls. Many load-bearing cordwood structures are built round to take advantage of the inherent stability of round walls and avoid the need for corner supports. Infill cordwood walls use a skeletal frame (light wood frame, post-and-beam, timber frame) and the cordwood is built between the framing members. The frame provides stability and containment for the cordwood at the corners. Embodied carbon: Cellulose and wool JS. sequester more carbon than is produced. Fiberglass and mineral wool have high embodied carbon and will contribute significantly to the carbon footprint of your building. 0 m Energy efficiency: Good thermal properties. Loose-fill insulation fills cavities completely and prevents convective losses from gaps or voids. Thermal bridging will exist through framing members unless detailed to include insulated sheathing, double frames, or furring strips. Indoor environment quality: Dust and microfibers introduced during installation can be problematic. Cavities should be carefully sealed, and a thorough cleaning undertaken before occupancy. Material costs: Vary widely. Research locally available options. Labor: Contractors widely available. Blowing equipment available to owner-builders. Batt insulation How the system works Small fibers are bound together to form rectangular batts, sized to match conventional framing spacing (typically 12-, 16- or 24-inches). Batts are installed between framing members; friction against the wood keeps batts in place. Batts are cut to fit non-standard framing cavities and to conform to irregularities. Energy efficiency: Good thermal properties. Convective losses due to gaps and voids can reduce overall effectiveness. Thermal bridging will exist through framing members unless detailed to include insulated sheathing, double frames, or furring strips. Board insulation is produced in dimensions that correspond to typical framing dimensions, often 2×4 foot or 4×8 foot rectangles, of varying thicknesses (ranging from / to 4 inches). These sheets are fastened to the exterior of a wall or roof system. Some brands of board insulation come with tongue-and-groove edges to improve air sealing and thermal performance. Board insulation can be made from a variety of raw materials: Wood fiber board Waste wood fibers are bound together, often with a wax-based binder. Some brands may have a water-resistant coating on the panels. Cork board Shredded cork is agglomerated using naturally occurring suberin resin and formed into boards. Mineral wool board Mineral fibers spun while molten are glued into boards, often using formaldehyde-based binder. Fiberglass board Glass fibers spun while molten are glued into boards, often using formaldehyde-based binder. CRiTERiA CONSiDERATiONS Ecosystem impacts: Generally low ^ impacts for natural and/or recycled fibers. Higher impacts for virgin mineral materials. Embodied carbon: Natural fibers sequester more carbon than is produced. Fiberglass and mineral wool have high embodied carbon and will contribute significantly to the carbon footprint of your building. Energy efficiency: Good thermal properties. Typically used as exterior sheathing to prevent thermal bridging when used with cavity-fill insulation materials. Can be layered to provide additional thermal performance. Waste: Standard board sizing can require custom cutting and fitting and produce a large volume of offcuts that will need to be managed appropriately. Material costs: Vary widely. Research specific brands available locally.