Open wall construction is not limited to a single technique. The theories and principles behind the construction are unique, but the design uses existing methods and materials to allow for complete flexibility.
Methods discussed later describe only one of a number of design models created, and provides examples of the applications, which should be modified to meet local codes and personal preferences.
Wrap and Strap Open Wall Construction
This method is ideal for tradesmen because its application is simple, and the buildings thermal envelope can easily be maintained. It consists of a primary shell, which is the buildings structural skeleton, being constructed with standard platform framing techniques. The entire building is then wrapped in polyethylene, and then a second non-structural skin is attached to the exterior, which is insulated and finished. This type of open wall construction is used to retro-fit existing two by four constructions into super-insulated, air tight homes.
Platform Open Wall Construction
Platform construction methods require that the framing carpenters perform not only the structural framing, but install the windows, doors, insulation, air/vapor barrier, exterior moisture barrier and sometimes exterior finishes when used in paneled wall sections.
This method has been utilized in the pre-manufacture of exterior walls in sections which are handled by four workers easily. It is very effective for consumers, not familiar with structural design, but who wish to erect their own homes. It is also a much quicker method because the building can be factory built elsewhere, shipped, and erected with the vapor barrier, insulation, electrical, interior finishes, windows, doors and siding already installed. A unique ability of this method is the capability of the structure to be disconnected and moved in sections. This would have ideal applications for modification and additions, or to perform panel repair/replacement.
When building the foundation wall assemblies, it is important to use proper materials and consider the use of a raised, compacted sub-footing. Whether using a granular sub-footing, a full height concrete footing or a footing and sub-foundation to elevate the wall assembly, the foundation panels cannot be backfilled more than three and a half feet. Any more than this and the structural integrity of the wall will be compromised.
Finishing the wall by installation of the pressure treated plywood foundation cover and/or siding is then done. The plywood foundation paneling must be installed with the joints staggered against the underlying foam board. These panels are fastened with exterior screws, spaced eight inches around the edges, and ten inches within the interior area.
First Floor Walls
In design, it is important that if you do not use tie straps to hold the sections together, then the upper and lower sections should have overlapping plates to prevent separation. Also, depending on the particular design, the floor may have to be installed before continuing. It is recommended to install floors before continuing for this gives the carpenters a level area to construct the upper walls on and supports the exterior foundation.
Windows and Doors (exterior openings)
Probably the trickiest area to seal against infiltration and exfiltration. Although the area around the opening could be sealed with expanding foam, by pre-installing the windows in frames with a perimeter gasket, less work and material is required.
Because open wall construction relies on continuous wall insulation between stories, the interior floors in this method must be designed to remain within the building envelope. Utilizing overhanging walls, wider lower wall sections or ledger strips, the interior floors can be supported on sufficient bearing. Floor assemblies can be joists, truss floors, wood I beams or any other type of framing material.
Open wall construction was designed for use with truss components, but by strapping over ceiling joists, the same results can be obtained. For effective design, overhangs should be completely eliminated. They are costly and require extensive labor to complete. Roof edges should have angled fascia (called knife edging) to allow enough overhang to prevent excess run off from running down walls. Simple “two by” fascia, lapped over wall siding can be trimmed or painted in many colors and still provide adequate visual separation from the roof. By lapping the fascia over the siding gravity water cannot enter into the buildings wall cavities. But remember to leave enough screened eave area, to allow for adequate attic ventilation. Gables should also be completed with knife edging and reduced overhang. Sheathing and roofing materials are applied as per current building techniques.
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