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, 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 construction 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.
Construction of pre-manufactured walls are made in major sections of eight foot lengths, and minor sections, in divisions of one, two or four feet. Corner units are constructed to allow for varying exterior wall thickness. Wall heights include above floor thickness to maintain interior ceiling heights of seven foot six inches to nine feet.
Foundation
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 that three and a half feet. Any more than this and the structural integrity of the wall will be compromised.
Foundation walls must also be constructed with at least two by eight inch plates to prevent lateral plate failure caused by earth pressures. The bottom plate is of pressure treated material which is about one quarter of an inch smaller than non treated construction lumber. Because of this, the interior structural two by three wall assembly must overhang the plate by approximately one quarter of an inch. Exterior wall framing can be made with economy or number three graded lumber so long as it is pressure treated to resist rot. The horizontal economy grade stabilizer and remaining stud grade plates are non-treated lumber.
The interior framed two by three wall is constructed first. It is constructed with regular framing nails with the studding at sixteen inch centers. The wall section is then squared off and secured to prevent movement. Over this, a polyethylene air/vapor barrier is placed, with the edges folded over the outer wall members, and stapled along the outside faces. Do not staple the air/vapor barrier to the edges of the plates or the studs, this will only punch irreparable holes in it. Corner sections require that the air/vapor retarder be wrapped around the interior walls corner studding. At this connection it is necessary to ensure that enough of the air/vapor barrier is left so that it will provide a surface to seal to at the interconnecting sections. Corners should be neatly folded to prevent bunching which would create air passages and prevent proper joining of the sections.
Over this, a horizontal stabilizer is laid down perpendicular to the wall studding, and fastened with screws to each individual stud. The stabilizer should be placed forty-five inches above the sole plate for full batt insulation placement. The composite wall top plate can then be fastened to the interior wall top plate, flush and true, with normal ardox framing nails.
Individual pressure treated exterior wall studs can then be nailed to the to top plate at sixteen inches on center. The pressure treated sill plate is fastened, first to the exterior studding, then the sole plate of the interior wall assembly, the two end studs, then all exterior wall studs. These studs should be lined up straight, and fastened to the horizontal stabilizer with wood screws. Structural wind bracing (when required) is placed in the wall section, by being let into the exterior wall studs.
Batt insulation is laid in the middle cavity, perpendicular to the studs with approximately one half-inch overhang at the ends of the wall assembly. Batt insulation can now be placed within the exterior stud cavities. Depending on the covering of exterior, foam insulation and or sealed polyethylene to grade is applied. If foam insulation is used, the joints are sealed with vapor tape or acoustical sealant to the wood wall components as well as each other. It is a good idea to install the walls vapor trap along with the exterior polyethylene, as a single sheet, which can be sealed to the interior vapor barrier between the two sole plates. A moisture barrier is then placed over the wall assembly. It is secured by folding the edges over the studs and plates, and fastening with staples.
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.
Foundation vapor traps and groundsheets should be sealed by sandwiching them between the sill plate and the top of the foundation with a combination of sill gaskets and acoustical sealant. The moisture barriers are sealed with vapor tape at each joint. Anchor bolts should be left loose to allow for shimming or grouting if necessary.
The exterior walls are fastened together with wood screws, every two feet along the end studs of the interior wall. If required, metal tie straps are installed on the inside edge of the top plate. The walls can then be leveled and squared before proceeding with floor or roof construction.
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.
One way these walls can be installed with the windows and siding is by overhanging the wall sections. Construction of the interior bearing walls, vapor barrier, stabilizer, plates and exterior wall studding in this approach is similar to the foundation wall assembly. Where they differ is in the edging of the units.
Edging is the bottom cover, which hides and protects the overhanging framed members and insulative barrier. It can be decorative hardboard nailed to the underside of the sole plate which is sandwiched between the plate and the foundation or floor assemblies. This edging should be sealed to both the first floor and foundation wall or to the floor assembly vapor barriers.
This overhang method is ideal when combined with a face brick applied around the first floors exterior. The overhang allows for installation of bearing, platform framed floor assemblies while providing sufficient second story overhang for the brick face.
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.
The window or door is first squared off in its frame, and held in place by cross corner strapping. A half-inch plywood or aspenite frame is built up around the window. The aspenite frame width should be equal to the entire thickness of the wall assembly. This frame is then sealed to the window by sandwiching a sill gasket between it and the surrounding wood members, which is secured with wood screws inserted through the window frame. This allows removal for future repair or replacement of a window or door. Leave approximately two inches of the gasket extending beyond the interior edge. The resulting box allows fastening of the interior finish directly to its surface, without gaps or unsupported areas.
The framed walls and header should be attached with screws directly to the boxed window, with sealant applied between the box and the individual vapor barriers. After squaring up the completed section, bracing is let into the studs. This bracing is important to the operation of windows or doors, because out of square or twisted framing will cause binding. For this reason, casement windows and wood doors should be used to allow post construction adjustment. Once the bracing has been secured, exterior sheathing with the moisture barriers are installed together, and exterior finishes applied.
Interior Floors
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.
Floor construction, through wall overhang or wider lower wall sections, allows the carpenters to construct the floor assemblies as current accepted practices, with one exception. The header or trimmer joist may have to be placed inside the edge of the top plate of the lower wall. The main concern with this type of construction is that when installation of the upper-framed wall is attempted, there is a likely hood of dropping the lower wall off the platform and foundation wall when raising it. Simple blocking or tiebacks under the floor will prevent such a problem and ensure site safety.
Floor construction with ledger strips or ribbon boards permit finished floor levels to meet with the top plate of the lower wall assembly. The bearing ledgers or ribbon boards should be installed as per local building code requirements. Floor sheathing must overlap the wall joists to ensure a proper firebreak between floor levels. Although time consuming, header and trimmer joists can be omitted because end restraint is achieved by lamination to the lower wall studs.
Variations of the interior floor support concepts can include second plates, joist hangers or tie straps. Most importantly, design the connections for your tradesmen abilities. Framed floors are easiest to construct and install because the techniques and design criteria already exist. Only qualified personnel with the proper experience should attempt other more complex designs.
Roofs
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.
Raised heel trusses or plated ceiling joists will raise the roof to allow for eave insulation and venting. The attic vapor barrier should consist of 10-millimeter polyethylene. This thicker and more durable layer is less likely to be damaged during construction, or when running fish tapes for future utility lines. The vapor barrier is installed parallel to the truss members or ceiling joists. Wall edges are sandwiched with acoustical sealant between the top wall plate and two by two strapping. Joints landing on truss members or ceiling joists should be sealed between it and a piece of two by two strapping.
Over the ceiling air/vapor barrier, two by two strapping is screw nailed to the underside of the ceiling support members. Areas around penetrating utilities such as stack vents and furnace exhausts should be made with plywood sealed directly to the air/vapor barrier. Holes drilled or cut for the utilities must be carefully sealed to the plywood with expanding foam or other acceptable materials such as acoustical sealant.
