What Is The Open WallConstruction Technology?Methods discussed later describe only one of a number of design models created, and providesexamples of the applications, which should be modified to meet local codes and personalpreferences.Wrap and Strap Open Wall ConstructionThis method is ideal for tradesmen because its application is simple, and the buildings thermalenvelope can easily be maintained. It consists of a primary shell, which is the buildings structuralskeleton, being constructed with standard platform framing techniques. The entire building isthen wrapped in polyethylene, then a second non-structural skin is attached to the exterior, whichis insulated and finished. This type of open wall construction is used to retro-fit existing two byfour construction into super-insulated, air tight homes.Platform Open Wall ConstructionPlatform construction methods require that the framing carpenters perform not only the structuralframing, but install the windows, doors, insulation, air/vapor barrier, exterior moisture barrierand sometimes exterior finishes when used in paneled wall sections.This method has been utilized in the pre-manufacture of exterior walls in sections which arehandled by four workers easily. It is very effective for consumers, not familiar with structuraldesign, but who wish to erect their own homes. It is also a much quicker method because thebuilding 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 thismethod is the capability of the structure to be disconnected and moved in sections. This wouldhave 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, andminor sections, in divisions of one, two or four feet. Corner units are constructed to allow forvarying exterior wall thickness. Wall heights include above floor thickness to maintain interiorceiling heights of seven foot six inches to nine feet.FoundationWhen building the foundation wall assemblies, it is important to use proper materials andconsider the use of a raised, compacted sub-footing. Whether using a granular sub footing, a fullheight concrete footing or a footing and sub foundation to elevate the wall assembly, thefoundation panels cannot be backfilled more that three and a half feet. Any more than this andthe structural integrity of the wall will be compromised.Foundation walls must also be constructed with at least two by eight inch plates to prevent lateralplate failure caused by earth pressures. The bottom plate is of pressure treated material which isabout one quarter of an inch smaller than non treated construction lumber. Because of this, theinterior structural two by three wall assembly must overhang the plate by approximately onequarter of an inch. Exterior wall framing can be made with economy or number three gradedlumber so long as it is pressure treated to resist rot. The horizontal economy grade stabilizer andremaining stud grade plates are non-treated lumber.The interior framed two by three wall is constructed first. It is constructed with regular framingnails with the studding at sixteen inch centers. The wall section is then squared off and secured toprevent movement. Over this, a polyethylene air/vapor barrier is placed, with the edges foldedover the outer wall members, and stapled along the outside faces. Do not staple the air/vaporbarrier to the edges of the plates or the studs, this will only punch irreparable holes in it. Cornersections 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 itwill provide a surface to seal to at the interconnecting sections. Corners should be neatly foldedto 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 fastenedwith screws to each individual stud. The stabilizer should be placed forty-five inches above thesole plate for full batt insulation placement. The composite wall top plate can then be fastened tothe 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 sixteeninches on center. The pressure treated sill plate is fastened, first to the exterior studding, then thesole plate of the interior wall assembly, the two end studs, then all exterior wall studs. Thesestuds 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 theexterior wall studs.Batt insulation is laid in the middle cavity, perpendicular to the studs with approximately onehalf-inch overhang at the ends of the wall assembly. Batt insulation can now be placed within theexterior stud cavities. Depending on the covering of exterior, foam insulation and or sealedpolyethylene 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 toinstall the walls vapor trap along with the exterior polyethylene, as a single sheet, which can besealed to the interior vapor barrier between the two sole plates. A moisture barrier is then placedover the wall assembly. It is secured by folding the edges over the studs and plates, and fasteningwith staples.Finishing the wall by installation of the pressure treated plywood foundation cover and/or sidingis then done. The plywood foundation paneling must be installed with the joints staggeredagainst the underlying foam board. These panels are fastened with exterior screws, spaced eightinches around the edges, and ten inches within the interior area.Foundation vapor traps and groundsheets should be sealed by sandwiching them between the sillplate and the top of the foundation with a combination of sill gaskets and acoustical sealant. Themoisture barriers are sealed with vapor tape at each joint. Anchor bolts should be left loose toallow for shimming or grouting if necessary.The exterior walls are fastened together with wood screws, every two feet along the end studs ofthe interior wall. If required, metal tie straps are installed on the inside edge of the top plate. Thewalls can then be leveled and squared before proceeding with floor or roof construction.First Floor WallsIn design, it is important that if you do not use tie straps to hold the sections together, then theupper and lower sections should have overlapping plates to prevent separation. Also, dependingon the particular design, the floor may have to be installed before continuing. It is recommendedto install floors before continuing for this gives the carpenters a level area to construct the upperwalls on and supports the exterior foundation.One way these walls can be installed with the windows and siding is by overhanging the wallsections. Construction of the interior bearing walls, vapor barrier, stabilizer, plates and exteriorwall studding in this approach is similar to the foundation wall assembly. Where they differ is inthe edging of the units.Edging is the bottom cover, which hides and protects the overhanging framed members andinsulative barrier. It can be decorative hardboard nailed to the underside of the sole plate whichis sandwiched between the plate and the foundation or floor assemblies. This edging should besealed 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 floorsexterior. The overhang allows for installation of bearing, platform framed floor assemblies whileproviding 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 aroundthe opening could be sealed with expanding foam, by pre-installing the windows in frames with aperimeter 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 widthshould be equal to the entire thickness of the wall assembly. This frame is then sealed to thewindow by sandwiching a sill gasket between it and the surrounding wood members, which issecured with wood screws inserted through the window frame. This allows removal for futurerepair or replacement of a window or door. Leave approximately two inches of the gasketextending beyond the interior edge. The resulting box allows fastening of the interior finishdirectly to its surface, without gaps or unsupported areas.The framed walls and header should be attached with screws directly to the boxed window, withsealant applied between the box and the individual vapor barriers. After squaring up thecompleted section, bracing is let into the studs. This bracing is important to the operation ofwindows 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. Oncethe bracing has been secured, exterior sheathing with the moisture barriers are installed together,and exterior finishes applied.Interior FloorsBecause open wall construction relies on continuous wall insulation between stories, the interiorfloors in this method must be designed to remain within the building envelope. Utilizingoverhanging walls, wider lower wall sections or ledger strips, the interior floors can be supportedon sufficient bearing. Floor assemblies can be joists, truss floors, wood I beams or any other typeof framing material.Floor construction, through wall overhang or wider lower wall sections, allows the carpenters toconstruct the floor assemblies as current accepted practices, with one exception. The header ortrimmer joist may have to be placed inside the edge of the top plate of the lower wall. The mainconcern with this type of construction is that when installation of the upper-framed wall isattempted, there is a likely hood of dropping the lower wall off the platform and foundation wallwhen raising it. Simple blocking or tiebacks under the floor will prevent such a problem andensure site safety.Floor construction with ledger strips or ribbon boards permit finished floor levels to meet withthe top plate of the lower wall assembly. The bearing ledgers or ribbon boards should beinstalled as per local building code requirements. Floor sheathing must overlap the wall joists toensure a proper firebreak between floor levels. Although time consuming, header and trimmerjoists 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 tiestraps. 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. Onlyqualified personnel with the proper experience should attempt other more complex designs.RoofsOpen wall construction was designed for use with truss components, but by strapping overceiling joists, the same results can be obtained. For effective design, overhangs should becompletely eliminated. They are costly and require extensive labor to complete. Roof edgesshould have angled fascia (called knife edging) to allow enough overhang to prevent excess runoff from running down walls. Simple "two by" fascia, lapped over wall siding can be trimmed orpainted in many colors and still provide adequate visual separation from the roof. By lapping thefascia over the siding gravity water cannot enter into the buildings wall cavities. But rememberto leave enough screened eave area, to allow for adequate attic ventilation. Gables should also becompleted with knife edging and reduced overhang. Sheathing and roofing materials are appliedas per current building techniques.Raised heel trusses or plated ceiling joists will raise the roof to allow for eave insulation andventing. The attic vapor barrier should consist of 10-millimeter polyethylene. This thicker andmore durable layer is less likely to be damaged during construction, or when running fish tapesfor future utility lines. The vapor barrier is installed parallel to the truss members or ceilingjoists. Wall edges are sandwiched with acoustical sealant between the top wall plate and two bytwo strapping. Joints landing on truss members or ceiling joists should be sealed between it and apiece of two by two strapping.Over the ceiling air/vapor barrier, two by two strapping is screw nailed to the underside of theceiling support members. Areas around penetrating utilities such as stack vents and furnaceexhausts should be made with plywood sealed directly to the air/vapor barrier. Holes drilled orcut for the utilities must be carefully sealed to the plywood with expanding foam or otheracceptable materials such as acoustical sealant.Julian Arhire is a Manager with DtiCorp.com - DtiCorp.com carries more than 35,000 HVACproducts, including industrial, commercial and residential parts and equipment from Honeywell,Johnson Controls, Robertshaw, Jandy, Grundfos, Armstrong and more.affordable, architectural, Construction, energy efficient, House Plans, housing, ResidentialTechnologies