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SIP System Fabrication

SIP (Structural Insulated Panels) The Preferred Building System by Quality Builders, Developers and Home Owners


a. Architectural design drawings and details.
b. Engineering-structural calculations and structural details based on the design.
c. Title 24-energy compliance.
d. Fabrication drawings for SIP panel fabrication and assembly.
e. Ordering of “Stock” panel (known as blanks) from panel manufacturer.
f. Fabrication of panels for specific house design.
g. Delivery of wall panels for owner or contractor installation.
h. Delivery of roof panels for owner or contractor installation.
i. Delivery of various other exterior finishes to complete the weather-tight shell.

The typical “off-the-shelf” panel comes from the factory (manufacturer) with the top and bottom relieved for the plates (typically 11⁄2"). The factory provides the detail to connect adjacent panels side by side as part of their basic setup along with their spline material. Panels usually come with two electrical chases run horizontally at receptacle height and at switch height. There will also be a vertical electrical chase in the middle of the panel. (see previous illustration of “Stock” panel).

The fabrication shop takes these “stock” panels and will begin the basic fabrication process for the specific home being constructed. This fabrication process utilizes the fabrication drawings completed by the design and engineering team. These drawings are step-by-step procedures for both the fabrication of the panels and assembly of these panels in the field by the client or the client’s contractor. Fabrication consists of additional cuts, such as windows, doors, rakes, and roof-panel plumb cuts. For example, you might want to order gable-wall panels with the rakes precut and the foam relieved 11⁄2"or you might want panel #22 to have a 3" relief on the right side for a structural post to support a ridge beam or you might specify the electrical chases at 16" and 46" instead of at the standard height.

During planning and layout, it can be determined where cutoffs from one panel can be utilized to serve as headers or filler panels elsewhere, thus eliminating waste and saving money. At this point, it may be that a slight change in design can help your material and labor savings by more efficiently utilizing the panels. This is accomplished during the design phase as well as the production of shop drawings. It also determines the dimensional lumber and headers that are required in each panel, all of which are precut, labeled and installed in the panel. Accurate shop drawings are the basic road map for ordering, fabrication, and erection.

The panel to panel connections are a very important part of the SIP system. Besides being a structural joint critical to the integrity of the building, there are more linear feet of potential air leakage at the panel joints than at any other part of the system. Splines are used at panel joints to securely hold panels together and stop air infiltration. An approved caulking material is applied at all panel connections to maintain the integrity of an airtight SIP assembly.

There are three common types of wall connections, the thin OSB spline method has become the standard in the industry because it is inexpensive and has no thermal bridge. Next most common are the single or double 2x spline connection and the foam block spline. All panels come from the manufacturer precut to fit their spline system; the manufacturer supplies the splines. There isn’t any fabrication to be done with the factory splines. They are glued and nailed in place during the site construction process.

The two end panels on any wall have a solid stud at the outside corner for nailing and stiffness. These full height studs in an 8’ wall actually measure 92 7/8" long, allowing them to fit between the top and bottom plates. The corner stud is fabricated to fit in the panel at the fabrication shop by scooping the foam back 11⁄2", then applying a bead of glue to both facings and to the center of the core, and positioning the stud, then nailing the stud off through both facings with 8d nails 6"oc. If this is the panel that runs through to the corner, then there will be R-Control screw holes marked off every 16"oc with a felt marker and pre-drilled for the field crew to fasten the corner panels together using a 8" long control screw. The screws will fasten through the outside panel and into the 2x end stud of the adjacent panel during erection. This corner detail is suitable for exterior finishes such as stucco, shingles, log siding, and stone and brick veneers. When using wood sidings such as lap sidings, cedar or pine log sidings, a cap corner detail will be used to provide extra insurance for nailing the corners. A cap corner adds another 2x at the corner to provide additional nailing for siding or corner trim pieces.

The type of header required in a SIP structure depends on the Engineered Header span of the opening and the loads imposed on the header from above. This is determined during design and structural engineering. If the rough opening of the window is 4’ or less and there is at least 12" of panel above the opening, the header can be integrated with the panel or span two panels. In this case, the window is cut out of the panel, the foam relieved, and 2x material installed.

For walls with window and door openings wider than 4’, there will be a need to install a separate header. The header is fabricated from panel off cuts. In most cases a 4x timber is used at all openings over 4’. These headers will come loose and be marked as to which panel opening they are to be installed. Western Homes provides Weyerhaeuser insulated headers with Insulspan panels.

Rough window and door openings are cut and foam is scooped out and 2x headers, trimmers and sills are installed. The foam for the trimmers and sills is relieved back 11⁄2". The headers are relieved back 11⁄4", glued and nailed starting with the sill first using 8d nails at 6" oc, then followed by gluing and tacking the header followed by the side trimmers. A complete window is fabricated in a few minutes.

In California, Oregon and Nevada, as well as other states, there are specific seismic connections required. It will be necessary to install hold-downs to lock the SIP into the foundation or to the wall below. The typical hold down is a Simpson STHD8RJ, which is cast in place during the concrete pouring of the foundation and nailed off on the outer side of the panel (see illustration).


The floor and foundation plans will have the locations of their respective wall connections shown. The shop fabrication drawings will also note what panels are to be connected. Because a SIP wall behaves more like a structural unit than a stick-built wall, fewer hold-downs are typically needed to ensure the connection to the foundation.

It is rare that a special chase has to be placed in an exterior wall panel if the design of the home has been thoroughly thought out. Many times vent piping and water piping can be placed in the interior walls, and with under cabinet “loop vents” there is rarely a need to install plumbing in the SIP wall. If for some reason it is necessary, the plumbing chase can be cut out using a two handled, hot-resistance wire tool during fabrication. The simplest and most effective way to route plumbing supplies and vents is to build a chase, the same approach used in masonry and log home construction.

Roof panels excel beyond the stick-system by doing most of the fabrication on the ground rather than up in the air. A significant advantage of SIP roofs is that they do not have the same ventilation requirements as a stick-built roof. A critical and expensive step in the planning of a stick-built roof is how to ventilate it, and these details can be expensive, time-consuming to build, and questionable in their performance. But because of the consistent density of the foam core of a SIP and its low moisture permeability, ventilation issues become simpler, whether or not a cathedral ceiling is planned.

The SIP roof comes with fasteners (panel screws), adhesive, and each panel will include 2x sub-fascia installed in the panel. Depending on the type of roof cut, the ridge blocking may come already installed in the panel or pre-cut and marked where it should be installed. The connection splines can come in one of three ways: OSB spline material for no load roofs, double 2x spline connection, or “I”-joist connection using TJIs, Micro-lams, or Para-lam material where higher snow loads dictate. A typical SW roof system that provides an insulation value of R-33 will be 81⁄4" thick up to R-52 for a 121⁄8" panel. The very best application for SIPs is on the roof, where all of a buildings heat tries to exit and where the most amount of time is spent going from the ground to the carpentry task at hand. With some planning and basic roof math, there isn’t a roof designed that would not be cost-effective and energy-efficient to fabricate first on the ground, out of SIPs. Although the materials for a SIP roof are somewhat more expensive than a conventional framed roof or a pre-fabricated truss roof, the over all savings is realized in the time savings (labor) to build the roof, as well as the long term energy efficiency.

The subcontractor who will have to adjust his approach the most, when working with SIP buildings, will be the electrical contractor. Basically, installing wiring in a SIP home is a lot like running wire in a commercial job, except that the electrician doesn’t have to install the conduit because the panels come with factory-cut chases. When the builder erects the shell correctly, he has already drilled most of the required holes in the exterior walls. On raised foundations, the sill plate must be pre-drilled for the vertical electrical chases in the panels by the contractor. If requested, the top plates will be marked for their specific location in each panel and will come pre-drilled to line up with the vertical electrical chases in the panels.

The techniques for installing wire in the exterior walls comprises only part of the overall scope of the job, the rest of the tasks are the same. Having the electrician involved in the planning prior to installation of the wiring will save time and money. For instance, if a receptacle outlet is located 14" up from the floor, a switch is located 44" up from the floor, and an exterior light fixture is pulled from the same vertical chase, the electrician’s life is made easier.

A typical 4’ wide panel has a 11⁄2" diameter horizontal chase cut into the foam core about 14" up from the bottom of the panel and another horizontal chase about 44" up from the bottom. These locations will accommodate most receptacle circuits and switch locations. In addition, a 11⁄2" diameter vertical chase is in the center of a 4’ panel. This basic grid allows the electrician to run almost all of the wiring for an electrical plan without significant drilling or cutting of panels.
Besides the standard configuration, factory-installed custom chases can be ordered to virtually any specification to accommodate the most elaborate wiring plan. In addition Western Homes offers a third horizontal chase located about 3" above the plug height chase to accommodate residential cabling systems (cat 5 wiring) to create a “smart” house layout.

Cables cannot be surface-run and are required to be run in conduit on the outside of the building. If the main panel is in a basement, the approach is standard practice. But for a slab building or if the main panel is scheduled upstairs, the installation is different and the main panel will have to be located in a mechanical room. There are specific code requirements for clearances from the panel to other objects, as well as in which rooms a panel can be located and will be determined by local building codes.

Before the wiring is run, devices need to be located and openings made for the wire runs. To prepare for switch boxes, receptacle boxes, and fixture outlet boxes, the electrician will need to make openings in the facings of the panels. These can be cut either with a jigsaw or with a hole saw. The boxes most commonly used in SIP walls are plastic boxes with side mounting brackets that can be screwed to the OSB. Steel boxes with adjustable mounting ears are handy for offsetting the face of the box to accommodate different interior finishes.

There is not much of a concern when a building has a basement or raised foundation because most circuits are looped from below into the vertical chases provided in the panels. But this method cannot be used with slab construction. When the SIP structure is built on a slab, wiring has to be pulled horizontally to feed receptacles and switches, and some way must be found to go around the corners.

One common solution is to pull wire up the vertical chase and along the top plate, then drop down in the next chase around the corner, but only certain top-plate details allow for this. Another approach involves drilling the corner studs at the same height as the bottom chase of the panels, where the power feeds are run. The end stud of the panel that butts into a full-length panel needs to have a hole drilled in it at an angle so that the wire will sweep through it and into a connecting hole in the adjacent panel. Upon request these holes should be drilled during the fabrication stage. If not, they will have to be drilled in the field prior to installation of the corner system.

A typical well-built SIP house (designed by Green Sense Builder), with 51⁄2" EPS core walls (a 61⁄2" panel), a 71⁄4" EPS core roof (an 81⁄4" panel), a passive solar design, an ICF basement, and argon-filled low-E dual glazed windows, would allow as few as 1.0 air changes per hour (ACH) compared with the average of 20 to 30 ACH for stick-built houses. As a measure of air leakage through the exterior envelope of a building, ACH is the best indicator of a building’s air-tightness and energy efficiency as it gains or loses heat to the outside environment. The fact that SIP buildings are typically many times as tight as conventional stick-built homes adds up to significantly lower heating and cooling requirements, lower initial installation cost, and much lower operating costs over the lifetime of the home.

In climates where temperature can fluctuate 60 degrees or more in a single day, systems such as radiant heating are thought to be marginally effective because their slow, even heat delivery cannot keep up with drastic meteorological changes. But a well-insulated, airtight building can retain a given temperature for an extended period of time, regardless of outside air temperature, thus placing a smaller demand on the heating and cooling systems. As the required size of these systems decreases, the options for the type and configuration of the system increase.

The most common energy source for heating and cooling systems is natural gas, propane, or electrical resistance heat units. To a lesser extent is air-source or ground-source electric heat pumps, or various types of pellet fuel, geothermal sources, or solar-powered systems. Heating and cooling strategies can be divided into general categories of active or passive and subdivided into ducted systems and radiant approaches.

The most common approach relies on ducting for the supply and return of forced air. This system consists of some type of furnace to produce hot air, a compressor or chiller to produce cool air, and an air handler to distribute the conditioned air. The return side of the system can also be easily tied into heat-recovery ventilators (HRV) to maintain fresh air quality.

Vaulted spaces add to the feeling of volume in a room, and the dramatic spaces created can be a big selling point for new homes. The tight qualities of a SIP house will mean that warm air will tend to be trapped at the high side of the vaults. To gain maximum efficiency mount the cold-air returns with a damper grill high up on a wall of a vaulted room. A more flexible approach is to have one grill high and one low in the same chase and have them both dampers. This way, hot air can be returned to the cooling unit from the top grill and re-circulated in the summer, while in the winter the hot air can be left to condition the room and the cooler, lower air can be returned to the HYAC system to be warmed from the lower grill.

When sizing the HVAC system the contractor must take into account the ACH factor of SIP construction and the higher R values of the walls and roof. Most residential HVAC contractors use a basic rule of thumb formula calculated per square foot of living space and cubic feet per minute per square foot to airflow. Indoor air quality unlike some engineered wood products. SIPs are relatively benign as far as off-gassing of harmful chemicals. However, because of the airtight nature of a SIP building, there will be a need to deal with interior pollutants from such items as carpets, furniture, and paints, as well, as appliance fumes, radon, and excess humidity. Most of these air pollutants can be controlled by mechanical ventilation. The goal is to exhaust the stale indoor air and replace it with fresh outside air while controlling the humidity.

The money and effort spent installing a well-designed, efficient whole-house ventilation system in an airtight SIP house is well worth it, in terms of indoor air quality (IAQ).


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