Building with Structural Insulated Panels (SIPs) was first developed in the 1930s, as an alternative home-building technique. Yet it is hardly known by the general population. Interest has been growing in the use of SIPs in recent years, mostly due to the potential energy savings that SIP homes are promoted as offering.
SIPs consist of an insulating closed-cell rigid foam core, sandwiched between two OSB (Orientated Strand Board) facings. The panels are designed to be interlocking, either using splines to connect adjoining panels or nailing the OSB to structural lumber plates. A combination of structural adhesive and nails are used for connecting panels together.
You can’t just go out and buy SIPs, as they need to be custom designed and manufactured for your construction project. This starts with the architectural design, which must be done with SIP construction in mind. It’s a good idea to hire an architect to do the design, who is familiar, as designing for SIPs is different, than designing for stick construction. While the same designs can be executed, the manner of putting them together is different.
It is critical that a SIP designed home be well-designed, without any errors in the drawings. Whereas most contractors are accustomed to dealing with problems they encounter in drawings for conventionally built homes and making the necessary modifications on site, it is often impossible to make those modifications on site with SIPs, as the panels are prefabricated at the factory. Changes often require ordering new panels from the factory, adding cost and delaying the project.
Other areas where errors in the drawings can cause serious problems are in the areas of the electrical and mechanical installation in the home. Unlike stick framing, channels for wiring and plumbing must be molded into the panels, while they are being manufactured. If they are not, and the tradesmen for these trades must cut channels themselves, there is a high risk of them weakening the panels.
But properly designed, SIP homes are able to be erected faster, with roughly 50% less labor. While the panels themselves cost roughly 15% more than materials for conventional construction; the labor savings makes the overall cost of the home considerably lower.
SIP homes have a life expectancy roughly similar to those of more conventional stick framed homes, roughly 60 years, making it possible to get loans for the construction of these homes and normal conversion to a typical 30-year mortgage.
Benefits of Building with SIPs
As with any building methodology, there are advantages and disadvantages of building with SIPs. People who choose to use this building technique, generally do so because the advantages of building with SIPs appeal to them.
Building with SIPs requires roughly half the labor that normal stick framing does. This not only reduces labor costs, but it also reduces the overall scheduled time required for erecting and drying in the shell of the home. Workers can begin the process of installing electrical, mechanical and interior finish sooner, shortening the overall construction time for the home.
Due to the reduced labor requirement in building the home’s shell, the cost of SIP homes is slightly less than that of stick framed homes. How much less depends a lot on the specific design of the home. Because of the manufacturing techniques used in making SIP homes, a boxy design is easier to build. The more deviation from a basic box, by adding areas where sections of the wall protrude or are indented, the more the cost of the home increases.
Perhaps the greatest advantage of a SIP home is the insulation that it offers. SIP walls have a higher R-value than conventionally built walls, especially if XPS foam is used in the core, rather than EPS foam. But the greater advantage is that the insulation is uniform throughout the design, leaving no gaps which are difficult to insulate.
In addition, there are fewer points where insulation has to be forgone for structural elements, as the panelized design provides structural strength throughout, along with insulation throughout. Typical problem places, like the joint between the walls and roof, aren’t a problem with SIP construction.
The combination of the laminate construction of the SIPs, splines connecting panels together and adhesives used in the joining of panels provides a highly airtight home. This can even be improved in some cases, where urethane foam is squirted into joints in the panels.
A layer of house wrap is applied over the OSB skin of the SIPs, providing an additional airtight seal. Because of the design of the home, this layer of wrap can continue from the walls, right over the roof, without break, eliminating one of the biggest air leak points in any home.
Much of the heat loss in a home is due to air leaks. By creating a more airtight home, there is less loss of heat, resulting in lower heating and cooling costs.
Earthquake and Hurricane Resistance
A finished SIP home is not only nailed together, but bonded together as well, making it stronger than a stick frame home in some critical ways. Some of the places which are typical problem areas, such as the joining of the roof to the walls, have more physical contact, than with stick framing, reducing the risk of earthquakes and hurricanes destroying them.
Disadvantages of Building with SIPs
While there are several good advantages in building with SIPs, there are disadvantages as well. before deciding on SIP construction, you should understand these potential problem areas and either have a plan for dealing with them or make sure that they aren’t going to be an issue for your home. As we are talking about the exterior shell of your home, it’s not like the SIPs can be removed and replaced with something else, if they don’t work out.
Many of the problems can be eliminated by hiring an architect who has a good track record in designing with SIPs and contractors (including subcontractors) who are familiar with working with them. There are a number of potential problems which crop up, for no other reason than using workers who assume that the methods they use in stick framing will also work with SIPs. In some cases, their actions can severely compromise the structural integrity of the home.
One of the main materials used in the manufacture of SIPs is OSB. This is a great engineered wood product, which has been used successfully for decades. However, it is highly susceptible to moisture. If you’ve ever seen a home roof where the edges are curling up, chances are that the roof is sheathed in OSB, rather than plywood.
If a home is being built from SIPs made with OSB and it rains, the OSB will soak up moisture, degrading the panel. The same can happen if the SIPs aren’t properly spaced off of the foundation. The edges of the OSB panes are the most susceptible to moisture, wicking it up from the bottom, where it weakens the OSB.
There are other materials used in the manufacture of SIPs, including fiber-cement panels. You may want to talk to your architect or SIP manufacturer about your options, before placing your order.
The foam core of SIPs is a popular nesting material for pests of all sorts, who will burrow into it for the purpose of removing that material for their nests. The application of insecticides and/or boric acid during construction can greatly reduce the risk of this problem.
Because SIP built homes are so airtight, they typically require a smaller HVAC system than stick framed homes. Usually about a ton less. This would need to be calculated and specified by a HVAC engineer who is familiar with building SIP homes. Additional vents, to bring air into the home, may also need to be added.
Because the panels are factory made, it is extremely difficult and can be expensive to make modifications to the home, once construction has begun. The contractor’s ability to make modifications to the panels, on the construction site, is limited. While there are some things they can do, especially if they are highly experienced in working with SIPs, there are always limitations.
Danger of Structural Damage
Because of their design, SIMs are the structure, rather than framing being the structure. They have been designed to meet building code requirements, so that’s not an issue. What is an issue is if a subcontractor makes modifications to the panels, in order to put in electrical wiring, ductwork or plumbing. A horizontal cut in the panel, making space for a pipe, can destroy the structural strength of that panel, resulting in your home not meeting building code requirements.
Vertical cuts do not cause this sort of problem, only horizontal ones. The selection of subcontractors, to ensure that they know how to work on a SIP home, is the only way you can be sure that this risk is properly managed.
SIP panels, while lighter than a comparable sized stick framed wall, are heavy. Many panels are considerably larger than the 4’ x 8’ panels that most carpenters are used to working with. A crane or forklift is necessary for working with these larger panels, unloading them from the truck, moving them around the job site and setting them in place.
While it is theoretically possible to build SIP homes with curved walls and non-standard angles, manufacturers are not set up for this. Therefore, designs are limited to boxy homes, with 90 degree angles.
Not all SIPs meet the fire safety requirements of the building code. In these cases, the application of a layer of drywall on the inside of the wall is usually enough to ensure that they will meet the requirements. This can be a concern with some architects, who might want to use SIMs, without the required layer of drywall, for artistic purposes. Unless some other fire-rated product is applied to the walls, to protect them, this can create a fire safety issue.
While SIPs are rated at a 60 year lifespan, making them comparable to stick framed homes, if they are attacked by moisture, it can greatly reduce the lifespan of the home. While there are methods which can be employed to reinforce OSB panels which have been compromised by moisture, they are difficult and costly to implement. There is no simple solution, allowing the home to be used.
Building with SIPs
Building with SIPs is considerably different than building with stick frame construction. It starts with the panels being fabricated at a SIP manufacturing facility, where the exterior panels and foam core of the sandwich construction are cut and laminated together. Each panel is numbered to match the plans, with the number painted onto the panel.
Once the panels arrive on the construction site, they are sorted for ease in finding them. Walls are assembled by standing the panels on a double wood “plate” (much like the plate at the bottom of a stick frame wall), with the plate fitting into a channel in the bottom of the panel. The panel is attached to the plate with adhesive and nails spaced every six inches.
From there, adjoining panels are attached to the first with splines. While there are more than one kind of spline used, the most common is a piece of OSB that slips into slots behind the OSB on both adjacent panels. It is glued there, then the panels are nailed together, by nailing every six inches through the OSB skins on both sides of both adjacent panels and into the splines.
The wall is finished off by adding a 2”x 4” cap, which sets into a groove in the top of the panel. The ends of these boards must be at least a foot from any joints between panels. Once again, the skins of the panels are nailed to the cap.
Floor structures can be made by the same method that wall panels are or traditional wall construction techniques can be used. Roofs are also made of SIPs, allowing for very quick erection, with insulation already built into the roof.
All panels will have channels molded into them for the installation of electrical wiring, plumbing and HVAC ductwork.