By now there have been many articles in the popular "shelter" magazines about SIPs. Articles have also appeared in the newspapers, SIPs have been seen on TV (that makes 'em real!), and even the architectural press has yielded up some inch-columns on our favorite "new" material. Stories about "green" construction almost always mention SIPs. We are beginning to finally gain acceptance within the building community insofar as "name recognition." People now have heard of SIPs and have a basic idea of what they are and how they work. It is the exception rather than the rule that a reference to Structural Insulated Panels is met with a blank stare.
Additional evidence for this is that there is growth in the industry and an increasing confidence about creating "stick translations." - taking that builder's set of conventional construction drawings and converting the building into SIP construction with the help of a proper set of "cut" drawings. Most of the people responsible for creating the "cut" drawings utilized by the factory for pre-cutting the panels and the field crew for site assembly have had little difficulties - if any - performing the small amount of engineering required for "panelizing" a conventional stick structure. These buildings have been going up without any problems for decades and the process has lulled the design community into thinking that this is what is do be done with SIPs…and not much else. But the real fun is in fully exploiting the full capabilities of SIPs and doing things that sticks could never economically do. My list of SIP applications that take panels well beyond where most stick structures even think of going is as follows:
- Cathedral ceilings
- Floors Over Unconditioned Space
- Curved Roofs or Walls
- Box Beams & Cantilevers
- Point Load Distribution
- Shear Diaphragms
- Seismic & Wind-loading Resistance
We'll take these in order, briefly, as each really deserves it's own chapter in a proper book.
1. Cathedral Ceilings
This is where I suggest that anyone wanting to stick his toe into SIP waters begin because it is the most cost-effective application out there. The new codes require a minimum roof R-value of 38. In order to achieve this with sticks you need to utilize a minimum of 2 x 12 rafters, regardless of load/span requirements, in order to fit in 12" nominally thick batt insulation. One must than put down lathing for a self-venting wood shingle roof or jump to 14" deep composite members to allow for venting over the insulation and under the plywood roof sheathing deck. The cost of these can be pretty steep. If we use SIPs instead, we may use a 6 inch SIP with a urethane core, an 8 inch SIP with XPS core, or a 10 inch SIP with an EPS core. See the following chart:
When SIPs are used as lintels they may be used as engineered box beams which are capable of carrying enormous loads. In most houses for most openings conventional lintels become totally obsolete. This saves layout time and expensive material.
Many times we are seeing columns on a stick-framed job that are built up of many sticks…sometimes as many as 6 or 8 members between windows or at corners. This may burn up quite some board feet, especially if these are 2x8s or greater. SIPs may do very well as columns in place of all these sticks and -- depending upon the wall design -- may come as just part of the wall between windows or doors and still handle considerable loads.
4. Floors Over Unconditioned Spaces
This is an application that allows one additional design possibilities where none may have existed before. In our office (before SIPs) habitable spaces and floors over "the outside" or even a garage were to be avoided, sort of like plumbing in an outside wall. It could be done with special consideration and expense, but best economical practice precludes this solution. Stick floor construction generally allows for severe infiltration that makes it almost impossible to deliver the comfort and economy we expect from a well built habitable space. With SIPs being such an effective insulator, and most importantly so resistant to infiltration, this may now be considered without any worry.
5. Curved Roofs or Walls
Yes, we know these are also possible with stick construction, but again -- at what cost? Curved SIPs, not available from all manufacturers but from many, offer the possibility of historic "bowed" roofs, radial roofs, stair turrets, towers -- or, whatever! Bear in mind that each different radius calls for a different jig, so the more panels from the same jig the more reasonable the final cost.
6. Box Beams & Cantilevers
When SIPs are properly analyzed as box beams the loads they can carry as longspan beams or cantilevers are surprising. The load/span charts created under contract to SIPA by Thomas Bible, P.E. has a section on box beams that show 25 1/2" deep and 6 1/2" thick SIPs capable of carrying 864 pounds per linear foot for a clear span of 16 feet. This is with LVL flanges. The same beam as an 8 foot cantilever is capable of carrying 3,456 pounds out at the end. Eight foot high walls, understood to work as cantilevers, may carry over 6,000 pounds at the end of a 4 foot cantilever. Needless to say, this kind of reckoning should never be seat-of-the-pants, but properly calculated by someone who knows how to do this.
7. Point Load Distribution
The central idea behind SIP construction is that it is thin shell engineering. Like an eggshell, point loads are dispersed over large areas of the surface; the stress then at any given point is very small. Beams that have to carry such great loads that they are designed as steel may still, in many cases, be safely carried at panel walls with out having to post down to the foundation. Edge blocking that distributes these high loads along a significant run of panel may be the key to having the wall SIP alone safely receive the end of a steel beam. Again, these details should be properly designed by someone who knows how to achieve this. Much of what I see in the SIP world is way over-engineered so as to call for too many special framing and joint reinforcing members that destroy the inherent economy of working with SIPs in the first place.
8. Shear Diaphragms
All wall and roof planes become shear diaphragms when properly constructed with SIPs; that is, the connections between the panels are correctly dealt with. If so done, 90% of the stresses are transmitted across the SIP joint. If we think of a traditional gabled box, it may be understood that the two roof planes act as diaphragms that will resist the outward thrust on the top of the wall. Th roof-to-wall connection should be engineered for this, in most cases the standard SIPA detail of sloped wall plate and panel screws through the roof at 8" o.c. will be fine. We see that is possible, in some cases, to eliminate the usual collar ties or ridge beam usually called for as a structural resolution for this form. Never utilize this application without checking with a qualified engineer or architect!
9. Seismic % Wind-loading Resistance
The new national codes are very proscriptive about calling for all kinds of ties and strapping for stick construction in areas where seismic and wind loads are severe. Ordinary SIP construction automatically takes care of most of these situations. Assuming your sole plates are properly bolted down to the foundation, the uplift resistance may be calculated per fastener when one specifies the edge distance, the plate material, and the fastener type. For 1" edge distance (7/16" OSB skins), no.2 or better douglas fir, and no.8 x 1 5/8" screws, we use the value of shear resistance of 100 pounds. When you figure the fasteners on both sides of the plat, and their spacing, you may come up with a value for the wall section or per linear foot of wall.
As we can see, it may be considered a chore to run all these engineering calculations, but the rewards are great. The new codes have also imposed calculations and documentation requirements for stick construction that had been ignored before. If one has to "run the numbers" anyway, I think that working them through with SIPs enables you to come up with some exciting designs and structures that can't be duplicated with stick construction. Only if you add engineered steel and exotic connections to sticks that all help to send the cost way beyond that of SIPs, will they be able to match the efficient performance of SIPs. Go forth and try some of these applications, but do so carefully with the help of a knowledgeable engineer or architect. Eventually we hope to see full recognition of the mechanical/structural properties of SIPs start to engage the design community and encourage the development of new structures that may better reflect the classic architectural concerns of expressing resolution of the issues of time and place.