Engineer highlights key SIP application issues
John Page, with J.E. Page Consulting Engineers (Florissant, MO) has provided third-party engineering services for SIP structures for about a dozen years. At present, about 50% or more of his business comes from SIP-related projects; that share is on the increase, thanks to an extensive working relationship with ThermaPanel Homes. He also teaches structural engineering at Washington University in St. Louis, and has plans to use SIPs as investigative design challenges for students.
Here are a few of Page's key observations about SIPs and associated engineering design issues over that time period:
- The average structural insulated panel and its strength characteristics haven't changed very much over the last dozen years.
- SIPs are very strong. "They provide a big factor of safety, bigger than expected much of the time" said Page. "It's nice to know that you've got excess strength there." Because of the flexibility of the panels, load capacities are often limited by deflection concerns, but the panels strength is often much higher.
- Unfortunately, SIPs'strength characteristics are often being under-appreciated when it comes to design applications.
- Frequently, it makes more sense to frame some sections of walls, especially those sections loaded with windows, than it does to use SIPs throughout. Otherwise, you can end up with only a few inches of SIP panel used inefficiently with a lot of interspersed framing. "There's room here for a composite approach," said Page. "But lots of framers don't like to talk about SIPs and lots of manufacturers want to completely eliminate separate framing from their wall systems. The fact is, plain panels cost less than custom panels with factory built in framing, so use plain panels where it makes the most sense, and use some framing where that makes the most sense." Message: use SIPs except where it's faster and cheaper to frame around openings.
- If people want to optimize their use of roof panels, they shouldn't opt for complex hip roof designs. "Yes, you can use SIPS in all roof design applications, but they are much more suitable for single slope and single ridge roofs. Cathedral ceilings and clerestory windows are some of the especial benefits of SIP roof framing which can be lost when cookie cutter type plans are used.
- Panels over long roof spans will deflect over time, and it may well be noticeable. Today's homeowners are used to looking at roofs built with roof trusses, which are very rigid; when they see visible deflection in roof panels, they tend to call manufacturers and some end up consulting with Page. "Panels are more like our old rafter systems; they will deflect [sag] some immediately, and then deflect some more over time," said Page. His recommended design solution: double the dead load and add it to the live load to determine total load capacity of panels when using specific limiting deflection criteria and understand how much deflection this represents.