Building with Structural Insulated Panels

Better than the comparison of materials alone suggests, and the reason is geometric rather than chemical. In a timber frame the insulation fills the bays but the studs cross the wall from side to side, and timber conducts heat several times more than the insulation beside it: the result is that a considerable share of the wall surface performs worse than the declared value, and the effect shows clearly in thermography as a regular pattern of vertical stripes. In a SIP the insulating core is continuous, interrupted only at the joints between panels and at the openings, so the actual performance is much closer to the theoretical one. Two clarifications, though, keep the comparison honest. The first is that the advantage concerns uniformity more than the conductivity of the material itself, so it narrows as the frame is built with thermal-break techniques. The second is that a SIP has almost no thermal mass, so it heats up quickly and offers little in summer: in a warm climate that is a genuine limitation, and it must be resolved with other elements of the design.

Not the way you would in masonry, and this is the point that most often catches out those coming to SIPs from traditional building. A SIP panel is a structural sandwich: its capacity comes from the two facings working together, held apart by the core. Cutting a chase in one facing means cutting the element that carries the load, and doing it in the middle of the panel, where stresses are highest, is worse still. There is a second, less obvious consequence: the cut opens a route through the insulating core, and the core of a SIP is also the airtight layer. So an improvised chase produces a structural problem and a hygrothermal one at once, and the second is the more insidious because it manifests years later as condensation inside the panel. The correct route is to define the service runs at design stage, having the chases formed in the factory within the core, and to keep the remaining runs outside the panel, in a service cavity created inside the finishes.

It is the failure mode that has given SIPs their bad reputation where they have been built badly, and it is worth understanding because it is entirely preventable. The facings are wood-based panels: if they stay wet they swell, lose strength and can be attacked biologically. The problem is that water entering a joint ends up inside the sandwich, where it neither evaporates nor is visible: there is no ventilated cavity to dry it and no surface showing a stain. Decay therefore proceeds silently and is discovered only when the panel is already compromised, often through localised deformation of the finish. Three measures, taken together, remove the risk. The first is to protect the panels during erection, closing the shell quickly and never leaving them exposed to rain. The second is to have a genuine rain barrier outside, that is, a ventilated cladding, so the joint is never reached by water in the first place. The third is a well-made airtight seal on the inside, because much of the moisture that condenses in a joint arrives from indoors as vapour, not from outside as rain.

They behave well, and for a reason that has nothing to do with strength: they are light. Seismic forces are proportional to mass, so a building weighing a fraction of a masonry one attracts far smaller forces to begin with. On top of that, SIP panels are excellent shear diaphragms, that is, they resist the horizontal actions that tend to distort the building, because the facings work in their own plane exactly as a bracing panel does. The critical point, as always in lightweight construction, is not the panel but the connections: between panel and panel, between wall and floor, and above all between wall and foundation, where the risk is uplift, that is, the building tending to lift and slide rather than crush. The hold-down connections must therefore be designed and verified, and they must be executed exactly as drawn, because there is no redundant mass to compensate for a missing one. The second point to watch is regularity: like all lightweight systems, SIPs work well when the layout is compact and openings are distributed sensibly, and much less well in very irregular geometries.