I'm in the early stages of planning a new house, and am trying to select the best roof option. I'm leaning towards steel SIPs, but have been having trouble finding any details on how to mitigate thermal bridging from the exterior along the steel skin. If a steel SIP roof panel is extending out forming an eave, the lower skin would provide a low-resistance route for heat to enter and leave the building. Is there a standard way to mitigate thermal bridging in metal SIP panels? One option might be to bevel the end of the panel so the lower skin doesn't extend outside, but I don't know if this is viable. The only other option I can think of would be to insulate the underside of the eave on the exterior of the building (with some sort of similar exterior insulation at the gable ends). I'd appreciate any thoughts on how this problem can be addressed.

Some steel SIP companies allow the bottom steel skin to be saw cut where it bears on the top of the wall. This breaks the path of conduction while still leaving the bottom skin that extends beyond the wall. If the company does not allow the bottom skin to be cut, then the option you mentioned about insulating the bottom of the SIP should suffice. However, the soffit (overhang) will appear thicker. Will you be building in the southeast where steel SIPs are available?

If I understand correctly, you're saying that rather than bevel cut the panel to keep the bottom skin inside the outer wall, the bottom skin would just be removed near the end of the panel and only the foam and top skin would be extended outside. That seems like a viable solution, though I suspect it might require some additional structural support for wide eaves. When you say that the SIP companies allow this, do you mean that it would be a factory cut made to the delivered panels, or would be cut on site by the local installer? I'd most likely be building in the mid-Atlantic in Maryland or Northern Virginia but I'm very early in the planning process. I was assuming steel SIPs were available throughout the country but it sounds like that might not be the case.

My suggestion assumes that you want the steel SIP panel to extend out enough to make a soffit (for example, a soffit of: 12", 16", or 24" etc). If so, then the bottom metal skin of the roof panel can be saw cut on site where the panel hits the middle of the bearing wall. The only metal skin removed is the width of the saw cut (1/8" ???). The saw blade depth should be set to the thickness of the metal skin plus about 1/16" so that the foam in the panel is saved as much as possible. .... The roof panels are attached and supported with matching metal from the SIP supplier screwed to each side of the wall and the bottom side of the roof panel. The metal must be bent to match the angle created by the intersection of the wall and roof panel. (Two pieces of bent metal is used to attach the roof panels to the wall - one piece outside and one piece inside.) This detail works well when the bearing wall is also steel SIP. If your wall is something other than steel SIP, then there must be some material on each side of the wall to accept stitch (short) screws on about 6" centers. .... This detail eliminates screws through the top skin that might leak and reinforces the cut panels. .... Some SIP companies may be able to cut the end of the roof panels to create a vertical eave, otherwise, field cut. The end of the panels can be capped off with matching metal from the SIP company. The upper edge of the bent cap should be slipped under the top skin of the roof panel. The bottom edge of the bent cap should be below the bottom skin of the roof panel. A drip edge may still be needed. .... Get approval from your local engineer or the SIP company before using any of my suggestions. .... Use my email address as shown in my signature below each posting for future questions. Use the @ symbol for "at" and period "." for dot.

Thanks Alton. I really appreciate your help with this. I hadn't even considered the idea that all you really need is a thin cut in the steel skin to break the thermal bridge effectively, so that definitely seems like a great way to handle this.