Posted By yetanotherjohn on 15 Mar 2012 02:14 AM
The point on the heavier roof is well taken. Of course, a steel SIP panel falling on you isn't going to make your day either. The home would be in the new madrid earthquake fault zone. I hadn't really been focusing on earthquakes much. From what I can find out, properly reinforced concrete can be pretty good at resisting earthquakes. A lot of the assumed features of the building (e.g. shape) are already optimized for earthquakes. The problem is that "properly reinforced" essentially means more rebar which increase cost of material (more rebar) and labor (to install the rebar).
Ultimately, the structural engineer is going to have to lay out the options here. Part of this is the calculation of the chances of a fire (probably highest) vs tornado vs earthquake. But I appreciate the point you make as one more dimension to consider.
As far as the air barrier goes, you right that a steel SIP can be sealed correctly and make a good air barrier. But that is assuming it does. The cast in place concrete roof and wall naturally makes an air tight barrier that depends a lot less on the level of craftsmanship of the installers.
On thermal bridging, my understanding was that the steel SIP had steel connections between the two steel panels of the SIP. As such, to the extent there is a steel connection between the two sides of the SIP, temperatures on one panel will bleed through to the other panel no matter how much insulation is between. O the other hand, I haven't study metal skinned SIPs much, so they may have a way to beat the thermal bridge.
What seismic category will the home be in? Category "D" or "E" or "F" ?
Ideally you want a lighter weight roof in an earthquake zone area. In a tornado area, the concrete roof would be better.
A steel SIP does not have steel connections between the panels. The EPS foam is sandwiched between steel panels but the two panels never connect. They are open ended.
The steel SIPs connect via a T&G connection. The steel SIP is very air tight, they don't let air transmit between them. There is 6-12 inches of EPS foam and two steel panels. Air does not get through, even at the T&G connection.