Since air is such a good insulator, then why wouldn't it make sense to disregard the use of any kind of foam that is conventionally used inside of SIPs and instead just have a giant air gap between two completely separate steel envelopes? What I'm describing is, for example, a 40' x 40' building placed inside of a 50' x 50' building. I chose a steel building as an example only because they seem to be the absolute least expensive rigid structures that are available.

The problem with air as an insulator is that if gaps between two parallel surfaces are wide enough the air within the gap can develop convection currents. Those convection currents can very quickly and very efficiently transfer thermal energy from the warm surface to the cold surface.

When dual (or triple) pane insulated glass assemblies are constructed, the typical gap between the window panes is limited to less than 1/2" and then filled with argon. The thin gap is an attempt to eliminate convection currents and the argon is used because it is a better insulator than air. And even using those techniques to improve performance, dual pane glass assemblies still provide only a fraction of the thermal resistance of 3.5 inches of EPS foam.

Right, you wouldn’t want to use air…you would ideally want to have a vacuum to reduce the heat transfer…like a thermos bottle. More importantly, it is largely the foam that is responsible for SIP structural strength…much like the web of a steel I-beam is largely responsible for I-beam structural strength. This is why some folks worry about how SIP structures will fare when they are subjected to fire or the foam panel adhesion just degrades over time.

You may want to recheck your sip foam and steel web analogy. Neither one is the source of strength. Both the foam and web serve to keep the sources of strength (skin and flange) in a position to provide the most strength.

Posted By syedkarim on 30 Sep 2014 12:18 AM
Since air is such a good insulator, then why wouldn't it make sense to disregard the use of any kind of foam that is conventionally used inside of SIPs and instead just have a giant air gap between two completely separate steel envelopes? What I'm describing is, for example, a 40' x 40' building placed inside of a 50' x 50' building. I chose a steel building as an example only because they seem to be the absolute least expensive rigid structures that are available.

Building two buildings is not cost effective

You may want to reread what you wrote Torben. What you wrote is precisely correct. Without the foam or web maintaining the position of the structural elements, there is little structural strength. So if the adhesive or foam fail in a SIP, the SIP structural strength quickly approaches nothing.

EPS is 95%+ air