I should edit that- I meant that even with a poorly fitted window or 50+ mile winds, the closed cell nature of the concrete doesn't let air circulate through it

I should edit that- I meant that even with a poorly fitted window or 50+ mile winds, the closed cell nature of the concrete doesn't let air circulate through it, while fiberglass does. Such is, my point was that foam boards and concrete are compared to blown / rolled fiberglass, even though the fiberglass has more failure modes for insulating. A poorly fitted window will seriously compromise the R value of the surrounding fiberglass. As for the carbon footprint of cement, solid is pretty high, foam not so much. I don't know about commercial stuff, but most DIY'ers seem to use a little under 100 lbs portland cement to make a 55 gallon barrel's worth once the foam is mixed in. Rough math puts a 55 gallon barrel of concrete at about 1000 pounds. Edit: I just realized that 10% seems low, so what I saw may have used a 55 gallon barrel but not completely filled it. In any case, depending on the structural integrity that you need, the weight reduction (and therefore carbon footprint from curing) is significantly reduced.