Q: I understand the carbon footprint for the plastic foam insulation is pretty high. 475 Building Supply and PHIUS are recommending cellulose and wood fiber bd which can be carbon sequestering. Short of that they are also recommending ROXUL, much lower carbon footprint.
A: Spray-foam insulates well and hence helps to reduce global warming. However, the gas used to make the bubbles in the foam is often a gas called a hydrofluorocarbon or HFC. If these gases sound familiar it may be because of their cousins, chlorofluorocarbons, or CFCs, which became infamous for causing the hole in the ozone layer. CFCs were banned in 1996. They were replaced by hydrofluorocarbons, which don’t deplete the ozone layer but were later found to cause global warming. Hydrofluorocarbons can be about 1,000 times as strong as carbon dioxide in causing global warming. Hence, installing spray-foam has a high carbon footprint even if it insulates well. Because of this, some companies are now using hydrofluoro-olefins or HFOs to make the bubbles in spray-foam insulation. HFOs have very low global warming potential. If you are going to use spray-foam insulation I recommend these HFO-blown foams.
Rockwool boards are also great insulation and are fireproof, which is distinct advantage over sprayfoam which burns with a thick black smoke.
Dense-packed cellulose is also a good insulator. However, I am unconvinced by the argument that it is sequestering carbon out of the atmosphere because that wall will eventually be demolished and either decompose in a landfill or get incinerated. Hence, its carbon is returned to the atmosphere. I think it is more accurate to say that using dense-packed cellulose delays the carbon-dioxide emissions compared to the tree falling down and rotting in the forest, but I think it is inaccurate to think that it is permanently sequestering carbon dioxide in the sense that other carbon-sequestration technologies (such as turning carbon dioxide into rock and burying it underground) do.
All types of foam, whether sprayed or boards, have similar end-of-life issues as dense-packed cellulose and are likely to get incinerated (releasing their carbon dioxide) or buried in a land fill (where the carbon will be permanently sequestered) when the building is demolished.
Overall, I think that fiberglass, rockwool and dense-packed cellulose (treated with a fire resistant coating) are the best insulating materials, because they do not have the global warming potential of releasing HFCs into the atmosphere like sprayfoam does, they are fireproof, and they can easily be separated when the house is demolished and recycled or reused. On a renovation we are going to be doing shortly, we plan to use rockwool boards outside the air-barrier membrane (as the thermal-bridging prevention layer) and rockwool batts in the 2”x6” cavity walls for the insulation. On a different renovation we are planning to use dense-packed cellulose and rockwool boards. See my other answers to related questions on walls, SIPs, and controlling moisture in general in houses.