I've noticed that some people on here advocate the use of XPS below ground level - from the look of this, it looks like that needs to be CO2-blown XPS, otherwise it'll be doing more harm than good from the look of this:

http://www.greenbuildingadvisor.com/book/export/html/17147

... unless anyone knows differently, and you can now get lower global warming potential (GWP) XPS without dropping the k value too?

Following up on this very old post, I was horrified today to find that in the US they are (or were until remarkably recently) still using HFC to blow XPS. It was banned in 2015, but they argued an exception till 2021 and even that has been stayed except in some states that got fed up of waiting and made their own legislation.

I thought this had stopped about 15 years ago when PUR stopped being made with HFCs and switched to pentane.

It looks like manufacturers now use HFO instead so the GWP goes from 1400 to 48 or so. I presume this changed in the EU some time ago.

https://www.greenbuildingadvisor.com/article/xps-makers-announce-more-planet-friendly-formulations and
https://www.greenbuildingadvisor.com/article/extruded-polystyrene-insulation-with-low-global-warming-potential (from 2021)
explain the details of the legislation and the manufacture, but fail to find out what Kingspan use (which I suspect is HFO from their website saying 'GWP <50').

https://www.greenbuildingadvisor.com/article/choosing-low-carbon-insulation (also from 2021) has some pretty scary numbers for GWP per 1m2/R(si)
XPS (HFC) 46
XPS (HFO/HFC) 8.8
PIR 2.3
graphite EPS 3.5
rockwool 3.2
woodfibre -7
fibreglass 0.7-1.6
cellulose -0.8 to -2

So you _really_ don't want to be using HFC-blown XPS. It's not clear from the article if any manufacturers are still doing this, in the US or elsewhere, or if it is finally dead. But it's depressing to see how long they persisted in this practice despite alternatives being available.

One wonders why pentane or CO2 can't be used for XPS blowing the way it is for other foams?

It seems that XPS blowing agents with a GWP <1 are now available: https://sustainability.honeywell.com/us/en/applications/foam-blowing-agents/xps-insulation

And as someone mentioned in the comments: does HFO degrade to a PFA?, in which case this is really just swaping a really bad air pollutant for a really bad water pollutant. XPS is marvellous stuff as a building material, but at the very least one should be reading the EPDs carefully and not letting builders and surveyors do random substitutions 'because it's all the same'.