Cellotex contains glass fibres, whereas polystyrene contains only polystyrene, so the latter would be my choice out of the two.

In theory there are places that recycle polystyrene, although I seem recall another discussion here where most of the supposed recycling points weren't interested (unless you delivered it to them?).

Posted By: fostertomI was asonished to read that PIR biodegrades

You're not confusing Post Industrial Recycled waste with some old insulation by any chance? If not, I'd be interested to read the article.

The best way to cut polystyrene is with hot wire cutters - it’s dust free, and leaves a smooth surface not a bobbly one as when you cut with a saw, so you can avoid sanding too. You can buy them in various shapes, they’re also pretty easy to make.
Nichrome wire, 0.4mm thick, is a good starting point. You can even get rectangular sections of wire that can form small curves. Thicker wire snaps less, but needs higher current, longer wire needs more volts. You need a beefy power supply (the expensive part) - best is to buy/borrow a ‘bench’ supply that is adjustable over maybe 0-60v 0-10A. I have one that is configurable 0-30V 0-6A or 0-60V 0-3A, which is good enough for approx 30cm lengths of 0.4mm wire at around 5A, and 100cm ish lengths of 0.3mm dia wire at around 3A. Get the wire orange hot, then back off a bit.

I would try using extruded polystyrene, XPS

Posted By: tonyI would try using extruded polystyrene, XPS

+1
XPS would be my choice too, Styrofoam is widely used for sculpting.

Posted By: fostertommicrobes found that can digest PUR

Very many different molecules can be joined together with urethane bonds, also called carbamate, so there are very many different kinds of polyurethane. Some are easily biodegradable.

Unfortunately PUR insulation boards, are not (yet) a biodegradable kind of PUR.

In future, insulation might be made of a biodegradable version of polyurethane, then microbial enzymes could be used to 'melt' it down for reuse. The excited article in a certain newspaper last week about 'microbes that eat plastic', didn't really explain this! Science journalism is not their strong point any more ...

Share the sentiment, but don't hold your breath for it!

Man has been making and discarding steel and other metals for 1000s of years. Thermodynamics says there's a lot of energy tied up in steel, which an enterprising microorganism could swarm all over and gobble up.

But there still aren't m/any organisms that eat steel, afaik. Biochemistry says that it's difficult for an organism to get its digestive chemicals into steel.

Same with plastic. There are millions of different kinds of plastic, all with energy tied up in them which bugs could feast on, but many of the common ones are biochemically difficult for a bug to take a first bite out of.

Some biodegradable plastics have chemical features which are also found in natural polymers (wool, proteins, cellulose etc) so offer bugs a way to get started on digestion.

Some kinds of poly 'ester' or 'ether' are promising. The newspaper article about 'bugs eating plastic' was actually about a polyester-based version of PUR.

I doubt anyone recycles polystyrene dust. It will mostly be vacuumed or swept up and be mixed with other workshop dust.

There are some cellulose based insulation foams but I don't know what availability is like.

There are also some wood fibre based insulation products that might work but harder to use for the OP...

https://www.ecomerchant.co.uk/walls/insulation/wood-fibre-rigid.html

I think in the OP case I would sweep/vacuum up the dust, and put it out for general rubbish collection. Sad but.

Posted By: CWattersI would sweep/vacuum up the dust, and put it out for general rubbish collection. Sad but.

Not too bad - in future I'd expect that landfills will be re-quarried for all sorts of materials, as automated sorting becomes more able to distinguish and collect small quanties of an ever-growing range of materials incl different plastics, and economic. So landfilling much better than burning for energy.