Hi Lukasz - over 50 have read your posting and none have responded - perhaps an indicator of the question needing some clarification.
I am assuming that since your login includes "IOSH" you have a reasonable understanding of the Confined Spaces Regs and the definition of what constitutes a "confined space".
This means that you then need to analyse the specific scenario and consider what if anything about the work presents a "specified risk"
(a) serious injury to any person at work arising from a fire or explosion;
(b) without prejudice to paragraph (a)—
(i) the loss of consciousness of any person at work arising from an increase in body temperature;
(ii) the loss of consciousness or asphyxiation of any person at work arising from gas, fume, vapour or the lack of oxygen;
(c) the drowning of any person at work arising from an increase in the level of a liquid; or
(d) the asphyxiation of any person at work arising from a free flowing solid or the inability to reach a respirable environment due to entrapment by a free flowing solid;
So, you sheet a scaffold which is presumably in the open air. If in the UK at this time of year, very unlikely to result in heat stress, except as a potential result of putting somebody into lots of PPE, e.g. if they are dry blasting and need to wear a full suit and blasting helmet - but that is probably off limits for COSHH reasons.
I'm guessing that paragraphs (c) and (d) of the definition are unlikely to be relevant.
This leaves paragraph (a) - fire and explosion and (b)(ii) - aspyxiation due to oxygen deficiency or being overcome by toxic vapours.
How gas tight your sheeted scaffold is will affect natural ventilation within the enclosure, as will what you are releasing into the local atmosphere when either blasting or spraying, particularly if the latter involves flammable/toxic materials.
You could possibly ignore the Confined Spaces Regs entirely and simply apply as necessary DSEAR for the flammability risks and COSHH for everything else.
....or even ignore all subsidiary Regs and simply ask what are the risks and what precautions are "reasonably practicable?"
Inclusive of thinking about how to rescue someone who is incapacitated for WHATEVER reason.
Good luck, Peter