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A quick google finds some install guides.
The one I've read - linked above- insists that the PIR is fitted tight to the joists and supported using battens or brackets. Where a tight fit isn't possible, spray foam is used to close and seal the gap.
Suspect the answer to this is fit em really, really tight thus sealing them off, or leave a fuck off massive gap for ventilation which will reduced efficiency.
Howard
I have a load of PIR insulation board that I picked up for free that I’ve been planning on fitting between my ground floor joists to insulate the floor. I have access from below in the basement/crawl space so it should be relatively straightforward and I hope it’ll make a difference to warmth. Now I’ve got around to starting the job though, I’m second guessing.
Is it a good idea or am I setting myself up for potential damp issues with the joists? The space below seems to have fairly good ventilation with multiple vent bricks, I’m just worried I’ll cram this PIR around the joists and make them rot somehow.