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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.
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Do your joists already have signs of damp/rot? Or do they already run in close proximity to anything that can cause water to wick/condense onto them? Are they treated, or do you plan to treat them?
Timber needs to breathe, but having 1+ face exposed would be adequate. I'd check the moisture level on the cold side of the timber now, in February and at the height of summer and would consider how to improve it if particularly bad (dry rot begins in timber with 20% moisture content).
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I'm using woodfibre batts mitigate this. If there's moisture, the woodfibre can absorb it and the well ventilated subfloor will dry it out over time.
With a hygrophobic material like PIR then 100% of the moisture present is transferred onto the joists.
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Howard
JonoMarshall
gillies
@Pantsmaster
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.