PIR or Rockwool..

Hans_25 said:

PIR is more expensive and more difficult to fit, but provides superior insulation performance if done properly as Buster says. I'd go PIR unless you want a job done on the cheap.

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But is PIR more expensive & difficult when you consider everything. Much also depends on if you are doing it from underneath or have the boards up.
My logic;
1) you need double thickness of wool for same insulation or PIR
2) you need to draught/moisture barrier under boards with if using wool, otherwise you'll get draughts
3) you need to compress wool slightly to ensure under floor insulation remains in contact with the floor forever
4) you need to rigidly support the underside / seal the ends of wool to stop sagging, plus cap with something windproof otherwise you'll get wind wash
PIR does all the above for you, plus it has the silver reflective side (however good that is)!

With PIR you can knock long clout nails in the sides of the joist and drop the PIR in to rest on them so the board is raised just 1mm above the joists, foam spray any gaps (try doing that with wool), then screw the boards down and the PIR compresses/self levels on to the long springy clout nails, keeping the PIR pressed onto the underside of the floor boards. BC suggested nails, but I recently used some old curved/springy bed ply support laths screwed on the underside of joists to ensure that the PIR always pushes up with the spring of the bed lath and touches the floor boards forever.

100mm of PIR is practical but 200mm of wool in 100mm joists can be awkward!

But is PIR more expensive & difficult when you consider everything. Much also depends on if you are doing it from underneath or have the boards up.My logic;1) you need double thickness of wool for same insulation or PIR2) you need to draught/moisture barrier under boards with if using wool, otherwise you'll get draughts3) you need to compress wool slightly to ensure under floor insulation remains in contact with the floor forever4) you need to rigidly support the underside / seal the ends of wool to stop sagging, plus cap with something windproof otherwise you'll get wind washPIR does all the above for you, plus it has the silver reflective side (however good that is)!With PIR you can knock long clout nails in the sides of the joist and drop the PIR in to rest on them so the board is raised just 1mm above the joists, foam spray any gaps (try doing that with wool), then screw the boards down and the PIR compresses/self levels on to the long springy clout nails, keeping the PIR pressed onto the underside of the floor boards. BC suggested nails, but I recently used some old curved/springy bed ply support laths screwed on the underside of joists to ensure that the PIR always pushes up with the spring of the bed lath and touches the floor boards forever.100mm of PIR is practical but 200mm of wool in 100mm joists can be awkward!

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Insulation is the most practical and cost-effective way to make a building more energy efficient, keeping it cooler in summer and warmer in winter and providing substantial savings in heating and cooling costs. It also contributes to reducing noise levels.

Polyisocyanurate (PIR) insulation boards are rigid insulation panels consisting of a closed-cell foam core laminated on both sides to a facer. Of the many different insulation products available in the market, PIR boards have one of the highest thermal performance per millimeter. This means that a thin and lightweight PIR board can achieve a similar insulation value as other thicker, heavier insulation products.

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