Thickness is the number on the brochure. What actually decides comfort is how the material behaves when the wall gets damp and how hot the house gets in August.
Mineral wool is the default for good reasons
It is cheap, non-combustible, fills irregular cavities without gaps and lets vapour through so a wall can dry. Its weakness is that it does nothing for summer comfort: it is light, so it stores almost no heat.
It also settles if it is not restrained properly in a wall that travels by road. Ask how the batts are held, because a settled cavity leaves a cold strip along the top of the wall that nobody sees until the thermal camera does.
Wood fibre buys you the summer
Wood fibre is three to four times denser than mineral wool, which means it takes far longer for outside heat to travel through it. In a lightweight compact building that phase shift is the difference between a loft that is usable in August and one that is not.
It costs more, weighs more and needs the same ventilated cavity behind the cladding. For a house that will be lived in year round rather than let in the shoulder seasons, it is usually the better money.
Rigid boards buy thickness back
PIR and phenolic boards deliver the same resistance in roughly two thirds of the depth, which matters when every centimetre of wall is a centimetre of floor. The catch is that they are vapour-tight, so the wall build-up has to be designed around that, not adapted to it.
They also need careful taping at every joint. A rigid board wall with sloppy joints performs worse than a properly filled mineral wool one, whatever the datasheet says.
The vapour layer is part of the insulation
Insulation only works dry. A vapour control layer on the warm side, continuous and properly sealed at every penetration, is what keeps it that way. One badly taped socket box undoes a metre of perfect wall.
Ask to see photographs of that layer before the wall is closed. It is the single most informative image in a build record and the one thing you cannot inspect afterwards.
Floor and roof are not the same problem
The floor cassette sits above a ventilated void and loses heat to moving outside air, so it wants the same treatment as a wall. The roof takes the summer sun directly and is where phase shift earns its keep — the argument for wood fibre is strongest there.
Ask for the build-up of all three separately. A specification that quotes one figure for the whole envelope is a specification that has not been thought about.