Hi, I am new to this group. I have a question about insulation to my roof in a loft style room. Please see pictures attached. Any advice what is better to use, 100 PIR or rockwool slab 100? Many thanks. Alfred 

to get same insulation with rock wool as PIR you would need at least twice the thickness ,maybe 3 times the thickness depending on type -look up the "u" values"

 so to me it would be PIR  -then another vapor barrier before the cladding .

 

do i need to put any vapor barrier on rubber first? or just PIR straight away? do I need to leave some space for air circulation between PIR and cladding?

if you are happy that the black stuff is water proof ,then fit vapour barrier below insulation just before cladding so it stops any moisture from occupation getting into roof space 

 i am guessing that black stuff is draped across the rafters and then counter batons above and then slates -

If so then that is supposed to run any moisture down the roof and out the eaves. and not be be totally air tight

pir will not absorb moisture  to any degree --

 if you wish to lave a gap it must be above PIR   and must be well sealed  making a cold space above it between it and the black stuff which is what you your roof is designed to have with that type of construction -its cold roof 

 

 

Mineral wool every time. 

 

Theoretical U values are only a small part of the equation.

thank you guys .