room- roof insulation

[Alfred]

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 

[scottishjohn]

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 .

 

[Alfred]

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?

[scottishjohn]

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 

 

 

[Iceverge]

Mineral wool every time. 

 

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

[Alfred]

thank you guys .