slate roof insulation

[robert a]

Hi

i have a cottage built in 1890 which has a slate roof on sarking boards ..... a Scottish thing.
it has two upstairs rooms with dormer windows essentially a loft conversion.
the rooms are lath and plastered on walls and ceiling which i have stripped off exposing the stud work , rafters and underside of the sarking boards.
The room has never had any insulation and as you can imagine is fairly nippy in winter, so before i re plaster board i would like to add some insulation.
the rafters are approximately 6 inches deep
It hasn't got to meet any standards and has to be on a budget, just wondered what best option is PIR , muli foil , rockwool ect ?
ideally want to keep within the rafter depth rather than going over the front as this will encroach on head room , thinking 75-100 working on theory that something is better than nothing.
would i need a vapor barrier between the plaster boards and insulation ?

Thanks in advance

 

[turpis]

In terms of effectiveness, PIR is king (U value ~.022), then Rockwool SLABS (~.038) then standard loft insulation roll (~.044).
In terms of budget it's as you'd expect, PIR is the most expensive, insulation roll is the least.
And in terms of installation PIR is the most tricky to install (need to cut accurately, fit tight against rafters / fill gaps with foam), Rockwool slightly less but still difficult because the slabs are rigid, loft insulation roll easiest because you can cut a little over and tuck it in.

And really, if you've got nothing there already any of them are going to make a big difference.
I wouldn't worry about a vapour barrier as it's a bedroom so low moisture, assuming you don't keep the windows closed all year and dry your washing in there.

 

[catlad]

Pretty similar to turpis
I would use 100mm rockwool slabs between the rafters and put 25mm pir over the rafters or more if possible, then plasterboard.
If you tape the joints in the pir before plasterboarding, then you will have a vapour barrier.