Hi All. First post here and trying to get some opinions. We are about to purchase a Cat 3 listed house in Scotland and am trying to work out what materials will work best for different elements of the house. Listed consent issues aside for one part of the house there is a single story cottage, with rubble stone walls and a vaulted ceiling, currently un-insulated.

I am trying to work out what approaches for insulation might work best and wondered whether the following 'hybrid' insulation strategy for the the roof might work:

50mm+ ventilation gap to slates/sarking
100mm flex woodfibre between rafters
breathable membrane under rafters
50mm celotex under rafters (apart from woodfibre perimeter to avoid impermeable insulation touching wall)
plasterboard

All 4 walls have no internal insulation and are limewashed rubble.

I would also plan to create a small curved coombe ceiling to maintain an air gap at the apex between both sides of the roof. Depending on the exact structure of the roof covering I could also add additonal conservation vent slates assuming LA happy with this but that will depend what's under the slates as to whether its necessary. Additonal ventilation provided in the warm space wil be via 2 new velux windows.

Does this construction look ok?. I'm trying to get a good balance between cost and performance but happy for any comments!
Thanks

Welcome Den,

Similar query to Ed's, plus I don't fully understand this:

''50mm celotex under rafters (apart from woodfibre perimeter to avoid impermeable insulation touching wall)
plasterboard''.

What do you feel is the issue with the impermeable insulation touching the wall? As the walls are un-insulated and potentially subject to condensation I think I would be more worried about the wood-fibre touching the walls (it may be water-vapour permeable, but my feeling is that it won't be helped by liquid water, which it could then transfer, like a sponge, to the plasterboard). I could be wrong!

I think if it were me I would be treating the face of the Celotex as a vapour control layer -VCL- and taping at all joints and perimeters. Some people feel it's OK to have the VCL a little bit into the 'sandwich', although I usually do it 25mm in when I have to, not more than that (just a 'gut feeling' on my part), but as Ed says, that would mean having a VCL under the wood-fibre, not a breathable membrane.

And I echo Ed's feeling that maybe the breathable WF is odd in what has been made a non-breathable set-up by the Celotex. However I suppose one could argue that, if the VCL were less than perfect the WF could do its 'moisture-buffering' job prior to passing the WV into the ventilation gap to dissipate harmlessly.

If you are putting a VCL between the Celotex and the plasterboard why go to the trouble of taping the Celotex joints?
Any reason for using Celotex rather than one of the other insulation boards used for insulation e.g. EPS
What is the rafter spacing? Anything more than 60cm will risk the plasterboard sagging over time. The solution would be to cross batten the rafters at the insulation joints (which I presume are 50cm wide slabs) then fix the PB to the battens.

I am planning a similar hybrid roof insulation system. Wood fibre batts between rafters and Celotex below. My reason was because a) concerns about future gaps around the celotex between rafters and also lack of breathability in an old roof, and b) I couldn't use a full wood fibre build up and still meet Building Regs because of the head height constraints in the loft. Actually the roof still doesn't meet Building Regs but Building Control approved it based on a whole house SAP calculation.

I am also hoping the wood fibre will offer some improvement vs Celotex on buffering the summer heat.

My vapour barrier will be under the celotex.