I wonder if anyone would be kind enough to give me their opinion on the roof build-up I am planning to install in a new build single storey extension i'm currently building. The extension is a vaulted open plan living and kitchen area with pitched roof (circa 45deg's). I have 150mm s/w rafters and intend to insulate between them with (NBT) Hemp bats to the full 150mm, i will then batten the underside and fix 12mm gyproc plaster board to finish. On the upper edge i plan to install 60mm of Pavatherm Plus (T&G), followed by counter battens directly to the Isolair wood board (part of the pavatherm plus build up) and Spanish slate to finish. Firstly is this a very expensive option? Could i simply use the isolair board (T&G) at say 35mm to avoid cold bridging and take out the Pavatherm layer as i'll already have 150mm between the rafters? Will i need a dampproofing layer beneath the slates, a breathable membrane or a cavity? Sorry for the long message i'd appreciate any opinions.

Thanks Chris, coincidentally enough the house is also for my mum! And i'm not sure about the Hemp & Pavatherm for the same reasons.
The extra awkward bit for me is that the house is in Guernsey in the Channel Islands, the only Natural Building Products supplier is an agent of NBTechnologies so i'm fairly tied down to their products or the standard Builder's Merchant's gear from down the road. Trying to import stuff yourself gets very expensive. Same goes for the slates, although as you say it would be much nicer with reclaimed so i am actually looking into it.
Ideally i'm trying to avoid EPS &co. but i may not have a choice...

Guys,
Lookijng more and more into prices i think i'm also not going to be able to afford all that i'd like, i've cut the build up down to simply 150mm of hemp between the rafters with a layer of 35mm isolair on top, giving me a u-value of 0.24, not as little as i'd have wanted, but enough i guess. However it's still going to cost too much like this so i'm now considering 150mm of rockwool between and the isolair on top, i could also insulate the 25mm (or so) of service space beneath the rafters with rock wool to further resist cold bridging. I'm wondering what you think and wether it will get me to the same sort of u-value? It would cost a quarter of the price!
Any comments appreciated

I Oliver,
Knauf have a compression resistant mineral wool sarking board, that I use this on top of rafters. I use Rw3(rockwool) in the rafters.

On the underside of rafters you should have a vapour check to stop uncontrolled drafts, then a service cavity or I have used 40mm steico therm and routed out services.

Fit a vertical batten centred over your rafters on top of the sarking board and a permeable membrane on the vert batten.

Fit a horizontal batten. this layer should be vented at he top of the fascia at the gutters to let air in and using a vented ridge tile to expell, pay attention to the venting detail, sketches for your roofer are a must

Thanks for the response Rimu, however i'm hoping to build a completely breathable roof avoiding the use of venting at either the fascias or the ridge, as well as specific cavities. From what i can judge (and i'm fairly new to anything other than fairly standard techniques!) the good thing about the isolair sarking board is that it has both insulative and water proofing qualities (as well as being 90% recycled material) and comes with t&g edging (so if well put together fairly draft proof - and as i do all the work myself i like to hope that will be the case) It also allows me to simply apply one layer of batterns to take the slates. Have i understood right?????
My thinking on insulating the (horizontally lain) services cavity was to counter any cold bridging through the rafters due to fixings.
Re: the knauf products there is also the never ending problem of being in Guernsey to do this job and it's not easy to source materials from afar, the isolair seems to be one of the few products i can get from the uk relatively competitively.
Lastly, i need to commit to a set-up ideally today or tomorrow at the latest to not get held up with lead times etc.... Therefore as ever any feedback appreciated asap! cheers

Maybe I didn't explain the ventilation well, your roof will be water vapour permeable from the inside to the underside of the slates. You need ventilation in the vert batten layer to allow water vapour to escape otherwise you will get condensation on the underside of the slates.

Permeable membranes are relatively cheap and easy to fit, i'd put one on anyway.
TF200, or tyvek.

Oliver

I think you are correct in being able to use isolair in lieu of tyvek. It serves the same purpose as it is water resistant as well as breathable.
I think your solution is very green but I would be interested in the costs as when I looked at these products they were so much more expensive than other alternatives(slightly less green).

Please note that these products are all imported from europe which isnt that green.

Why not install your isolair on top of the rafter, plasterboard below and blow in warmcell in the void between the rafters. This will give you a comparable u value and will be easier to install than cutting all the hemp batts as well as probably being cheaper.

aye, just covering my bets.
I'm always very cautious with manufacturers claims.

The membrane is also for protection of the Isolair which is expensive, when it comes time to re-slate and batten,the membrane can be replaced.

Also with the t&g slabs take your time lining them up, only one or two fixings in the first row untill you get well up the roof, incremental error can cost alot of time.

There are many good membranes that are alot cheaper than tyvek.

Oliver Will your roof make up with just 150mm of rockwool get your required U-values?
I'm useing a similar system to Chris Wardel but with 150mm of phenolic foam (kingspan K7). Phenolic foam has about twice the R value as rockwool and with 150mm I get 0.18w/m2k U value on the roof (minimum required in Scotland)

Otherwise interesting to read about that isolair stuff, thanks will check it out.