Strip off the roof and put down 20cm SIP panels ontop of the massive timbers. Batten and tile.

Yup, take everything off down to the supporting structure. In theory you could leave the sarking and bitumen if its difficult to remove or the sarking needs to stay in place.
The sips come with a variety of finishes for the interior side. OSB, pine panelling, plasterboard etc. Depending on the distance of the roof span, the panels can be installed to be self supporting, anchored at the eaves.

How about a combination of mineral wool and multifoil? New generation products carry full BBA Certification, have Building Control and NHBC approval and can meet u-value requirements within the rafter space you suggest.

Hi Bella, It's not as difficult as you may think.

Have a look at http://www.euroform.co.uk/genx_applications.shtml for options.

Sounds good. Compressive strength is fine for downslope battening over. Who says fire retardant is necessary? - routinely done similar without.

EPS is reasonably vapour permeable, so you could later add any amount more of insulation below, and your initial above-sarking EPS shouldn't be resistive enough to create interstitial condensation. By rule of thumb the inboard vapour resistance should be 3 or 5 times stronger than the outboard resistance so ideally your later inner insulation should have stronger resistance than the initial outboard EPS, or else there should be a weak vapour barrier somewhere inboard. But you'd prob get away with it if the later inboard insulation was no less resistive than the initial outboard. - which means the later shd be EPS too, not sheep wool or anything less resistive.

Use grey/silver 'Platinum' EPS, not the white stuff - much better insulation for pro-rata price. Don't use any other type of plastic insulation - all the others are vapour impermeable. XPS (coloured stuff) is no better insulation than Plat EPS, costs much more and is impermeable.

Is it a vented loft under, or room-in-roof with plaster lining under the rafters? Will there be a bathroom or other water vapour generator?

Does the roof have parapet gables, or chimney stacks or other things that the roof has to be flashed against? i.e. is there a restriction on how high you can jack up the roof's top surface? If not, then why stop at 50 thk insulation? 100 over and 150 between rafters would be top-spec. Glue it to the OSB with spray contact adhesive, not more than 30% coverage, otherwise you'll seal off your breatheability. Foam fill the EPS edge joints as you go.

Make the OSB into your airtight (but vapour breatheable) membrane by gluing the joints. 9 OSB3 would be fine, all joints supported on a rafter or nogging, bubble-glued and screwed @ 150c/cs (polyurethane expanding gap filling glue as sold for glueing T&G chipboard flooring). Good to glue it to the rafters too. Or T&G OSB, which makes it 18thk - then edges won't need noggings.

What pitch is the roof? - affects how many/what kind of long fixings of downslope battens you need, thro the EPS and OSB. Make sure they all hit a rafter and don't merely puncture the OSB airtight membrane.

I use reflective type breather felt, 'draped' over, not flat under, the downslope battens. Shiny side down- adds worthwhile extra reflective insulation. Draped to not quite touch the EPS, joints well taped to keep wind from blowing on the EPS.

Thanks Tom, thats just the reassurance I was after..

It was actually the manufacturer who responded to my initial email enquiry regarding the compressive strength, to say that their standard EPS 70 (the stuff off the shelf in the local yard) was not suitable for external, due to no fire retardant - which did strike me as odd, as by the time that lack of fire resistance would become an issue the roof would effectively be gone? Even if it did burn, it would only result in the battens sinking on to the OSB below? I'm not sure if this is just a liability limiting response?

There are a few challenges with this job, the first being that it was fairly unplanned, the original roof was probably 200+ years old, and basically just powder held together by wood worm and fungus. Due to a sequence of events outside my control it was now or never to rip it all off, which means I haven't really had time to get all the materials I would like organised (being "off shore" it takes time and planning to get the things you need without paying through the nose if they are anything other than standard fair). The other problem is that there is no money and being water tight is the only priority for the client. I will be paid eventually, but it could be a while so I'm just trying to strike a balance between the best thing to do for the long term future of the house and its occupants vs. not going bust!

Architecturally there are also some restrictions, chimney stacks, gable detailing, etc, so 50mm is the best we can hope for in the current situation. I was just trying to figure out if I was going to be doing more harm than good in my enthusiasm to get something on it while we had the chance. I have no idea if it will be insulated in my lifetime, almost certainly not by the current owner, so I was just trying to make sure I did the best for whoever eventually comes to finish it off. Roof is aprox 34 deg, was thinking just to use a standard heavy gauge screw 150mm, down through batten, EPS, OSB (12mm SmartPly - off the shelf). Current configuration is rooms in roof, no bathrooms, but then the house hasn't changed much in the last 100+ years, so there isn't really any bathroom to speak of. Eventually I would assume some sort of bathroom between the two main rooms (in the same bay as the stairwell), but again this is an emergency measure I'm just trying to make the best of.

So given the inboard to outboard resistance rule of thumb, if the rafters were eventually filled with more EPS, then a foil backed insulated plaster board, the condensation risk would be fairly low?

With regards the eaves detailing, I have rafters coming down on to a timber wall plate set over a new RC wall beam, the original roof was just laid in to the rubble at the top of the roof, no wall plate, no straps, just sitting there for 200 years. Any suggestions on the best way to terminate rafters and insulation over a solid masonry wall 600mm thick?

Rafters and OSB is already on (OSB sealed with silicone), it was a race to get it all done in a week, its just what to do next...

Is there much advantage to spray glueing the EPS on to OSB?

Not likely to find foil backed membrane in the local merchant, any tips? I'm a membrane on to sarking man usually, then counter batten over, what do you think is the advantage of draping?

Many Thanks,

James.

THe fire retardant is just that. It does not make the EPS fireproof. Its good practice to use the FRA grade where large volumes of the product are to be stored together e.g. obviously at the stockists but also on site. It is certainly advisable where large areas of the foam are exposed during construction. Roofing would be a very good case. Once the construction is finished there's no reason it would be anymore of a risk or contributor in fire than any other plastic foam

At 34 pitch, such screws prob fine - lots of, as cantilevers to resist all slate/tile weight sliding down the roof. Also fix opposing battens together where they meet at apex. Spray glueing the EPS keeps it in place meanwhile, and allows its edge joints to be foamed, which enables full potential of insulation without multiple open or micro-gap convection by-pass weak spots. If not shiny breather, then yes, flat on the OSB. Foil backed pbd is a total vapour barrier, so is overkill if you want the whole to be breatheable construction. If not breatheable, i.e. with strong inboard VCL, then you might as well use impermeable higher performance plastic insulation e.g. Cellotex. EPS gives best bang for buck even tho thicker, but if thickness is constrained ...