How about re-thinking the dormer shape and extending the dormer rafters down into the main roof rafters and creating what some call a gable fronted dormer. With just a small vertical element at each end thereby creating a greater under tile area for insulation outside of the original side dormer. walls.

A few thoughts that occur to me;

- sheathing would provide racking/strength to the side panels, which will be lost when you remove the tile battens, so I would OSB them. The OSB could be added internally, if you felt that was a more vapour open buildup.
- I sense there is "some" risk, in that you would be building a timber frame with no ventilated cavity, but it's a relativey small area, albeit exposed at roof level.
- a major part of the role of the VPM on timber frame construction, is to protect it from rain during construction, whilst waiting for the outer skin. If you'll immediately add the EWI, then the VPM will not add anything.
- a lot of the issues with heat loss at dormers is the junction elements (ie, where it butts the rest of the roof). So adding the EWI will tend to cover the main roof rafters, and elements of structure immed around the roof penetration, so that's a bonus (assuming you take the EWI tight to the existing roof, and relocate the flashings out to the EWI face (or close to it).
- insulating the void above the dormer could include some rigid insulation like PIR/PUR at the roof/wall junctions, where you won't get much wool at all, angling the rigid board and foaming in place. Then utilise wool, once you have enough depth to make it effective.
- owlman's suggestion would alter the look of the property (better? worse?) and I guess would need local gov't approval of whatever form in Hungary? You would only gain internal space if you removed adjacent roof rafters, which will have structural implications.

@ GreenPaddy
I hadn't envisaged doing anything more than simply extending the dormer rafters either side and an obvious continuation of the dormer roof insulation. I assumed that upsetting the internal decor to fit the new gable dormer was a no-no.
The original side faces of the dormer would all stay the same albeit maybe with additional insulation. This would create two forward facing right angled triangles left and right of the old window. For those, re-use old vertical tiles?
Personally I wouldn't bother with local gov, I'd just do it, but that's me.

>>>>>>> "my doubt is whether the wall timbers would give enough support for the EPS if it was just foamed and mech. fixed to the timbers or would a backing of OSB be needed?"


I have just been through a similar-ish thought process, although not for a roof or with EPS.

I was looking at gluing sheets of insulation onto a timber frame, using beads of foam glue along each timber, and then putting a finish onto the sheet insulation.

I was convinced (by someone I thought was experienced) that the narrow ribbon of adhesive would tear off the face of the insulation. Instead a wide bed of adhesive was needed, eg onto sheathing.

I changed plans and cut the insulation to fit inbetween the timbers, this took ages. This messed up the plan to use the insulation boards for draft stopping (wind washing) and so meant a lot of filling gaps. The timbers turned out not exactly level, so there was more faff adjusting to get the finish to fit onto them, wouldn't have been a problem onto flat insulation.

I now wish I had stuck with plan A. I should have done a test panel to check if the insulation would stick to the timbers without tearing.

Your dormer cheeks may have to be treated as you would the bottom of an EWI'd masonry wall, in terms of protecting the insulation in the splash-up (in this case from the roof) zone. I have a dormer to do too, and thought of EWI and then more-or-less dismissed it because of these concerns. Of course there may be a work-round, but I have not thought of it yet.

It looks like OSB sheathing would be the better option giving more rigidity to the EPS and easier detailing for the flashing etc. One of the rooms is a bathroom so higher humidity here but as the surface area is small, as owlman said, take the hit on your vapour concerns.

I'm not too concerned about splash up or maintenance, the sides have some roof overhang and are a v. small triangle and the front is painted wood - render must be easier than that! The building faces S.W. so gets a good amount of sun so algae growth won't be a problem (the adjacent roof has 18 PV panels that produced 8755 kWh last year).

Hopefully it will be a fit and forget, unlike the existing timber T&G.

Walls current - 0.32 will be 0.17
Roof current 0.31 will be 0.16
Windows timber frame with Ug 0.7 stays as existing.

Currently heating with gasifying wood boiler with wood from my own (on farm) sustainably managed forest (overseen by state forestry authority), for the future A2W heat pump fully funded by additional PV.

So the question arises - how much carbon (or/and money) do you invest to reduce the footprint of todays heating or the future heating plan?

Posted By: owlmanFor upgrading as part of otherwise required general maintenance, yes

This is where I am, re-tiling needed and something has to be put on the dormers so it might as well be insulation as anything else.