Iceverge

Toe nail them to get them in place, then 150mm screws would be my idea but I'm not a carpenter. I would run them at 90 Deg to the rafters. Alternatively you could just nail gun 45mm service cavity battens. And extend the 220mm rafter by the same on the other side of the membrane and use more cellulose and less mineral wool.

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. How about this. Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

Rather than rafter thickness do you have an overall thickness of the roof please? When considering 0.12 Vs 0.13 might be worth sitting down and doing some calcs to really get a handle on the little difference it will make. 0.01w/m²K will really pale into insignificance Vs a little better airtighess or improved COP on a heat pump. Say 100m² of roof, a pessimistic average delta T of 15deg for the heating season of 4 months. About 2900hrs x 0.01w/m²K x 15⁰ = 435Wh or 0.435kWh. Run it through a heat pump at a COP of 4 and it'll cost you about 3p per year.

I remember a discovery documentary about rifle barrell design has remained static with 100 years after centuries of constant change. It basically reached an end point. Similarly, now nearly all cars have 4 wheels, wind turbines have 3 blades, every phone is a landscape shaped touchscreen. Unicycles and blackberrys are rare. There is a practical finishing line. Building regulations are a study in slow slow creeping incrementalism. They're heading towards PH and will get there evetually despite the standard being already 3 decades old. Why not skip to the end? Everyone else is trying to sell you one of these.

Discourse is very much shaped towards minimising climate change rather than coping with it. I wonder if it'd be a better use of brain power for most of us.

What U value do you need and what's the motivation for that particular number? How thick a roof build up can you afford? It sounds like you have some tricky junctions. As there'll be inevitably plenty of onsite carpentry a cut roof from off the shelf lumber would have lots of advantages. Time, material availability, cost and workability. However you'll have more significant thermal bridging (especially that steel) so a layer of continuous insulation somewhere would be helpful. What type is the wall construction and what stage is the build at currently?

Air con with a function for heating while you're at it. It'll be far cheaper to run in the winter than electric underfloor heating.

Interesting, what was the logic in going in so far? I assumes it was a done deal once the main untampered thread of the screw was gone beyond the inner face of the OSB.

The breather membrane is just to make sure that any water doesn't find it's way into the structure. The PIR if taped correctly is keeping the drips out anyway but belt and braces. Correct Intello is for airtightness.

This is a massively important point. Good airtighess is hugely hugely important to stop moisture laden simply blowing through cracks and condensing inside the wall. When you have good airtighess and planned ventilation you can get away with murder!

If you look towards the end of the article it shows the wall buildup. 100mm block 50mm vented cavity 50mm PIR Breather membrane 11mm OSB 220mm studs full fill with mineral wool. Intello Membrane 100mm service cavity with mineral wool Plasterboard and skim. Any moisture, even from construction, won't be drying outwards through a PIR sheet with two foil facings. It will have no choice but to go inwards through the Intello. I expect the buildup was approved on the basis that as a passivhaus the membrane sealing should be excellent. I would prefer no condensation in my wall and a continuous layer of EWI is a nice method to achieve this as it keeps the dew point outside the structure. It also makes a tremendous difference to the structures U value by mitigating thermal bridging. Either the house cools rapidly through uncontrolled ventilation in which case there's no differential vapour pressure or else when unoccupied there's no moisture being added internally. I don't see the realistic issue. These calculators assume that there's zero faults in construction and zero construction moisture. I would be wary of the layer of OSB sandwiched between the insulation and the PIR foil facing. In any case PIR performs poorly at low temperatures regarding its thermal resistivity. Some like EPS would be just as good on a freezing winters day. However it's susceptibility to fire has moved me to prefer rockwool or woodfiber.

Surely all that matters is that no localised condensation occurs. From this point of view I think EWI is beneficial, even one of low vapour permeability, as it keeps the structure above the dew point, therefore no condensation. In any case if you have a good airtighess layer, no internal vapour of note will get anywhere near the dew dew point of the structure regardless of the vapour permanently of the materials. Moisturise damage through diffusion just doesn't happen, it's all through poor airtighess.

I have come across external layers of PIR on TF structures like this one but with an active vapour control layer included. https://passivehouseplus.ie/magazine/new-build/longford-self-build-goes-certified-passive-on-a-budget They in effect dry inwards.

Of course it can, otherwise every car and train in the land would be full of mushrooms! Good luck finding a certifier willing to stake their indemnity insurance on the workmanship on a building site and the variable ventilation behaviours of the general public though.

Screws for me in this situation. Nails will have very little pull out resistance from an 18mm layer of OSB. Much less than a solid rafter for instance. The only exception I might think of is if you were to glue the battens to the OSB before nailing but you probably (hopefully) have a membrane in the way.