Iceverge

Can you not lower the wall plate?

Raise the tie for the rafters, add 125mm PIR on the underside of the sloped sections, Full fill the rafters with mineral wool/cellulose and pur 400mm above the flat section. OSB above the rafters and batten and counter batten above for ventilation. Add a 100mm service cavity below the rafter tie for MVHR. Forget about any storage in the roof. It's simply much too small. Even with your original design it's only 1000mm high at the apex.

Out of interest @JohnMo, given that you were aiming for a higher level of performance would you do the kit again or make something up yourself?

It'll be fine if it doesn't tear. I had some Protect VP400 exposed for about a year on my garage wall.

The feel of how high a ceiling has to do with the ratio of the floor area to the ceiling height. Our 2.4m high utility (2.6mx3.3m) feels much taller than our 2.7m bedroom (5mx4.5m) so I wouldn't worry in your case. Nothing to stop you installing slim profile surface mounted downlighters. A wire with some acoustic sealant poking through the plasterboard is much less of a fire/noise risk than one punched through. Someone did a mass Vs cost chart ages ago. Standard plasterboard came out on top for noise. I'd be really careful about noise transfer on this one. Washing machines make themselves known.

Give me a cross section please of the house. I think I have an economical solution in mind.

If you do want a warm attic I would move the airtighess layer to above the rafters as a taped sheet of OSB that wrapped around the rafter ends and through the wall plate to the inner wall. Have you a plan view or your roof and a bigger cross section?

A 25 deg roof will be near impossible to insulate correctly to the eaves and airtighess will be a mare too. As @ADLIan says you could do a cold attic. You could still store stuff there. I would put an airtighess membrane and then a 20*70mm battened service cavity below the joists before a double layer of plasterboard. The MVHR could penetrate the membrane but you'd need to be diligent with taping the penetrations. Mineral wool or preferably cellulose over the top.

What's in the room above? For noise reasons ( washer dryer boiler) I would consider double boarding it and getting rid of the downlighters.

Congratulations. It's an exciting stage to be at. I wouldn't dismiss a good relationship between a builder and architect. There's a good chance one will design something buildable and the other will be able to execute it properly. All the problems I see from builder and designer being completely disjointed. Do you have a good idea of what you want yourself? Any styles you like or would be acceptable in the area. Asking a few for prelim design isn't unreasonable but you should have a look at their portfolio first and interview previous clients.

I use the Meteoblue app. You can step ten weather radar forward and get an idea if you'll get a bath.

Funnily enough like @G and J says I was thinking about someone who had retired. 3 day week of chippy work, plenty of tea, time to do it properly. I'd keep at that until they carted me out in a box.

Congrats. Any after pics....?!

Again, I would encourage you to bulldoze the original building. If you think it's making your life awkward in the design process just wait until you start building. This is my favourite version, although I always like to see what it would look like without the jewellery of plants, cars birds etc. For that we have chatgpt. With a view to an arts and crafts house I would consider altering a few small bits. I would "thicken" the roof profile, incorporating some overhang for the gables (better for rain protecting and avoids water staining of the wall too) The balcony railings on the left hand side I would change to a low brickwork wall, perhaps with some decorative patterns built in. The entrance doorway and glass I would simplify somewhat. The large glass window around the door is very office like. Some features like brick buttresses might be cool.

For reference I battened and counter battens the corrugated sheeting for ventilation. It happily made the roof lights more or less flush. I taped and staples the membrane appropriately over the flashing. Laid battles as required for the sheeting and cut it as square as I could to the edge. Seemed to work ok.

That's the back of my garage roof. I used the " just start sheeting and see what happens approach" as it was rarely going to be seen. I cut the corrugated sheet where it landed and moved on. I used the standard keylite flashing supplied. With some planning it should be possible to get the sheets to have equal gaps on both sides but for a shed I don't think anyone has ever noticed

I wouldn’t be too fussy either way. The steico boards should be watertight for a few weeks on their own and staple holes aren’t a massive issue if you have no rips. Best plan I think is to get your joiner to put the membrane on but keep the staples to the top of the membrane on the slope where they would be under the overlap from the upper membrane or put them in line with the rafters so they’d be covered with the counter battens later. Try to keep the staples “in the open “ to where there is an overlap of membrane. Probably complete over kill to be honest.

Flexible acoustic/airtight sealant or a square of airtight tape.

ICF is pretty excellent on its own though. Out of interest why not just screw it to the webs rather than dot and dab? Speed?

I'm a wet plaster fan, primarily for airtightness but also it gives shoddy workmanship less places to hide. You can see and patch settlement cracks easily too. I'd be interested to hear had anyone gotten to passive levels with boarding out. I suppose there's no reason it couldn't be done but any dry lined houses I've been in have had a breeze out of the sockets on a windy day.

I've used left over 75mm MVHR ducting for ducting some services in my garage. Would this suit?

£100k I would have said. Plans would help.

That's actually respectable. Is there an advantage to downgraded to 3?

Have the trusses been ordered? Can you provide a cross section?

What kind of airtightness numbers do you normally see?