Redbeard

I think you really, definitely, want an air/vapour-tight layer at FF ceiling level. Detail it with say Intello membrane (See Buildhub Market Place for a cracking bargain!) and air-tightness tape. If you have to have the MVHR unit up there how about 'engineering' an 'MVHR house' ? You decide on the size needed and you stop the insulation for that square/rectangle. Build a stud 'MVHR house' (with a maintenance door) and make it air-tight and well insulated. Don't insulate the bottom (at FF ceil level). It wants to be in the heated envelope. For aesthetic reasons of course you will still have plasterboard across at FF ceil level (golden rule of pragmatic eco-build - the end result must not look stupid!) but there will be no insulation there to impede heat tfr. There will be a good, tight connection between the surrounding loft insul'n and that on the 'walls' and 'roof' of the 'house'.

Walls are '70mm solid wood'. How can you run cables 'inside the wall blocks to hide it away'? Or do you just mean stapled to the inside of the wooden wall and sandwiched between it and the insulation? Is it too late to suggest you insulate externally? That way there's no interstitial condensation risk. You said in your OP 'We just purchased a summer house'. Does that mean that any of us who are suggesting tweaks (roof sheets, for example, or EWI) are advising you to change something you can't change? (If you wanted EWI your roof would have to be bigger, and if your bought building already has a roof you won't want to be buying pre-insulated metal sheets.) Can you give us more on what's possible to change and what's a no-go? Thanks.

Not quite what you mean as this is a plug, but at 85mm it may work: https://ewistore.co.uk/shop/external-wall-insulation/spiral-anchors/

Particularly if the roof covering is EPDM (or perhaps GRP if finished nice and smooth I'd have thought you could adhere a collar to the roof covering, and the pipes would be an interference fit. Anything less tight than that and you maybe have a Polo roof.

Are you installing the insulation from the top (lifting the floorboards) or below?

I did EWI and small extension last year (and this year, but I haven't re-started yet!). Already had my own PL ins'ce as I had only just given up regular contracting. Changed home insurers before I began as my exg insurer was being silly. Can't remember all the detail, but it felt less than helpful. Paid for P.L insurance for my chosen 'helpers' .

+1 to @Nickfromwales. I was about to ask if you had considered the 'Critical Friend' route? I worked with ('between'?) clients and their architect, the 'cut of whose jib' they liked, but who was not 'an energy architect'. Effectively he did the design (I cannot design my way out of a paper bag) and I 'did the energy bits'. We also had a few hours with a PH consultant mid-way through the process. It is not (and was not intended to be) PH, but it's a nice 'tight' house.

Hello. You say "There's a gap between the existing ceiling (which is of lathe and plaster construction) and the floor of the extension (which is suspended insulation under TNG etc..). I wanted wanted to ensure this gap is well insulated internally (so where it meets the staircase) as well as externally (which I assume will be stud / block work / guttering etc...). " I don't quite understand that explanation, because you refer to 'plans for an extension' (which implies it is not there yet) but you also refer to "a gap between the existing ceiling (which is of lathe and plaster construction) and the floor *of the extension*". I am guessing that when you say extension in the second example you actually mean the loft conversion. (Maybe...) Can you enlighten me? Pictures/drawings would be good. Apologies if I am just being obtuse.

@Conor posted while I was writing mine, but I wonder if PD regs in NI are different to the mainland and still refer to PV on 'roofs' in CAs?

Planning Portal says this: " All the following limits must be met: Panels should not be installed above the highest part of the roof (excluding the chimney) and should project no more than 200mm from the roof slope or wall surface. The panels must not be installed on a building that is within the grounds of a listed building or on a site designated as a scheduled monument. If your property is in a conservation area, or in a World Heritage Site, panels must not be fitted to **a wall** which fronts a highway." (My emphasis). This seems to me to imply that they are (may be) PD on a **roof** fronting a highway. This apparently does not stop my LA discouraging PV on the front of properties in conservation areas. I am sure this will hinge on individual interpretations of 'so far as is practicable' (below). My memory says that some years ago PD rules *did not* allow PV on front roofs in CAs but as far as I remember that changed (probably somewhere around 2010) to the current position. See GPDO Part 14 A.1 and A.2. https://www.legislation.gov.uk/uksi/2015/596/schedule/2/made Conditions A.2 Development is permitted by Class A subject to the following conditions— (a)solar PV or solar thermal equipment is, so far as practicable, sited so as to minimise its effect on the external appearance of the building; (b)solar PV or solar thermal equipment is, so far as practicable, sited so as to minimise its effect on the amenity of the area; and (c)solar PV or solar thermal equipment is removed as soon as reasonably practicable when no longer needed.

+1 for Back to Earth. They will do a WUFI assessment.

...and if you don't knock it down... I would use rigid wood-fibre, and get a WUFI (or similar) dynamic condensation risk analysis done first to inform the 'safe' depth you can use. Some say only 60-80mm, some say 100 OK in some circumstances, but bear in mind that, depending on the exg fabric the resultant U value will be worse than 0.3W/m2K, probably. Some would say use plastic ins (PIR nearly twice as good for a given thickness as some rigid WF) and just get the vapour control right. It is this bit - getting the VC layer right - which, while hardly rocket science, can often be done badly. Difficult for you to control from abroad! My stance on wood-fibre is that if it is designed to operate without a VCL that's one VCL not to get messed up. I have used it a lot, internally and externally. Is the pointing done in lime, by the way?

@Jimbo23 said: ''@redbeard You're correct, 1920s exactly. 15-20 years lifespan is fine. Does that mean leaving it completely alone or does that include repairs (i.e. re-add missing hip/ridge tiles, repointing etc)?'' The 15-20 years was my latest target for *my* (slate) roof. No, it's definitely not leaving it compeletely alone! I don't think any 100+ year-old roof can be expected to be fine if left alone for 15-20 years. Even if it's just minor fettling it will need some TLC. My latest target for my roof is after a lot of trips up the roof. Till I started getting old I went up every year for a darn good look round. I cannot make a guess at yours from the pics, but you need to look at it critically, like you are *expecting* the tiles to be knackered. Let them prove to you if they are not. Appreciate you don't own it yet, but I think someone really needs to go up there to have any degree of certainty.

You'd get longer out of it, almost certainly, but we cannot see the condition of the tiles. I am guessing circa 100 yrs old (?) - looks '20's - and I don't know the life of Rosemary clay tiles. Certainly I have seen some 'dying' in less than that time. My (slate - so almost certainly 'destined-for-a-longer-life' roof had, I reckoned, 5-10 years left when we moved in. 35 years (and a goodly bit of maintenance) later I reckon it's got 15-20 years. If you want 30 years and don't want to re-roof I suspect you might look elsewhere. If you want something which is not too bad, will last while you save the money and will improve a lot when you re-roof it then this may be for you.

Agree with @Iceverge. I am a great fan of floating floors (base/insulation/OSB/finished floor, with or without some membranes along the way) but perhaps the degree of 'out-of-level-ness' will have a bit of a bearing. May be hard to level it all out with sand and then not spoil the level as you lay? (Envisages a series of pulleys on ceiling each supporting a piece of insulation, jigsaw-style!)

Did/does BCO require a radon sump as well? The few jobs I have done which required a membrane also req'd a sump.

I take it they are clay tiles? It may be just my eyes but they look wider than 'standard' Rosemarys. I did a porch roof with 1920's clay Rosemarys which had been stripped off an replaced with concrete tiles, which probably won't look as good in 20 years as these looked after 90 (when I used them). Equally I have seen some clay tiles very badly spalled after far less time. My slate roof is 120+ years old, and one I was looking at earlier this week 150+. Both have had a lot of care and maintenance, and I would be pleasantly surprised if a clay-tiled roof lasted as long. But follow the others - close inspection and opinion on more than just the tiles, too.

"Why a cavity behind the plasterboard I'll put my money on air circulating behind the plasterboard." Service void, and yes, if the air barrier is poorly detailed you could be right, but with fingers crossed it could be tight as a drum.

Thanks for that. Well, provided the air-barrier has been well detailed at all joints, perimeters and penetrations you may hopefully have a good air-tightness envelope. Were all the windows and doors well taped in?

@GrantMcscott said: "We have plasterboarded and kitchen is in and some bathroomes but no carpets and some walls still need to be taped ect. When is the right time to do an air test?" I think I'd do it now. I would have suggested doing it before the kitchen was fitted and arguably before the plasterboards went on - when the main envelope was tight but before you'd added enough stuff to get in the way if you found leaks. You mention taping. Are you just joint-filling or full wet plastering on plasterboards? What is the house built of? If it's masonry do tell us it's not dot-and-dabbed plasterboard... please.

Danwood build said: 'DHW?' Domestic Hot Water

Not sure you can stop short of dubbing out. is the 20mm 'out' random or defined? Could you accommodate it with a 10mm toothed coat on the wall and (with possibly some tacky PVA to stop 'slippage') 10mm on the board? Use a nice soft lime mix.

Just taper the joint as @carrerahill says and the plasterer or joint-filler can tape and fill/plaster over it. I did dozens of jobs like this and it was never a problem. If you thought it might be then perhaps do the mitre, paint the 'raw' end with PVA and bed the joint between that and the flat ceiling in something like Sticks Like Sh*t.

Pro Dave wrote: "I fear we may have frightened @Tricky from replying? I do hope not." Yes, me too. Do 'come back' @Tricky. We want to help you get a comfortable home which is safe from the elements.

Think I'd drain everything down. Does the full reno include full re-piping? If so you don't even have to worry if you have not got lowest-point drain-cocks. Just cut the pipes.