Redbeard

So area is 72m2, exposed perimeter 25m. P/A ratio = 0.35. Therefore likely base-case U value 0.56W/m2K (R = 1.78m2K/W). On the face of it adding 50mm PIR (R=2.27m2K/W) seems to give a U value of around 0.25 (a pass!), but how it actually performs will depend on a number of factors including whether the B & B floor is ventilated.

Or drop the fan off the ceiling. If it has a duct attached you know it has one (though you do not know where it goes to), and if it doesn't.... If it is the cheap wire-wound plastic duct make sure it does not go up and down (over uneven insulation, for example) or else each dip can become a slime-sump.

Is the fan in a stud wall or a ceiling? Can you see any ducting in the loft? If not, i suggest isolating the supply (or asking a 'trician to do so) and removing the fan to have a look. It sounds as if you suspect that he's just fitted the unit and left it to fill the void with steam. Is that what you think may have happened?

Is there anything wrong with the roof tiles? underfelt is only a second line of defence in case the tiles fail. I would not rush to repair this unless there's a leak. All the growth suggests that the tiles are not young, and it's impossible to tell from a pic. Would I be right in guessing 1960s or 1970s ish?

@ETC, sorry, got that now. I had missed the ref to the lay-up on top of the B & B. That would work quite successfully. Without knowing the house we can't know the perimeter/area ratio, so cannot calc the U value with, say, 50 - 60mm insulation, but it will be a hell of a lot better than it is now.

@ETC, I take it you mean 'work from the existing *first* floor level down'. Of course we don't know the ceiling heights, but losing 200 might be significant.

I have no experience of Scotland but in England it was still OK in 1990 (+) to use 'teeming and ladeling' with Bldg Regs, so you were allowed to get away with not insulating the floor if you (allegedly?) bumped up the wall insulation. I worked for a hsg ass'n at the time and although we used to build our owmn we were outbid on one site and ended up buying the pair of spec-built semis which were built on it. I soon got in the habit of taking off my jacket and one pullover before going in because the house was always at 25+ degrees. Why? Because they were always trying to get warm feet. Never did, really; just sweated from the knees up. Re the suggestion of floating floor the problem is that it puts your stair out of compliance with Bldg Regs unless you have the space to make the bottom stair a 'landing' (can't remember the dimensions)

I worked on a house which had some (but not all) TF 3G sliding sashes. We got 2-and-a-bit m3/m2/hr. Not too unhappy with that. Under test conditions you could hear the seals beginning to 'give' as the tester cranked the fan up. But we still got 2-and-a-bit. Performance vs aesthetics; depends what you want most.

Make one! With a bit of careful thought you are likely to make one which will be much more air-tight and much better insulated than a proprietary one.

Yes, you can definitely use breathable membrane as the 'hammock' and it arguably performs a 'belt-and-braces' function as an 'anti-wind-wash layer'. If you have nothing better to do for the next few hours read these. The BEIS Best practice document is interesting insofar as it is one of a series (IWI, solid floors, suspended floors and rooms-in-the-roof) which were published in (I think) 2022 but (as far as I am aware) barely publicised at all. I may not agree with absolutely everything in them but they have clearly used knowledgeable sources. Seach 'BEIS Best Practice' for the whole bunch. In terms of material it is an expensive option, and it drops bits on you, but in terms of (semi-) rigidity flexible wood-fibre is soooo good. No dropping-out or drooping. Cut 20-30mm oversize, squeeze like a concertina and pop it in. I do 'support' it, as I want a membrane, but I am certain it would stay more-or-less forever without additional support. Under-floor insulation guidance: https://www.gov.uk/government/publications/insulating-suspended-timber-floors-best-practice https://www.greenbuildingstore.co.uk/information-hub/insulating-suspended-timber-floors/ https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors http://www.sofiepelsmakers.com/suspended-timber-ground-floors.html

Splendid, ST! Thank you. The next step is Auto-Crayon-Aided-Design, where the crayon guides itself.🙂 @JohnBishop, ST has depicted my suggestion perfectly. However now I have seen your pictures you need this less than I thought (indeed perhaps not at all if your pics are representative of the situation over the whole eaves). The point of 'my' membrane is to ensure that you cannot block the vent gaps. AFAICS, if they are what I think they are, your black plastic formers do exactly that; if you push the insulation hard against them there still remains airflow. So, message to anyone in future with this issue, if you have nothing to stop the vent path being blocked then mine is a fair idea. If you have plastic formers or similar like @JohnBishop's, you probably don't need my suggestion.

We got under 2.5m3/m2/hr in a refurb of a 19th century house with all sorts of issues and I was a bit disappointed that we did not get better. I would be really disappointed with 5 on a refurb, let alone a new-build. Depending on what the consultant knows about your knowledge I'd have expected simple things like explaining what a solar diverter is, but also looking more broadly at air-tightness, explaining what can be achieved and why you want it, and so on. Does 'finally written...' imply a long wait?

@ProDave said "Get 1m lengths and put nuts on before cutting into 3 or 4 (I'll explain if you go ahead with this). Rod, nuts, washers, angle grinder." Good call. And when you (OP) forget to put the nuts on you'll be off to buy a grinding disc for the angle grinder for 'fine tuning'.

Yes! I do not have pics, I am afraid, as I am one of the world's worst photographers. Basically temporarily clear away the insulation from the eaves. Work out the vertical height (depth) of the insulation you want. cut a piece of breathable membrane which is a bit more than the length of the hypotenuse of that triangle. Lie uncomfortably on your front with your head nearly wedged in the eaves and a staple-gun in one hand and the breathable membrane in the other. Staple to the underside of the rafters tightly, so that as you push the increased-depth insulation up against the membrane the ventilation gap (the gap between the membrane and the underside of the roof covering) remains fully open. If there is a stub of wall to seal the membrane to at the 'bottom of the hypotenuse' then even better. This 'deflector' should allow you to insulate tightly and deeply without risk of getting air-flow between the layers of insulation or compromising the ventilation. I am sure someone with skills in CAD could draw that up in a moment.

"Does it mean I should put some extra insulation?" No. Not if the insulation is, as I assume, on the ceiling of the room(s) below, and the vents are on the slope at the top of a cold void. There should be a howling gale up there anyway for water vapour control. As long as it does not infiltrate to the 'warm side' of the insulation, no problem. If you want to put more insulation in top up to 400 - 500mm, but make sure not to block eaves ventilation. You could staple some membrane at the 'point of the triangle' in the eaves to deflect ventilation air above the insulation.

I am no stone wall expert but I would be trying to find out whether the wall was *originally* rendered externally (granted that it probably would have been in lime then). I have seen a few houses where people have removed the 'horrible render' only to find that the house was originally built with render, mainly because the stone was not of high quality. Take it off, point up the stone, and they had a damp issue because the stone was poor. The system you describe above sounds OK but I would prefer it if it were ventilated, which basically means at least 4 air-bricks per wall in diagonally-opposite corners. If it is not vented then that thickness of insulation is potentially cooling the wall a lo, without anywhere for water vapour to go. Ventilate it and you effectively have the same situation as in a cold roof.

It is also relevant to note that Thermafleece used to (I don't know if they still do, but I guess so) require that a breathable membrane was used to separate the sheep's wool from the wall itself, for exactly the reason which @SimonD is suggesting - that the outer (currently cement-based) render *may* fail at some point in the future, in which case the sheep's wool does not want to become a sponge. I second the ventilated (and I mean properly cross-ventilated) void. See for example Stirley Barn, done by the Green Building Company (Green Building Store's building arm).

@MarkAldo11said: "New regs propose vents everywhere, trickles in the windows, so there’s plenty of areas for air to escape rather than breathing through the walls." Important point is that 'breathability' (water vapour permeability) is not ventilation, and even less to do with air-tightness strategies (someone will find connections I don't have in mind while typing quickly, but this is broadly true). For example you can have a very airtight breathable house, you can have a very airtight non-breathable house, and you can have a non-breathable (or breathable) house which is not very airtight.

A few lengths of CLS, unless you actually need a cupboard. If you just need a plinth to mount a sink on make an open table out of CLS screwed together.

Chequer-plate alu? But as others have said, can't overlay the ply if it is at all soft. It doesn't look very happy.

Along same lines as @Russell griffiths, I got some fibre cement boards as verge undercloaks. I just put the leftover one in a 'safe place' yesterday, and cannot find it to measure, but I think they were 3-4mm. Thinnest tile-backer I have seen is 6mm.

Is there access to the void above? If so, access it and search. If not, get the Stanley knife out. Explore.

Hi and welcome. You wrote: "Any hell much appreciated" We promise not to give you any!! And now seriously... Generally: Please Please Please Please Please Please insulate, as much as you can, now. You will have loads and loads of time to regret not doing so, or doing it insufficiently, later. Having said that... Floor. Yes: Insulate. Any is better than nothing at all. At risk of having to do a bit of door-chopping (can we have pics, as I might not be so blithe on that if it's a uPVC door) how about 25 or 50mm PIR (Kingspan/Celotex-type), damp-proof membrane as vapour control layer, taped at all joints and perimeters, T&G OSB18mm with glued T&G, then your tiles. Walls: Gov't best practice guidance (search BEIS, best practice, solid wall retrofit) suggests that if you are using anything that is not 'vapour-open' ('breathable') you should have a *ventilated* cavity behind the insulation. Some have been doing it otherwise for many years without issues but I understand they (the gov't) have v knowledgeable advisors and good reasons (interstitial condensation risk) for their suggestion. Anyway, more from me or others later. Got an extension to finish!

Not just an air gap, but a *ventilated void* (which I would take to be served by 4 air-bricks in diag opp corners on a rectangular wall) according to BEIS https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice Obviously then air-tightness becomes even more important than it is anyway, or else you just feed cold air into the room via leaks at perimeters.

@newhome said: " I’m not sure about the floor difference as yet but thought that I could place the frame 100mm higher than the floor level to create that if needed? If it’s just to stop potential flow from liquids then surely that would creat that effect?" I think it's about heavier-than-air gases, and specifically petrol and perhaps LPG w.r.t. garages. I don't quite understand the 'frame higher than the floor level' bit. Surely there's either a 100mm difference or there isn't, and if there isn't, one has to go down or the other has to come up. We are after all, talking about a potential 100-deep 'pool' of petrol vapour. (Note, in my head it was 150, but 100 has been mentioned, hence I am repeating 100. Haven't searched for the info.)