Redbeard

Excellent news and very speedy service.

Gov't Best Practice guide for rooms-in-roofs says 50mm ventilation gap between the insulation and the membrane. I fealise youtrs is noty but it sounds like the set-up is similar.

Please do try to keep relevant content on here wherever possible, or summarise if you have an “Eureka!” moment, as solutions gained off air do not benefit the masses 🙏. Gratsi! Yep, did that for just that reason - replied on the open forum. Thanks for the reminder anyway.

It's about thermal by-pass. If you do EWI and somehow cold air gets into the 'sandwich' inside of the EWI you just have a very expensive fat layer of render.

Most on here would suggest filling the residual cavity if there is any suggestion whatsoever that the cavity may be less than 100 per cent free from air ingress. That could be a significant issue. You don't want to have a bunch of expensive insulation hanging on the outside looking pretty (only). NB I have not read all the thread, juts the 1st page or so, so I may have missed relevant stuff. Edit. Tried to send you a message, but 'EinTopaz cannot receive messages'. Do you or the moderators know why?

I want to add Larsen Trusses and 250mm+ of mainly flexi wood-fibre to my Victorian roof. 75mm rafters, 270mm purlins. He wrote me as report stating that given the reductions in weight by removal of exg ceilings etc and the commensurate increase by insulating would result in some net gain. However while he said that such timbers would not be approved in a new-build he is satisfied that it will take all built loads and the worst-case snow load. That will keep BCO happy. It sounds like you already have the 'ammunition' you need.

Sensible reply as promised. I am really sorry you have had this issue. The manufacturer's info does not seem terribly comprehensive, and how does that 'high mortality seal' (what?!) at perimeters only deal with air-tightness? The instructions for bringing cables through the insulation make no mention of the need for air-tightness, or I may have missed it. Also one of the pictures seems to show (OK, may be my old eyes) a gap between sheets wider than I would wish (i.e. bigger than not a gap at all). There is ref to a condensation risk assessment in exposed areas. There should be one in any case but, as per @Iceverge's post, any 'failure' - quite possible at 100mm - might well attract a recommendation for a VCL from Glaser Method calcs. WUFI or similar may have different views. Edit: Like you, OP, I would be worried about the same firm doing the job again. One might argue that the reality of your installation may have shocked them so much that they will never do a job like that again, but an inescapable fact is that someone signed that off as OK. How confident could you be of 'second time lucky? This is hugely disappointing given that it was presumably done under the auspices of PAS 2035, which came out of the Bonfield Review into public funded retrofit after some spectacularly awful failures. Worrying too.

I think the 'high mortality seal' only applies in marine environments. Sorry! Sensible reply later!

I take it you are talking flexi WF? I found a handsaw and, as per @torre, compression, worked well. For rigid WF Pavatex sell a blade for a reciprocating saw but TBH it was not a success for me. Circular saw with dust extraction worked for rigid WF. 2 passes - one from each side - for thicker sheets.

So, we've got (?) sub-75mm rafters and some Recticel. The current gov't standards recommend a 50mm ventilation gap with sufficient vents on each side. What thickness of Recticel did they use? I fear that problems may have been built in under that nice-looking roof. (and of course its failure to run off into gutters is at least one). I agree wholeheartedly with @Nickfromwales. They need to be invited back to tell you how they will make what they have done actually work in practice. (Of course we don't know the specifics of the 'contract'. It would certainly be useful to know, for example, is there was any 'you do this and I'll do that' agreement, which could alter the facts of what we appear to see).

Has the roof been stuffed full of insulation? If so you could end up with condensation on the underside of the roof 'deck'. What did the installers tell you?

Apparently it only works when you sit it on a 230V hot-plate.

Are you doing this from the top or from below, with the boards in situ? I assume from the top as you refer to taping as VCL (on the warm side). Why not carefully cut joist-width slivers of 25mm PIR and glue to underside of joists, then when you slide the 125 PIR down it should meet at the bottom of that 'fillet'. Yes, the joint won't be tight as a duck's whatever, but an extra piece of softwood won't be very insulative either.

I forgot to mention that. Maybe it's a basic H & S measure - glue your left glove to the stack with silicone while you use more of the same stuff to make a right mess just below...? (Remember not to take hand out of glove).

Oh yes, poor insulation and a cold void will do it! (a) The supplier should come back and remedy that (unless insulation of ducting was specifically 'by others') and (b) What did they say/promise? I'm trying to establish whether they in effect gave you a 'performance specification'. If so, the 'actual' has not met that spec. You say that you did not expect the MVHR to provide for your full heat load, but not too far off. What did they tell you it *would* provide?