Annker

Very useful video on some of the design issue relvent to the situation being discussed here in this thread.

Great thanks for clarifying. I'll update the post with photos in due course for anyone following with a similar issue,

@Iceverge Once again thanks for your response. I like airtight paint, I had its inclusion in mind for the first option above and I think it would be the best option to confidently seal all gaps. The joints in the timberwork are all tight (my own work) so FM330 would not even be necessary to fill any gaps. "The important thing is that is joins the layer above and below the ceiling." If I understand you correctly, joining those two layer together will not be possible as the plywood subfloor runs continuously between the lower and upper stud wall, thus physically separating the VCL above and below the ceiling. I can ensure that the separate VCL below and above the ceiling are fully sealed & bonded to the respective underside/topside of that plywood subfloor but short of cutting a track through the plywood they wont be physically connected to one another.

I'm still trying to figure out the best option to resolve the problem of the missed VCL installation at joist level. My issue is struggling to understand the princple of where the VCL should be in the assemblage, therefore I'd appreciate any comment on the two sketched sections below. My understanding is that ideally the entire joist would be internal side of the VCL. That cannot now be achieved here, however, as per below I could continue the VCL up the wall, cut in-between the joists, taped back to the rim joist and around the perimeter of each individual joist pocket perimeter, then fit insulation within the joist pocket. In this arrangement the VCL is cold side of the insulation layer. An alternative sketched below is where the 60mm PIR board (installed over the studwork) is cut around the joists and continued to the underside of the plywood subfloor, then similarly the VCL is continued to the underside of the plywood subfloor and taped accordingly around the joists. In this arrangement the VCL is warm side of the insulation layer. The first arrangement seems to closely mirror other details I have seen online, however does it not pose a condensation risk, where internal born warm moist air may condensate on the VCL behind the insulation? The second arrangement seems a safer in terms of at least limiting the amount of internally born warm moist from entering the insulation layer, and what does get through can still dry into the cavity. However I have not seen any similar detail online. So thats my hurdle in a nutshell, any comments are greatly appreicated!

Thanks for the response Mike, I have a few cans of FM330 in the store so these voids will get filled in the morning👍

That was my post you referenced above. One solution I am considering, although unsure if necessary or suitable is to seal the area with a liquid airtight paint. It is easy to apply, relatively cheap to buy and will certainly eliminate any drafts.

Just querying this suggest Nick. My understanding has been that installing a secondary insulation layer beneath a warm roof (i.e a hybrid flat roof) should be avoided as they poses a condensation risk. Although I should add that though is solely from what I have read online.

@lookseehear That sounds like a good solution, I believe lime against timber shouldn't encourage any decay. Would airtight paint not be a better solution vs using a tape? I imagine slapping on paint is much easier way to close up the small gaps the lime plaster misses.

OP here. Just asking for a sanity check. I have a number of various gaps and penetrations that need to be air tighten prior to IWI. Some are large and will be awkward to detail with tapes, membranes or even airtight paint. I am wondering if I could just bulk fill them out with Illbruck FM330, it certainly would be much, much easier to apply and have confidence that it seals up. I imagine where the gaps are simply missing bricks or where steel is adjacent FM330 is a suitable choice. Question is will FM330 also be suitable to use where timber joists are pocketed into the wall, or may it cause a dampest issue against the joists?

Read a bit of this thread earlier in the week. Good to read you got that builder out the gate. Work at this stage can be rescued relatively easy. In this situation it's not essential that elements and connections look pretty, just need to be structurally sound and perform. In contrast bad work done at fitout stage can be much harder to rectified, and may forever look like a dog dinner You caught it in time, and have every opportunity to deliver a fine looking project now.

Say with MF5 tophat section the primary support channel goes in a maximum 1200mm c/c. Can I ask what is the maximum c/c of support that GL1 requires, and therefore spacing of the GL6 support bracket? My plan is to install the GL1 perpendicular to the existing joist, and therefore put a GL6 bracket on every 4th joist which works out a little less than 1200mm c/c

I found the BG spec sheet for the system. GL6 is the bracket. It's exactly the system I want. Simple, fast solution and easy to get right👍 Just to add for future reference BG only list the GL6 which has long leg, other manufacturer & suppliers offer a GL5 with a shorter leg which is a better option when looking to keep tight to existing timber joist.

I actually think the 30mm is borderline ok. The only place I believe it could show up is any margins around joinery fitted in the alcove. And as the alcove are ~ 1/3 of the width of the room the ceiling in each of them will only be running ~10mm off. Can't see any issue for the plaster finish either, the ceiling is a flat plane. I don't care to remember how many concrete soffit/ceilings I battened out in the past. We did a lot of apartment blocks that way. 2x2 battens fixed to the soffit with express nails and then counter battened again with 2x2(without nail guns). Dusty, noisy, over head work. I hated it.

Thanks Nod, I wasn't aware of that system. I'll have a look around their website, sounds like it could be a very good option.

Just to add in another question for opinions. In one room the 1st floor joists are running 30mm off level front to back. There is no subsidence and it seems they were just installed this way; they aren't bowed down too much and otherwise the ceiling is flat. The ceiling is about 3m high and 30mm fall is along a 4.7m long run. I wonder if such a fall would be noticeable and should I go to the effort of level out this ceiling? 30mm is a decent amount to have installed a joist off level but I'm not certain this will be noticeable in the finished ceiling. (I may level the floor above though).