Annker

That is a good point to make, and yes I am broadly aware of the relationship between RH and temp. Its worth mentioning that atm my house is very much a bare bones building and I believe at a lot of times during the day it is colder inside the house than it is outside it. My observation was moreso a generally one ,that on humid days (not necessarily rainy days) the patches appear darker.

Thanks Mike. Rationale for installing a continuous VCL makes sense. As with any renovation, each room ceiling will have its own challenges to overcome; Plan was that 1x existing ceiling only needs reskiming and 1x ceiling only needs overboarding. Elsewhere 1x ceiling needs a new MF ceiling installed to get it back to level, remaining ceiling are in a state in which a VCL can be install as is. I have the ideal princple in my head so just have to tailor it to each situation. Understood on the eves tray. A full re-slate of the roof will be done next year so ventilation of the roof space will be entirely dealt with then.

In the photo I had only offered up that uncut 4x2 to help illustrate the detail. You are correct Mike, I will be running a top plate along the studs as you've expertly shown in isometric! Just to clarify, you would opt to continue the VCL below all the entire ceiling area, or just ~300mm or so into the ceiling area proper? From my research it seems that where the loft space is a cold roof, installing a VCL at the ceiling level has its pros and cons. It is suggested by some that the ceiling/cold roof space interface may provide a function similar to a dehumidifier removing moisture from the habitable room; installing a VCL would prevent that function from operating. Although in a bathroom perhaps it it best to have a VCL in the ceiling as the required mechanical extraction will remove moisture at a good and controlled rate. Perhaps in terms of ensuring the IWI assemble is air tight, a good option would be to just wrap the top of the studwork in Intello Plus; taped to the wall and tape to the top plate. (As illustrated by the blue line in the mark up below) Seems to tick the boxes of building with the princple of vapour open and air tight to the forefront.

Timber for the IWI studwork has now been acquired and loaded out, walls are going to get built, it seems like years getting to this point! However, there is an airtightness detail I have yet to get my head around and resolve. The parge coat, taping around joists and other penetrations has eliminated draughts that could potentially transport moisture into the IWI build-up. I suppose moisture that is transported through the ceiling and into the loft will be ventilated away in the cold roof in the typical fashion. But what about the junction wall the IWI assemble meets the ceiling? Should my airtight layer continue up from the external brick wall and across my 1st floor ceilings; or can it terminate at the top of my external wall? I can think of three options to deal with this: Intello plus AVCL installed extensively across the entire ceiling and taped to the external brickwork A ~300mm rip of Intello plus installed around the perimeter of the ceiling again then taped down to the external wall brickwork No AVCL required along ceiling level at all; any moisture in the the upper section of the IWI being ventilated away through the loft I can justify all three options in my head for various reason so would appreicate to heard any suggestion

I get so much valuable information from people on this forum and feel I contribute little in return so I'm going to post up photos of the IWI install as I go, it may provide some insight to others doing a similar install. Below are the various methods I used to seal around embedded joist ends. Where the background wall was reasonably sealed around the joist I used airtight paint. It is easy to use, fast to apply and gives a comprehensive covering. In instances where the background was gappy such as around joist hangers I started of just taping the hell of them. The Tesco Vana tape is good, you can mould it around a little and it gets a good adhesion to the parge coat and especially so to timber. (I also primered backgrounds with Tesco primer) However in some other instances the mortar joints in the brickwork between joists was very gappy and would require jointing. And in some of these instances there are timber lintels in the mix so I decided to fit and and tape sections of Intello plus, this meant that the surfaces to be adhered to were predominantly timber which the Vana tape does a great job of. Lastly there were a few instances where the joists ran parallel and too close to maneuver a tape in to seal around. Here I just build filled with Illbruk FM 330. These void seem dry so fingers crossed this bulk filling wont end up trapped moisture against the joist.

Below the floor is a cellar, damp but certainty not flooded and is it well ventilated. I have installed an additional 6 or 8 wall vents and have good cross ventilation throughout the cellar. Note. Prior to erecting IWI studwork I plan to run a rips of Intello AVCL along that gap for airtightness, although I'm unsure which side should face the room or cellar or if that matters @Mike, just to copy you in: I have a humidity meter in the house and have noticed that the patched darken when the humidity rises (no sh*t Sherlock!) However, I think that observation is important in the absence of any sign of external source of water ingress to the building. Yesterday was a particular humid day I had left a rear sliding door open for a while and the humidity meter was reading 95% (matching the weather report). Is it plausible that the those patches were historically damp, and are now sensitive to changes in humidity. Perhaps due to a concentration of salts. Just to reiterate they certainly do not feel damp to the touch, so rightly or wrongly I'm not that concerned about treating them.

Thanks Mike, useful information. Seems to be a lot of research on that website will have a proper read through later on. Your point on RWP positions is spot on. There were leaky RWPs adjacent to the damp patches, now no longer an issue but likely they were leaking for years prior to I repairing so it's entirely likely that was the initial source of the dampness. I know from past experience that saturated brickwork can take an age to dry out. The bricks certainly aren't saturated now and I'm happy my vapour open buildup will handle any remaining dampness. Additionally, I wonder if salts would cause a problem for me, as my newly finished wall surface will be separated from the salt affected patches on the wall.

In the past week or so I have noticed seemingly damp patches showing through the parge coat at the base of the ground floor walls, pretty much around the entire external perimeter of the house. The damp patches do not feel noticeably damp to the touch, however they are certainty visible and register ~25% on the damp meter, as opposed to ~12% where dry (I am aware that this damp meters measure salts as a proxy) Points to note: There is a reasonable dry cellar below the ground floor, it is damp but certainly not flooded. External ground level surround the house ranges between 1 foot to 2 foot below internal floor level and is freely draining. Brickwork external face does not appear to be wet, certainly no obvious correlation with the patches Underground drainage is in good shape Roof line is also in good shape. So the issue is I cannot see any obvious source of this apparent dampness, could this be a case of the much debated rising damp? I'm concerned that if I just crack on and install the IWI as is, I may have mould issues down the line. Appreciate to hear if anyone else has observed a similar situation. I imagine the IWI can handle some moisture but how much? Current IWI design is Rockwool batts within a timber stud perhaps using Steico flex wood fibre boards instead of Rockwool may be a safer in these seemingly damp areas? TIA

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).

I've sometimes used that method in the past but it is considerable more work and harder to achieve a level finish (I'm a carpenter so experienced at this sort of work) The other alternative is sister along the joists with 3x2, possibly could get away with using 2x1/12. This would be a little less expensive material wise (vs MF system) but more labour. I have 2x row of solid bridging therefore x3 sistering pieces per joist. Less work than counter battening below the joists but still more work than MF system options.

Bit of a niche question. On my renovation project I have stripped back all lath and plaster ceilings to bare joists, in preparation for new plasterboard and skim. The underside of the existing joists are up and down, and in some areas two rooms have be knocking through to form one bigger room. Therefore I need to level the underside of the joists for the new continuous ceilings. I've previously installed MF ceiling in new build project however unsure of how to go about it in a renovation job. My plan to maintain ceiling height, is to screw the MF7 primary support parallel directly to the existing joists. Essentially the MF7 will be within the joist void and therefore the ceiling height will only be lowered be the thickness of MF5 top hat section. Is anyone familiar will the system and know if this an accepted arrangement?