Annker

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?

Looks like a good solution but as I've only about a dozen pockets to do it's probably too expensive an option.

Yes the "Tony tray", I had it well researched but must have been rushing and its inclusion was forgotten. By top plate I'm referring to the horizontal top plate of the studded wall. Basically just end the VCL along the ceiling line, don't fit it in around the joists and tape it along the top plate of the studded wall. The option to have the VCL cut in around the joist will be a bit of a faff alright, however if its required I will do it.

Appreciate any ideas on the following issue to overcome. When constructing my stick build TF extension I mistakenly left out a strip of VCL that I believe is now (at fit out stage) needed to link together with the VCL proper. The design for the VCL was either going to be the PIR foil layer taped as required, or the addition of a sheet VCL. My designed TF build up is broadly as section below: I'm at the stage now of measuring up insulation and looking back at an old thread I see I missed installing a strip of VCL around the joist ends as per image below: So now I am wondering how best to detail/install the wall VCL at the ceiling/floor joist to wall junction as its now built Do squares of VCL need to be cut in around each individual joist void pocket and taped to the joists or can the VCL be simply taped along the continuous top plate? Similarly, at 1st floor ceiling/roof level I'm unsure how the wall VCL should be terminated. I have a VCL installed externally above the osb, so again I will not be now able to link it to the internal wall VCL.

Understood, that makes sense to me.

Sounds good, thanks it's very useful to hear others experience in the absence of your own. I think I will still opt for insulation within the studwork for a couple of rooms. Namely two rooms that have bay windows. Here it would be preferable to keep the depth of buildup to a minimum around the bays otherwise the proportions of the individual wall panels start to look off. Here I may keep the studwork 10mm-15mm away from the brickwork and pack a little insulation behind the stud.

My concern is that I have some large area walls, 3m high ceiling and almost 5m long. In such large areas do you think flexi batts would maintain their installed position long term? I have actually just ordered a bag of mushroom head insulation fixings, I was going to experiment with these to see if they may help keep either rockwool or flexi wood fibre boards in place.

Not choosing rigid specifically over flexi on a material basis; rather that the rigid boards can be bonded to the brickwork in continuously layer as opposed to flexi batts that need to be fitted between studs and therefore the insulation layer missing at every 400mm c/c stud. Although perhaps the lower u-value of the timber stud vs insulation discounted?