nostos156

Perfectionist by heart I'm afraid, and when you deal with stuff like insulation/vapour barriers and such where they over-emphasise how breaks in it can massively negatively impact things, it's the perfect storm, ain't it?

I tried looking this one up, but results seem to be mixed. So looking for a sanity check. For a quick background rundown: old wall was an exterior facing attic one, which was constructed in the old style: solid outer wall, with thin vertical battens that were attached to the wall by hammering in wooden posts right into the old mortar, then laths, then plaster. Pretty damn cold in the winter, and any wardrobes close to that wall had mould on the backs of them. (Although only at the bottom of the wardrobe and I did note draughts, so there was probably an issue with the old skirting as well). This wall has a chimney behind it (so probably the draught source) but when I tore the wall down it was completely dry. I bricked it up and drilled holes for ventilation to keep it dry, that the cavity will take care of. Have since torn that old wall down and built out a new one with 2x2s, with a 50mm cavity from the stone. 50mm PIR was laid in between the studs (at 400 centres), and then to avoid cold bridging from the wood I put 25mm over the whole lot. Liberal use of foil taping. Finished by screwing down plasterboard directly on top (did not opt for battens to keep overall thickness at a minimum and did not need a service void). I didn't know how many screws were appropriate, answers seemed to vary, so I erred on the side of caution and did them at 150mm around edges, 300mm in the field. This has evidently ended up in a fair bit more screws than I see some people use, especially around joins. It got me (probably over) thinking how much of an effect these screws are having on cold bridging, and if I goofed? Like I said, probably overthinking it, as I mainly insulated the wall heavily so that I could bang a wardrobe up against it once more and not worry about the appearance of mould any more. Just hoping those excess screws won't have defeated the point of all the insulation, particularly right behind the wardrobe where it sits at those joins.

The calculations suggest that a paltry 3mm of basically fluffy wool insulation wouldn't do bugger all to the actual insulation of a house, but anecdotes seem to suggest otherwise. That being said, I wonder how this stuff takes fixing things to the wall on it. Presumably it dimples a fair bit and would look naff?

Yeah, I got 50mm insulation in January, and it took me until around April before I got it all installed and foamed in. Then it took me until the beginning of July before I got the 25mm stuff in (which was date stamped late November last year, so has had time to offgass already, since it seems the majority of it happens directly after manufacture). Haven't seen any evidence of shrinkage while in situ yet. From what I can tell, aside from bad batches, it's UV exposure that kills this stuff. Probably what happened to that stack.

Yeah, honestly I think this site is better as it will tell you exactly if you are getting condensation and how much: https://www.ubakus.de/en/r-value-calculator/?

Okay I figured out how to add studwork properly, so it has changed the values a bit.

Gave the calculator a go, I presume the dew point being inside the insulation layer is what you want? Interesting that I can't add studwork within the thicker PIR layer either.

What calculator is this?

I've been doing some quick reading on thermal bypassing and that looks like another minefield. Ultimately, there's only so much you can do within a given budget for an old house. The outside walls are solid breathable stone meant to wick away moisture from any ingress and so on, and it's probably far riskier to stop that on one side by not bothering with a cavity. It'll render the PIR not as effective as you say, but trying to retrofit modern standards meant for completely different housing types in much older construction is always going to less efficient. As long as it doesn't lead to condensation and rot risks by having the ventilation then I'll take that as a win.

Interesting topic to read, especially re: breathability of solid walls + risks of using solid PIR. In my own case, I'm insulating an attic space room exterior wall. I've battened out roughly 30-50 mm from the wall (uneven, as they just threw whatever stones they could find together, lol). I then put up 2x2 timbers and will put a 50mm PIR in between, likely them putting 15mm PIR over the top of it all to minimise any cold bridging (20-25mm is starting to eat away the room space too much for my liking, and 15mm seems like it'll do more than enough to prevent cold spots). As this is in an attic space where the roof is still original slates without a felt layer, it can get quite windy in there (and thus, in the cavity I created). I'm presuming this is more than enough 'breathability' to not worry about condensation risk behind the insulation, as was talked about earlier?

So I'm building the actual stud wall by this point and after putting the framing in and leaving a roughly 50mm cavity (as the wall is uneven it goes from 30mm at shallow parts to 50mm at deepest) from the stone to the framing. I am noticing an issue I never saw mentioned, so did not account for. Basically, the 2x2's seem pretty weak to being bent. Even with noggins on, pushing or pulling them shows a fair bit of movement, which worries me. The obvious solution would be to drill holes in the stones and insert a bit of wood behind the stud and screw it down (with the wood wrapped in DPC), so it is also attached to the wall (although this would likely involve dismantling what I've already done in order to be able to drill the stone behind the stud. Could avoid that by wedging in a longer block of wood and inserting two screws at either side, but then I'm doubling the amount of thermal bridging through the metal.) The last sentence comes to my next concern—with blocks of wood directly against the wall, doesn't that fairly increase the thermal bridging, especially with the fixings being metal? How do I prevent condensation risk on the metal fixings? Is there another way entirely?

That is fair. It hasn't been capped, just got a mesh covering on it. It definitely had prior evidence of mortar erosion as I could see it from the stone when I looked up, and there was a ton of dust that needed to be shovelled and bagged up before laying down the blocks, even if it was bone dry (and still dry during the periods of rain while doing all this we had). Will leave it at the 6 holes drilled then.

Well, went ahead and bricked it up, but decided to drill six 12mm holes in one of the concrete blocks I cemented down. Is that adequete airflow?

So, getting to the point of knocking the wall down. Discovered a boarded up fireplace (just had plasterboard over it, not bricked up). Kind of suspected this, but this was one of the reasons I was doing a stud wall and not attempting anything near the actual brick, as there's the classic brown staining above it. Was thinking of bricking it up, but I keep hearing about the need for a vent, even in an old stone and lime mortar house which is supposed to be breathable as hell. Since I'm leaving a gap between the wall and stud frame (as it isn't a cavity wall), could I just drill a few holes into the bricks I put down and call it done? There's a fair amount of air movement in the attic because the slates never had felt put down, so I'm figuring this should be enough ventilation without having to open a vent into the room itself and render the PIR pointless. My relative who is a brickie kind of shrugged and said the vent likely wasn't needed, but I'm not sure. FWIW, the fireplace was full of old dust that needed clearing out, but was perfectly dry, even after a rainy day (chimney is not capped, just had mesh put over it). Seems there was no recurring damp issues, even in the middle of the winter.

Single 40mm conduit (I could go 32mm but HDMI cables are a PITA), through a couple of studs. It's going into an interior wall in the attic space, so I can't get it in there unless I rip up floorboards and go from underneath. Going through the side would be easier if the holes are able to be bored. Pretty sure it is not a load bearing wall as it's parallel with the joists above it.

Wondering what the regs say on this, as I can't find information on it, just about joists. Googling gives me American standards saying you can drill up to 40% of a stud's width, even up to 60% if you double them. Other than that, I came across an apparent structural engineer saying up to 50mm in a 40x95mm stud is fine, but nothing clear-cut written in stone.

Yeah, I had issues with minor mould growth (located near the floor of the wardrobe) with an Ikea PAX that had a couple of cm of spacing from the wall. However, I think in this case it was due to the fact that there was a gap between the bottom of the skirting and the floorboards, which the carpet covered up. In especially windy days, you could feel a cold breeze coming through there… probably cold enough to condense things up around there. I just don't want to shove one up against the wall and have the cold bridge through enough that it would still cause issues due to thermal bridging. I don't know enough about R/U values to know if my plans would be enough to stop that or not. For example, the studs themselves could be a source of cold bridging. Insulated PB on top of the celotex would solve that, but I'd lose a lot more space going that route. If I went with a plastic VCL behind the PB, wouldn't I immediately break the barrier if I ever fish cables through that wall with my planned service void? At least if I just did some awkward foil taping around studs, I wouldn't be at risk of breaking it. As for permeability, the wallpaper in this room before the renovations started was vinyl stuff. The permeability of the walls was actually thrown out a long time ago, and it doesn't seem to have caused any issues from what I could tell. On the outer face of the wall is all solid stone, there is no render or exterior insulation. My understanding is that moisture can move through this.

Considering a DIY job where I will knock down an old lath & plaster exterior wall in an attic bedroom. The house is an old Victorian one with exterior walls of (very thick) solid stone. The wall in question is the coldest one in the room. The other exterior walls (with sloped ceilings) that face the slates have all had rolls of glass wool put in between the studs years ago, as well as in the loft above the room. The wall thickness is about 5 cm based on shoving a screwdriver in until it hits stone. From what I can tell, the timber stud work is the old kind where they hammered in wooden pegs to the stone, that had the very thin studs attached. Not sure what they're called. The basic idea is to knock that wall down, then build out a new 2×2 stud framing a short distance from the stone wall to create an air gap (25 mm is the minimum, right?). Using 2×2 as the room is not all that large and trying to minimise space lost. In between the studs, the plan was to use Celotex PIR at 50 mm thickness, but pushed back from the face of the studs slightly to allow for a service void for cables and such; rather than adding additional battening for a service void. The edge of the boards to timber would have spray foam to seal any air gaps from the cutting. Afterwards, I'd slap on standard plasterboard and paper over. How does this all sound? I've been informed that I should be using foil backed plasterboard for a vapour barrier, but don't those only really do something when the vapour barrier is continuous? Using it on one wall where it'd have seams due to the plasterboard seems like it wouldn't do much? Could I cut the foam down level to the PIR and then use foil tape at the edges and continue it around the side and face of the stud work instead? The main reason for doing all this is that this is the only good wall a wardrobe can go on, and I'd preferably want it pushed up against the wall. Would that kind of insulation be enough to stop condensation issues? If not, I am additionally planning to put a 3 mm layer of wallrock thermal liner, just to act as an additional cold bridge if necessary. That would hopefully be enough.