nostos156 Posted August 27, 2023 Share Posted August 27, 2023 Hi, So at first I tore down a really badly done exterior facing wall in a loft space. I rebuilt it out with timber and used PIR in between and over the top (50mm + 25mm), in order to maximise insulation for the least space. And also because I wanted to ram a wardrobe up against that wall without having condensation/mould issues on the back of it. Job done there, I hope. Well, I'd like to imagine 75mm PIR would stop the wardrobe issues. Anyway, the rest of the room was still old plaster/lath, and I was getting tired of trying to make the old cracked or blown sections good. Seriously considering an immediately regrettable choice of taking it all down and replacing. At the same time, I was thinking of adding some more insulation (there is fibreglass in the attic all around the room and on top, but…) in between studs and joists while I was at it. Decided that rockwool would suit me best, as unlike PIR, it does a good job of dampening airborne sound on top of insulating. I'm aware that the one wall I did PIR up won't help much, but I'm not looking for soundproofing—just dampening a bit to replace the one job that lath and plaster does very well and get something remotely similar. Most sources seem to indicate RWA45 is the common go-to choice for this application: insulates well, dampens well (thinking of going 75mm). However, as I'll be putting these in studs that will be completely open to the attic space at the back, is it self-supporting enough for that application not to settle over time? Is the denser RW3 a better pick? It would cost a bit more per pack, but given how my studs and joists are closer than the 600mm the packs come in, there will be a lot of jamming offcuts together if I don't want wastage/more orders; maybe the more rigid RW3 will cope better with this? I did read one excerpt from someone who mentioned the RWA45 goes springy when supporting stacked slabs, whereas the RW3 was completely rigid up to the ceiling… but that was for building sound trapping, not in the walls. Link to comment Share on other sites More sharing options...
JOE187 Posted August 27, 2023 Share Posted August 27, 2023 I would recommend the RWA45 packs. 100mm and 50mm always on offer online. Easy to cut to tight fit with insulation saw. I cut 2 pieces out of a 600x1200 and very small amount of waste. 2 layers of 100mm in between rafters will stay in place, no problem1 1 Link to comment Share on other sites More sharing options...
nostos156 Posted August 27, 2023 Author Share Posted August 27, 2023 (edited) 2 hours ago, JOE187 said: I would recommend the RWA45 packs. 100mm and 50mm always on offer online. Easy to cut to tight fit with insulation saw. I cut 2 pieces out of a 600x1200 and very small amount of waste. 2 layers of 100mm in between rafters will stay in place, no problem1 Sounds promising. You just left it like that into the open and didn't cover or strap it? Holding up absolutely fine with zero sag? No issues with having to shove a bunch of offcuts into areas? I'd probably end up having to squeeze two 200mm ends cut to fill up some areas to make full use of the packs. Edited August 27, 2023 by nostos156 Link to comment Share on other sites More sharing options...
nostos156 Posted August 30, 2023 Author Share Posted August 30, 2023 One last question. I asked Rockwool about how much overlap I should leave in order to friction fit. They said 50mm total to the gap, so 25mm on each side for an ideal friction fit. This seems way higher than what I was reading on places like here, where people were just using 5-10mm excess to friction fit? Feels like with 50mm it would want to start bowing in the middle! Link to comment Share on other sites More sharing options...
BenP Posted August 30, 2023 Share Posted August 30, 2023 40 was fine for us - also easy as you can offer it up to a stud and then cut on the opposite side of the next one along (essentially adding stud width to the dimension between studs) Link to comment Share on other sites More sharing options...
nostos156 Posted August 30, 2023 Author Share Posted August 30, 2023 (edited) 24 minutes ago, BenP said: 40 was fine for us - also easy as you can offer it up to a stud and then cut on the opposite side of the next one along (essentially adding stud width to the dimension between studs) So if I read that correctly - you offered up the 600mm slab to the stud, then used the next stud along as your straight edge to get a perfect cut every time? That's pretty damn smart tbh. My studs are rough sawn old style so probably closer to the 50mm. Hope it works in that case, some of them go more diagonally than plumb and your method would account for that well. Edited August 30, 2023 by nostos156 Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 30, 2023 Share Posted August 30, 2023 >>> Which flavour of Rockwool? Prawn cocktail for me. Link to comment Share on other sites More sharing options...
BenP Posted September 1, 2023 Share Posted September 1, 2023 On 30/08/2023 at 15:37, nostos156 said: So if I read that correctly - you offered up the 600mm slab to the stud, then used the next stud along as your straight edge to get a perfect cut every time? That's pretty damn smart tbh. My studs are rough sawn old style so probably closer to the 50mm. Hope it works in that case, some of them go more diagonally than plumb and your method would account for that well. Yep - but if it's 600mm it shouldn't need cutting Horrible stuff though - much more pleasant installing wood fibre, although 20/25mm extra is needed and it's much harder to squeeze in! Link to comment Share on other sites More sharing options...
nostos156 Posted September 1, 2023 Author Share Posted September 1, 2023 2 hours ago, BenP said: Yep - but if it's 600mm it shouldn't need cutting Horrible stuff though - much more pleasant installing wood fibre, although 20/25mm extra is needed and it's much harder to squeeze in! Old house, old studs. Everything's gonna need cutting I suspect. Will probably need to spec a decent insulation saw as well to cut down on fraying. Link to comment Share on other sites More sharing options...
JOE187 Posted September 16, 2023 Share Posted September 16, 2023 A bit late to reply, but make your own insulation saw!! Take an old standard wood saw and grind teeth on each side down, so it just has teeth and will cut through rockwool like butter!! Link to comment Share on other sites More sharing options...
Sparrowhawk Posted September 16, 2023 Share Posted September 16, 2023 1 minute ago, JOE187 said: A bit late to reply, but make your own insulation saw!! Take an old standard wood saw and grind teeth on each side down, so it just has teeth and will cut through rockwool like butter!! I have a collection of blunt wood saws so will give this a go without grinding and hope for the best. My studs are 20-30cm apart (every one different) so plenty of cutting to do. Link to comment Share on other sites More sharing options...
Kelvin Posted September 16, 2023 Share Posted September 16, 2023 I used a long knife and kept with regular sharpening on the electric sharpener it cut through knauf insulation easily with little effort. Link to comment Share on other sites More sharing options...
JohnMo Posted September 16, 2023 Share Posted September 16, 2023 42 minutes ago, Kelvin said: I used a long knife and kept with regular sharpening on the electric sharpener it cut through knauf insulation easily with little effort. I just used some decent pair of industrial scissors. Best purchase ever, used them on loads of stuff on the build. Link to comment Share on other sites More sharing options...
saveasteading Posted September 16, 2023 Share Posted September 16, 2023 1 hour ago, Sparrowhawk said: I have a collection of blunt wood saws It's a very cheap hobby, practiced by many. 1 Link to comment Share on other sites More sharing options...
Kelvin Posted September 16, 2023 Share Posted September 16, 2023 1 hour ago, JohnMo said: I just used some decent pair of industrial scissors. Best purchase ever, used them on loads of stuff on the build. Yes I have pair of those too. I found the knife made neat straight cuts much easier. Link to comment Share on other sites More sharing options...
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