zoothorn Posted January 22, 2020 Author Share Posted January 22, 2020 So what's the way to go here. I need to establish: do I put in rockwool before electrician runs cable within/ across joists: can I fill the 'cavity' of the actual joists (pic below): what rockwool, as I have to cut it bang on 1/2 point (of 600mm width).. IE if cutting is an issue/ nasty job etc. I have 4200mm x 3800mm total ceiling (wall to wall). Joists are 100mm width. Gaps between joists are 300mm W. The height of my gaps, is 205mm.. so I could squash in 200mm slab, but then hasn't the electrician not got ample room to work? also means slight pressure on the pB (a tricky job as it is, me + someone to do ceiling). So I might go for 140mm or 180mm rockwool. Thanks, zoot Link to comment Share on other sites More sharing options...
PeterW Posted January 22, 2020 Share Posted January 22, 2020 Sparks should run his cables in the webs and then you can go over / under the wires. He needs to de-rate the cables as it’s in the insulation but you only have sockets and a single light or two so I wouldn’t be worried. You can get FrameTherm in rolls - i would use that instead as slab is for walls normally. Link to comment Share on other sites More sharing options...
zoothorn Posted January 22, 2020 Author Share Posted January 22, 2020 13 minutes ago, PeterW said: Sparks should run his cables in the webs and then you can go over / under the wires. He needs to de-rate the cables as it’s in the insulation but you only have sockets and a single light or two so I wouldn’t be worried. You can get FrameTherm in rolls - i would use that instead as slab is for walls normally. Ok so I wait for spark to do his work 1st, before I do insulation? (both ceiling here -and- the walls too?). Can you explain what "de-rate cables as its in the insulation" means? (actually tbh I cannot make sense of the whole of the 2nd sentence). I will have about 5 overhead spot lights (flush-mounted round ~8cm types). Isn't frame-therm the 'orange loft stuff' tho that my builder suggested a huge pain to use, as it'll fall out? not sure. I really need the best-working-material here, or I see a nightmare keeping it up in place: I think this was the builder's thinking. Thanks, zoot Link to comment Share on other sites More sharing options...
ProDave Posted January 22, 2020 Share Posted January 22, 2020 Frametherm 35 on a roll is quite stiff and generally stays put. It is a lot less nasty to work with than most of the "loft" type glass wool insulation. Between the posijoists I wedged it in, then sotted any old offcuts of wood just to ensure it did not fall down before the plasterboard went on. Where possible I clipped cables to the side of the bottom timber of the joists so they were not in contact with the insulation. Just to show how stiff Frametherm 35 is, I used it in my warm roof. Just a push fit there, nothing else. I had one test piece that sat there 6 months before the plasterboard went on, and it did not move at all. Link to comment Share on other sites More sharing options...
zoothorn Posted January 22, 2020 Author Share Posted January 22, 2020 @ProDave thanks for pics- useful as I have same 'open-frame' timber-frame joists as you 1st pic. Ok so on this pic, you have what 1st pushed up the '35' stuff (before electrics?), then what splayed some apart & somehow got -within- the frames of the joists (?). Is that a white cable running diogonally across joists, if so how can this not be in contact with the insulation (& why can't it be in contact- shouldn't it be?) sorry a few Q's.. Link to comment Share on other sites More sharing options...
ProDave Posted January 22, 2020 Share Posted January 22, 2020 I did not bother filling in between the gaps. The white cable is tv aerials and network so not bothered by being in contact with insulation, taking the shortest route. Link to comment Share on other sites More sharing options...
zoothorn Posted January 22, 2020 Author Share Posted January 22, 2020 Ive googled de-rating cable.. but I'm suddenly in total confusion again/ cannot possibly cope with understanding it. Ok as it is, my spark will be doing the electrics (& another whose Part-P certified doing whatever it is he does for me) so the intricate electrical theory.. I have to put aside. So all I have to do, is the insulation. Its just a question of when to do it, what stuff to use, if I can get it into the joists, where the cables are relative to it (& whether I need to ask the spark anything regarding the proximity of cables/ insulation). Link to comment Share on other sites More sharing options...
zoothorn Posted January 22, 2020 Author Share Posted January 22, 2020 8 minutes ago, ProDave said: I did not bother filling in between the gaps. The white cable is tv aerials and network so not bothered by being in contact with insulation, taking the shortest route. What did you do 1st: electrics, or push the '35' stuff up between joists? Link to comment Share on other sites More sharing options...
PeterW Posted January 22, 2020 Share Posted January 22, 2020 You won’t get insulation into the webs so don’t try. Much easier to get the sparky to run first fix now and then insulate around it Link to comment Share on other sites More sharing options...
zoothorn Posted January 23, 2020 Author Share Posted January 23, 2020 17 hours ago, PeterW said: You won’t get insulation into the webs so don’t try. Much easier to get the sparky to run first fix now and then insulate around it Great- ok thanks Peter. That's the plan/ clarity I needed. So if the spark lays lays cable perp to the joists, across the lower wood joist 'rung', & I push up '35 stuff' above it.. do I need to be considering the insulation touching the cable, if I'm going 140mm D of '35 stuff'.. & so have a 40mm 'gap' below to cable? I'm not understanding the interaction of the cable to the wool (the 'derate' thing has confused me). Thanks- zoot. Link to comment Share on other sites More sharing options...
Ed Davies Posted January 23, 2020 Share Posted January 23, 2020 22 minutes ago, zoothorn said: I'm not understanding the interaction of the cable to the wool (the 'derate' thing has confused me). When a cable carries current it lets off heat. The more current the more heat [¹]. If it's exposed to the air it can get rid of the heat easily so won't get so hot for a given current. If it's buried in insulation it can't get rid of the heat so well and will get warmer for that same current, or to keep the maximum temperature the same it can carry less current. There are tables showing how much current different sized cables can carry in different circumstances (e.g., clipped to the outside of a wall, clipped to the top of a ceiling under insulation, buried in insulation, …). Your electrician will deal with this but it's worth bearing in mind if you're going to have any high-power tools in your workshop. [¹] Proportional to the square of the current, so, e.g., 32 amps results in 4 times as much heat per metre of cable as 16 amps so, for any given level of insulation, 4 times the temperature rise. Link to comment Share on other sites More sharing options...
zoothorn Posted January 23, 2020 Author Share Posted January 23, 2020 3 minutes ago, Ed Davies said: When a cable carries current it lets off heat. The more current the more heat [¹]. If it's exposed to the air it can get rid of the heat easily so won't get so hot for a given current. If it's buried in insulation it can't get rid of the heat so well and will get warmer for that same current, or to keep the maximum temperature the same it can carry less current. There are tables showing how much current different sized cables can carry in different circumstances (e.g., clipped to the outside of a wall, clipped to the top of a ceiling under insulation, buried in insulation, …). Your electrician will deal with this but it's worth bearing in mind if you're going to have any high-power tools in your workshop. [¹] Proportional to the square of the current, so, e.g., 32 amps results in 4 times as much heat per metre of cable as 16 amps so, for any given level of insulation, 4 times the temperature rise. Understand the principle fine. But how I could possibly put into practise is the thing, or know what the heck I should be doing.. in my situation. The walls are simple to put all this technical info into practise: I make a channel in the kingspan/ done. These are the higher current power tools' sockets too. Its this problematic 'push wool up between joists' I cannot get the idea of, if by nature its going to at the least bow down in middle twds cable.. & possibly after years actually sag down & come into contact. It is though, only light cables running within the joists, & thesedays presumably not the halogen jobs but led ceiling 'spots' too. Link to comment Share on other sites More sharing options...
ProDave Posted January 23, 2020 Share Posted January 23, 2020 In the ceiling space get the electrician to clip all the cables to the TOP cord of the joists if going in before the insulation. Then the insulation will not touch the cables or fall down onto them. Wiring regs say you only have to consider insulation if more than 300mm of the cable is embedded in insulation, so where it drops down for say a light fitting it will pass through less than 300mm of insulation so is not considered. Link to comment Share on other sites More sharing options...
zoothorn Posted January 23, 2020 Author Share Posted January 23, 2020 @ProDave so clip to the underside of the timber 'top' of the joist? (these joist timber 'tops' have my floor above, glued to them).. then the wool insulation below it? Understand the regs on 300mm wiring- great, thanks for that/ one thing I can tick off 'ok'. Link to comment Share on other sites More sharing options...
zoothorn Posted January 28, 2020 Author Share Posted January 28, 2020 On 23/01/2020 at 12:22, ProDave said: In the ceiling space get the electrician to clip all the cables to the TOP cord of the joists if going in before the insulation. Then the insulation will not touch the cables or fall down onto them. Wiring regs say you only have to consider insulation if more than 300mm of the cable is embedded in insulation, so where it drops down for say a light fitting it will pass through less than 300mm of insulation so is not considered. ProDave can you just explain what you mean by 'top cord' of joist? the underside of the top timber? What my builder guys (not the main one) suggested re. insulation, is go 100mm more (ie 400mm for my 300mm gaps) & bow it upwards to facilitate keeping in place. Which seems sensible- is this the general rule of thumb? But, a problem if so: if wool is bowing up & the cables are tied (as I assume you mean by top cord) to underside of top joist timber.. it will likely push up touching the cables. Link to comment Share on other sites More sharing options...
Hastings Posted March 5, 2020 Share Posted March 5, 2020 As another poster in another topic pointed out, bear in mind that as far as acoustics go (not thermal) in Mitek's published lab tests Posi Joists benefit by only a 5% (difference of +1dB Rw) in sound transmission when 100mm rockwool is added, compared to when zero insulation fitted between. So maybe don't try and put too much in there. Link to comment Share on other sites More sharing options...
zoothorn Posted March 7, 2020 Author Share Posted March 7, 2020 On 05/03/2020 at 12:20, Hastings said: As another poster in another topic pointed out, bear in mind that as far as acoustics go (not thermal) in Mitek's published lab tests Posi Joists benefit by only a 5% (difference of +1dB Rw) in sound transmission when 100mm rockwool is added, compared to when zero insulation fitted between. So maybe don't try and put too much in there. Thanks for this post / info Hastings. I'm back onto this topic as next job to do now. I'll aim for 100mm of Frametherm 35 then. The 1st fix electrics- done. I asked to clip to top tier of posi-joists.. & where possible like for 5x downlights, this is done. I do have a fair bit of cabling midway between joists tho, coming down & across etc from the top room + this lower room together > & out thru old thick wall into main house. What's the situation here: do I put wool nearly up to these cables, or leave a 1 ft gap? I don't know. Also my 1st fix spark wants a good ~10" cleared 'area' around & above each downlight (5w led, fire-rated). Is this warranted? I hadn't accounted for these 'holes'. Link to comment Share on other sites More sharing options...
zoothorn Posted March 18, 2020 Author Share Posted March 18, 2020 @ProDave could I pick this thread up (today buying my fluff). Your frametherm 35 stuff ProDave.. could you remind me of your joist spacing width, & what width fluff you therefore chose? thanks- zoot Link to comment Share on other sites More sharing options...
ProDave Posted March 18, 2020 Share Posted March 18, 2020 The frametherm 35 comes on 1 1200mm wide roll. you cut it to the width you want with a panel saw while it is still rolled up and wrapped in it's plastic wrapping when it is easy to cut. Most of my studs are on 600mm centres so you simply cut each roll in half. Link to comment Share on other sites More sharing options...
zoothorn Posted March 18, 2020 Author Share Posted March 18, 2020 Hi ProDave 7 minutes ago, ProDave said: The frametherm 35 comes on 1 1200mm wide roll. you cut it to the width you want with a panel saw while it is still rolled up and wrapped in it's plastic wrapping when it is easy to cut. Most of my studs are on 600mm centres so you simply cut each roll in half. Ok understand that. But, I was under impression that this stuff comes in 400mm x 1200mm rolls.. so in my case I'm sure someone said to always use more than my stud widths in order to get it to stay in? mine are 300mm centres, so wasn't 400mm said to be correct? I cannot find it tho. I know its simple to all here, but I'm all at sea having never done this. I don't have luxury of doing any of these small steps/ jobs again, or redoing, so takes me 2 days to do X (plus hours of research before) that might take anyone experienced 2 hrs. So in your case here, you had to put in many, many sections.. because they were only 400mm long-? (assuming your rolls were 1200 x 400.. divided into two, so dozens of 600mm x 400mm very very short sections. I just don't understand. Link to comment Share on other sites More sharing options...
ProDave Posted March 18, 2020 Share Posted March 18, 2020 If your studs are 300mm centres (check, that seems very close) then you cut the frametherm to 300mm wide. At 300mm centres, the gap between them will be more like 255mm so the 300mm insulation will squash in and be a tight fit. Link to comment Share on other sites More sharing options...
zoothorn Posted March 18, 2020 Author Share Posted March 18, 2020 6 minutes ago, ProDave said: If your studs are 300mm centres (check, that seems very close) then you cut the frametherm to 300mm wide. At 300mm centres, the gap between them will be more like 255mm so the 300mm insulation will squash in and be a tight fit. Ah dammit sorry I'm getting my centres & my gap widths mixed up. Inexperience. My gaps between joists are known @ 300mm. (so my centres are gonna be.. 400mm then is it). Link to comment Share on other sites More sharing options...
andyscotland Posted March 18, 2020 Share Posted March 18, 2020 Frametherm 35 comes in a variety of formats https://www.knaufinsulation.co.uk/products/glass-mineral-wool/earthwool-frametherm-roll-35 including pre-cut at either3*380mm widths or 2*570mm, so be careful what you order. If your joists are really at 300mm centres then the 380 might be a tiny bit wide so cutting your own is probably better. The length of the roll varies with the thickness of the insulation (fairly standard for all products, generally you get a roughly consistent weight/volume of rolled product so the thicker it is the shorter it is). 90mm frametherm is on a 6m roll from the look of it and 140mm on 3.9m. Link to comment Share on other sites More sharing options...
andyscotland Posted March 18, 2020 Share Posted March 18, 2020 2 minutes ago, zoothorn said: Ah dammit sorry I'm getting my centres & my gap widths mixed up. Inexperience. My gaps between joists are known @ 300mm. (so my centres are gonna be.. 400mm then is it). What thickness are your joists? If the gap is 300mm then for the centres to be 400mm means the joists are 100mm thick which seems high? Link to comment Share on other sites More sharing options...
zoothorn Posted March 18, 2020 Author Share Posted March 18, 2020 1 minute ago, andyscotland said: What thickness are your joists? If the gap is 300mm then for the centres to be 400mm means the joists are 100mm thick which seems high? The joist width is indeed 100mm. So if 300mm gaps between, & 400mm centres. Just measured to check. (The joist height.. I think n/a just for now.. is 200mm). Link to comment Share on other sites More sharing options...
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