Porthole Posted April 9, 2022 Share Posted April 9, 2022 Hi there I'm in the process of getting the loft insulated and boarded. The builder agreed to put 200mm rockwool insulation down and board the loft. We have noticed it is only 1 layer of 100mm rockwool equivalent between the ceiling joists. We are now also wondering how he should board the loft as you need ideally 270mm rockwool then this is higher than the ceiling joists so will be difficult to board. We've researched different methods such as loft legs or PIR boards with hardboard stuck to them to get a boarded insulated area in the middle of the loft for storage. I'm sure there are many views on the best approach but just wondered if you could provide some useful tips on what to do. I'm having to confront the builder tomorrow so your prompt response would be much appreciated. Thank you! Link to comment Share on other sites More sharing options...
Conor Posted April 9, 2022 Share Posted April 9, 2022 The legs are easy and cheap. When I did mine I also stuck in a couple extra 4x2 timbers between trusses to make it a bit more stable. 300mm loft insulation really is the minimum and you will notice a big difference after. 100mm between joists and 200mm above them. Do not compress the wool. Link to comment Share on other sites More sharing options...
Porthole Posted April 10, 2022 Author Share Posted April 10, 2022 ok thanks. The builder said that 200mm was what was needed for buildings regs - clearly that is incorrect. He suggested using 6 by 2 timbers and putting the insulation in between that so that it is more stable than loft legs. He thought that loft legs were not that great? Link to comment Share on other sites More sharing options...
tonyshouse Posted April 10, 2022 Share Posted April 10, 2022 I did 500mm of quilt loft insulation both between and over the bottom chord of my trusses. I had a 47x50 rail stood on legs off the top of the trusses such that there was act-mm clear ventilation space above the insulation. Incidentally I also managed to get 400mm in the wall plate zone. Link to comment Share on other sites More sharing options...
Conor Posted April 10, 2022 Share Posted April 10, 2022 25 minutes ago, Porthole said: ok thanks. The builder said that 200mm was what was needed for buildings regs - clearly that is incorrect. He suggested using 6 by 2 timbers and putting the insulation in between that so that it is more stable than loft legs. He thought that loft legs were not that great? That might be, but it's so cheap and the main regs are pretty poor tbh. I did 300mm top of the joists, that extra layer also covers over any gaps as you change the direction for each layer. The legs aren't great on their own, that's why I crossed braced with some extra timber, made all the difference. Link to comment Share on other sites More sharing options...
HughF Posted April 11, 2022 Share Posted April 11, 2022 Is there any merit in putting a breathable membrane over the top of the insulation, to prevent air movement within the insulation? Link to comment Share on other sites More sharing options...
Porthole Posted April 12, 2022 Author Share Posted April 12, 2022 ok thanks all. Builder is insisting on using 6 by 2 timbers perpendicular to existing ceiling joists. There will be 11 timbers at 3.6m each. I'm a bit worried this would add alot of extra weight to the loft - not sure if that is a good thing structurally - has anyone any experience of this? Structural engineer is on holiday right now. Thank you Link to comment Share on other sites More sharing options...
joe90 Posted April 12, 2022 Share Posted April 12, 2022 (edited) 8 minutes ago, Porthole said: ok thanks all. Builder is insisting on using 6 by 2 timbers perpendicular to existing ceiling joists. There will be 11 timbers at 3.6m each. I'm a bit worried this would add alot of extra weight to the loft - not sure if that is a good thing structurally - has anyone any experience of this? Structural engineer is on holiday right now. Thank you If they are at 90’ to existing ceiling joists this will load you existing ceiling a fair bit (IMO), I would not do it unless they are supported at intervals/ends. Despite building regs (which are dire at best) 270/300mm insulation is far better as said above. Most builders are not knowledgable on modern insulation/airtightness and just do the minimum allowed by regs 🤷♂️ Edited April 12, 2022 by joe90 Link to comment Share on other sites More sharing options...
Radian Posted April 12, 2022 Share Posted April 12, 2022 On 11/04/2022 at 08:11, HughF said: Is there any merit in putting a breathable membrane over the top of the insulation, to prevent air movement within the insulation? That's what I plan to do. I'm going to top-off my stack of original 200mm fiberglass and recent addition of 200mm mineral wool with Superfoil breathable insulation for a number of reasons - least of which is the R3.6 insulation value. Primarily it's to give a maintainable surface as all the fiberglass already in place has suffered over the years and is covered in dead insects, cobwebs and other detritus. No way can you vacuum or otherwise keep the surface clean. The other slight benefit should be the silver reflective coating helping to reflect IR off in summer and reduce emissivity in winter. Reduction in wind wash is yet another advantage. Link to comment Share on other sites More sharing options...
markc Posted April 12, 2022 Share Posted April 12, 2022 Unless the truss bottom cords are very small and not suitable for load bearing (in which case you should not board or use for any storage) then 90 degree timbers are not detrimental as they are fairly light compared to the flooring and any subsequent storage. ensure all trusses are screwed to the new timbers to distribute the loads and tie all trusses together Link to comment Share on other sites More sharing options...
joe90 Posted April 12, 2022 Share Posted April 12, 2022 1 minute ago, markc said: Unless the truss bottom cords are very small Ah, @Porthole is your roof built with trusses or not? Could make a big difference (I was not thinking of trusses 🙄) Link to comment Share on other sites More sharing options...
markc Posted April 12, 2022 Share Posted April 12, 2022 1 hour ago, joe90 said: Ah, @Porthole is your roof built with trusses or not? Could make a big difference (I was not thinking of trusses 🙄) And I just assumed trusses, but maybe not. Everyday is a school day 1 Link to comment Share on other sites More sharing options...
HughF Posted April 12, 2022 Share Posted April 12, 2022 3 hours ago, Radian said: That's what I plan to do. I'm going to top-off my stack of original 200mm fiberglass and recent addition of 200mm mineral wool with Superfoil breathable insulation for a number of reasons - least of which is the R3.6 insulation value. Primarily it's to give a maintainable surface as all the fiberglass already in place has suffered over the years and is covered in dead insects, cobwebs and other detritus. No way can you vacuum or otherwise keep the surface clean. The other slight benefit should be the silver reflective coating helping to reflect IR off in summer and reduce emissivity in winter. Reduction in wind wash is yet another advantage. Wind wash, that's the term... I'll have a look at that superfoil breathable product. Just out of curiosity, which part of Dorset are you in? I'm over in Purbeck (but I'm not working on that house at present) Link to comment Share on other sites More sharing options...
Radian Posted April 12, 2022 Share Posted April 12, 2022 1 hour ago, HughF said: Wind wash, that's the term... I'll have a look at that superfoil breathable product. Just out of curiosity, which part of Dorset are you in? I'm over in Purbeck (but I'm not working on that house at present) My house in Weymouth is built from your local stone 😀 Link to comment Share on other sites More sharing options...
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