LiamJones Posted October 17, 2023 Share Posted October 17, 2023 I'm doing a bungalow conversion, raising the eaves and replacing the roof to form 3 bedrooms upstairs. I initially looked into trusses, but decided against them after reading of troubles fitting insulation and achieving airtightness around the webs. Since, i've instructed a structural engineer to design a ridge beam and specify the rafter spec. I now need to make a decision on how i incorpate my insulation and airtightness into this. I've gone back and forward on cold vs hybrid vs warm roof, and i think i've settled with a cold roof for simplicity. I've also deviated from the original structural spec (with SE's permission), and gone to 44 x 175 rafters @ 450c to reduce PIR wastage (after watching Robin Clevett on youtube) Would anyone like to change my mind again? Link to comment Share on other sites More sharing options...
Conor Posted October 17, 2023 Share Posted October 17, 2023 Can the ridge beam be brought down so the rafters sit on top? Means no cold bridge at the ridge and a simpler fixing detail. 3 Link to comment Share on other sites More sharing options...
Iceverge Posted October 17, 2023 Share Posted October 17, 2023 Here's a drawing of what you have. I've added a 20mm service cavity. I like Robin Clevett apart from his instance on using PIR between rafters and stud walls. It has a poor decrement delay (phase shift), requires fantastic attention to detail to get a tight fit, doesn't perform particularly well in fire or noise situations. Returning to 195mm rafters at 600mm centres would be far easier + cheaper. Buy Rockwool flexirock ( or similar) batts and just shove them between the rafters. You'll get a tight fit easily and they can flex with the roof timbers as they contract and expand. No wastage either as there's always a corner that the offcuts can be stuffed into. At the moment I'm doing my garage. Note the ventilation gap above the membrane via batten and counter batten. I was going to use blown cellulose between the rafters but I got a good deal on the rockwool. I'm beginning to regret this TBH as my rafter spacing isn't suited to the batts and I'm doing too much cutting. I'm coming around to the idea of using as external layer sheeting as the airtight barrier like the Americans. I did my garden office/pumphouse recently and taped all the joints in the OSB externally. It was super tight and much easier and robust than taping an internal airtight membrane. That coupled with a layer of external foam would be super easy to apply and eliminate all thermal bridging. 1 1 Link to comment Share on other sites More sharing options...
LiamJones Posted October 17, 2023 Author Share Posted October 17, 2023 14 minutes ago, Conor said: Can the ridge beam be brought down so the rafters sit on top? Means no cold bridge at the ridge and a simpler fixing detail. Really good idea, I was going to run PIR under the beam and put in a collar tie to fix the plasterboard onto. But yeah that cold bridge isn't ideal. Getting away from hangers would be nice too! What does the detail look like, a piece of timber on top of the steel, with the rafters birdmouthing into it? I've also fired the question off to the SE. 2 hours ago, Iceverge said: It has a poor decrement delay (phase shift), requires fantastic attention to detail to get a tight fit, doesn't perform particularly well in fire or noise situations. Thanks for the detailed respone, given me lots to think about whilst eating lunch! You've just introduced me to decrement delay and i'm curious, the phase shift of the PIR build up i proposed is longer (7.6hr) than that of the rockwool version (7.4hr) you proposed. That prompted me to do a bit of reading, and articles like this one, show rockwool (i've assumed rockwool == stonewool) as having a shorter decrement delay than PIR. What am i missing @Iceverge? Link to comment Share on other sites More sharing options...
Iceverge Posted October 17, 2023 Share Posted October 17, 2023 Correct but it's readily available and would substitute nicely in there for your PIR. Decrement delay is a function of density and thermal conductivity. It is how long the heat takes to "soak" through a roof initially before it reaches a steady state of heat loss/gain. This would be more ideal in terms of decrement delay. The densepack cellulose really helps with airtightness too. Tiles. Tile Battens Counterbattens Membrane OSB 220mm cellulose between 220*45mm rafters @600cc Vapour membrane 50mm Hemp batts between 47*45mm battens at 90deg to the rafters @ 600cc for service cavity. 15mm plasterboard. Skim. Link to comment Share on other sites More sharing options...
Conor Posted October 17, 2023 Share Posted October 17, 2023 2 hours ago, LiamJones said: Really good idea, I was going to run PIR under the beam and put in a collar tie to fix the plasterboard onto. But yeah that cold bridge isn't ideal. Getting away from hangers would be nice too! What does the detail look like, a piece of timber on top of the steel, with the rafters birdmouthing into it? I've also fired the question off to the SE. Thanks for the detailed respone, given me lots to think about whilst eating lunch! You've just introduced me to decrement delay and i'm curious, the phase shift of the PIR build up i proposed is longer (7.6hr) than that of the rockwool version (7.4hr) you proposed. That prompted me to do a bit of reading, and articles like this one, show rockwool (i've assumed rockwool == stonewool) as having a shorter decrement delay than PIR. What am i missing @Iceverge? For the ridge beam, our guys machined a length of C24 wall plate timber to the same angle as the roof, and fixed it to the top of the beam, then (in our cause) roof panels just screwed in to the timber. In your case the roof joists would be fixed in to the timber as well, joiner will know what to do. Link to comment Share on other sites More sharing options...
Iceverge Posted October 17, 2023 Share Posted October 17, 2023 Or use a glulam. You could leave it exposed. Link to comment Share on other sites More sharing options...
LiamJones Posted October 18, 2023 Author Share Posted October 18, 2023 Dense cellulose is coming in about 3x as expensive as PIR to get similar u vals for the material alone, let alone paying a firm to blow it in! Link to comment Share on other sites More sharing options...
Iceverge Posted October 21, 2023 Share Posted October 21, 2023 I would say it's worth getting a quote from a supply and fit company. It was comparable when we built ours. PIR between rafters isn't a recipe for success. Link to comment Share on other sites More sharing options...
LiamJones Posted March 25 Author Share Posted March 25 6 months later, i'm almost ready to start doing this! Thanks for pushing me towards cellulose @Iceverge. Got a quote back from warmcel cheaper than the PIR. So i'm going this route. I understand i need something to hold it in place above and below. so i'm thinking the following roof buildup. What sort of vapur barrier to i need above? any recommendations on a particular one. How about vapour control below, i understand the foli faced PIR, taped, to achieve all i need in terms of vapour and air tightness? Cheers, Liam Link to comment Share on other sites More sharing options...
Iceverge Posted March 25 Share Posted March 25 26 minutes ago, LiamJones said: What sort of vapur barrier to i need above? any recommendations on a particular one. Breather membrane is what 's going there. Glidevale VP400 is good from screwfix. Obviously pro clima and Siga products are too but they're ££££. 27 minutes ago, LiamJones said: How about vapour control below, i understand the foli faced PIR, taped, to achieve all i need in terms of vapour and air tightness? That buildup will make it hard to blow in the cellulose. I would ditch the PIR. A membrane below would be better as per the above drawing. What is your maximum roof thickness and target U Value? I'm still stuffing rockwool between the rafters of my garage when I get a spare minute. It's a pain in the neck. Wish I'd done cellulose. What rough price did you get? Link to comment Share on other sites More sharing options...
LiamJones Posted March 25 Author Share Posted March 25 (edited) 28 minutes ago, Iceverge said: That buildup will make it hard to blow in the cellulose. I would ditch the PIR. A membrane below would be better as per the above drawing. The warmcel installer said he'd be happy to core through 50mm PIR between every rafter (600c). I do like the idea of PIR here to bring the u val down by 0.06 28 minutes ago, Iceverge said: What is your maximum roof thickness and target U Value? Rafters are 8x2, dont want to lose too much below as it'll be room in roof. above, less limited, although i dont like the idea of a warm roof. target u val, dont really have one, as good as i can get, whilst still having good decrement delay, good accoustically and good in the real world (e.g not PIR between) 28 minutes ago, Iceverge said: I'm still stuffing rockwool between the rafters of my garage when I get a spare minute. It's a pain in the neck. Wish I'd done cellulose. What rough price did you get? Yeah, really want to avoid that! £2313 for 120m2 @ 200mm, that's ex vat, but it should be 0 rated. Edited March 25 by LiamJones Link to comment Share on other sites More sharing options...
Iceverge Posted March 25 Share Posted March 25 1 minute ago, LiamJones said: The warmcel installer said he'd be happy to core through 50mm PIR between every rafter (600c). I do like the idea of PIR here to bring the u val down by 0.06 Should work fine so. No need to be completely fastidious about any small gaps between the boards and the cellulose will fill them Just make sure that it's a proper full fill. Airtight tape the holes to finish. An alternative to a separate airtight layer might be to tape the PIR face. I think @Thorfun did internal PIR. Maybe they have more tips. You could always put a counter batten above the Airtight membrane and bin the PIR. The cellulose would then be under the rafters too, breaking any thermal bridge. So many ways to skin a cat. @600cc you'll be better off with 15mm PB. 7 minutes ago, LiamJones said: Yeah, really want to avoid that! £2313 for 110m2 @ 200mm, that's ex vat, but it should be 0 rated. £105/m³ plus vat. That's a good price. Whatever my next project is I think I'll try to have 100% blown in insulation. So much less hardship and waste. 1 Link to comment Share on other sites More sharing options...
Thorfun Posted March 25 Share Posted March 25 23 minutes ago, Iceverge said: I think @Thorfun did internal PIR. Maybe they have more tips. i did indeed. on walls and roofs. tips? it's a pain to do especially on attic trusses (i can only imagine how much worse it would be on standard roof trusses!). hopefully the OP won't have this issue with room in roof though as the roof probably isn't built using attic trusses. we fully filled the walls/trusses with glass wool (horrible stuff) and then screwed PIR (80mm on walls and 100mm on roofs) in to the TF and roof. we then put AVCL membrane over the PIR. we used double sided tape to hold it in place until the service cavity battens were fitted and they held the membrane nicely. 38 minutes ago, LiamJones said: The warmcel installer said he'd be happy to core through 50mm PIR between every rafter (600c). I do like the idea of PIR here to bring the u val down by 0.06 i wish we'd done blown in cellulose. glass wool was horrible. it's great that the installer will core through the PIR to fill the roof. good luck. sounds like a great roof buildup. 1 1 Link to comment Share on other sites More sharing options...
LiamJones Posted April 15 Author Share Posted April 15 Any tips for the eaves detail on this build up, do i need ventilation between breather membrane and the tiles (e.g, through the 50mm batten/counter batten void)? i've seen eaves trays with built in ventilation, but to the underside of them, whereas i want the ventilation over the top of the tray? Link to comment Share on other sites More sharing options...
ProDave Posted April 15 Share Posted April 15 OV10 vent in place of or on top of very bottom tile batten https://www.roofingventilation.co.uk/Over-Fascia-Vent-10mm-x-1m-Harcon-OV10 Link to comment Share on other sites More sharing options...
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