Jilly Posted October 9, 2022 Share Posted October 9, 2022 Learning as we go as usual. We have a cold roof main building and are joining the flat, warm roof extension to it and the detail is troublesome. I've just realised that if we put deeper insulation above that specified we might have to go higher than the planning permission. I'm gutted and a bit worried. Is there any reason why PIR can't go between the joists as well? Link to comment Share on other sites More sharing options...
Iceverge Posted October 9, 2022 Share Posted October 9, 2022 any cross sections drawings to help us see what you've planned? 1 Link to comment Share on other sites More sharing options...
Jilly Posted October 9, 2022 Author Share Posted October 9, 2022 Link to comment Share on other sites More sharing options...
Johnnyt Posted October 9, 2022 Share Posted October 9, 2022 (edited) Our warm roof is pitched. The BCO was happy with pir between rafter as long as was not as thick the top layer. I think it was 80mm between and 100mm atop. Edited October 9, 2022 by Johnnyt 1 Link to comment Share on other sites More sharing options...
nod Posted October 9, 2022 Share Posted October 9, 2022 100% Yes Link to comment Share on other sites More sharing options...
Radian Posted October 10, 2022 Share Posted October 10, 2022 I've seen plenty of cautions about creating hybrid warm roofs A thorough condensation risk analysis should be carried out rather than just winging it. 2 1 Link to comment Share on other sites More sharing options...
ADLIan Posted October 10, 2022 Share Posted October 10, 2022 In a pitched roof insulation between and over the rafters (hybrid roof) may be acceptable with a careful choice of balance of insulation resistances and use an AVCL. In a flat roofs however this is not good practice. In above roof section there should be an AVCL (polythene for mech fixed and bituminous for fully bonded) on the ply deck. A foil backed plasterboard ceiling should not be relied on. Check insulation manufacturer instructions & BBA Certs 1 Link to comment Share on other sites More sharing options...
ETC Posted October 10, 2022 Share Posted October 10, 2022 Get the supplier/manufacturer of the PIR insulation to do a condensate risk analysis/interstitial condensate risk analysis before you do anything. I would normally expect to see approximately 125mm PIR insulation to meet the U-Value here in NI. Might be higher in mainland UK. 1 Link to comment Share on other sites More sharing options...
Jilly Posted October 10, 2022 Author Share Posted October 10, 2022 Thank you. I've just double checked the planning permission drawings and think we will have to go the cold roof route because of the roof height will be too great with a warm roof. Link to comment Share on other sites More sharing options...
ETC Posted October 10, 2022 Share Posted October 10, 2022 Just now, Jilly said: Thank you. I've just double checked the planning permission drawings and think we will have to go the cold roof route because of the roof height will be too great with a warm roof. Don’t forget that you will need through ventilation for BOTH cold roofs. You will need continuous low-level ventilation to the main roof at the junction of the main and flat roof. You will need low and high-level through ventilation to the flat roof. Personally I’d get a condensation risk analysis done - quick phone call to Kingspan - than muck about with trying to get ventilation. Whichever option you go for the main roof will need ventilated along the junction of the flat and pitched roofs. 1 Link to comment Share on other sites More sharing options...
Iceverge Posted October 11, 2022 Share Posted October 11, 2022 @Jilly I've eventually gotten around to having a look at this. Like @ETC there's something troubling me about the junction of the two roofs. If build as shown I'd be worried about the wind blowing rain from the flat roof up underneath the slates and into the house. The flat roof membrane should continue up the pitched roof to a filleted piece as shown below. Similarly, as said above you will need to ventilate the cold roof with something like slate vent tiles as shown below. Link to comment Share on other sites More sharing options...
Iceverge Posted October 11, 2022 Share Posted October 11, 2022 The more I look at the combo of the warm and cold roof the less I like the look of it. To be really robust it would really need an air control layer joined to the ply on top of the joists below the insulation on the flat roof to a completely continuous layer below the cold roof section on the warm section. If this isn't done with greatest diligence you risk warm moist air finding, it's way into the junction between the two roofs. There will be areas here which will neither from the dreaded combo of cold and lack of ventilation which is a recipe for condensation and rot. It's absolutely doable but it requires an attention to detail familiar to a builder who is proven get passivhaus levels of airtightness. Alternatively, you could add a vapour control layer below the bottom chord of the truss and the 225*50mm joists. It'd be much easier to detail but really should be done with a service cavity to avoid puncturing the layer, l(owering ceiling height). A strong knowledge of why having no penetrations is important is vital again with whoever is installing. If you can go to a continuous layer of external insulation on the roof (warm roof) all these problems go away. How tight are you really on ridge height? it'll lightly to be only 100mm extra . Have you ordered the roof trusses? What stage is construction at the minute? Link to comment Share on other sites More sharing options...
Jilly Posted October 11, 2022 Author Share Posted October 11, 2022 Thank you! The 220mm joists are ordered and going up. There was confusion on the drawings and we probably could have been ok with 150mm but it's too late now. The build up on the planning is only 200mm so we are a bit stuffed to go higher. BC have said we can put 150mm PIR between the joists and 40mm below to make the most of the deep joists, keeping 50mm for ventilation. The next excitement is how to manage ventilation on the joists going the other direction forming the cantilevered porch. Link to comment Share on other sites More sharing options...
Iceverge Posted October 11, 2022 Share Posted October 11, 2022 25 minutes ago, Jilly said: The build up on the planning is only 200mm so we are a bit stuffed to go higher. You could rip them down to 150mm at the thin end removing the requirement for separate furring strips and making the most of your ceiling height. 27 minutes ago, Jilly said: confusion on the drawings This is the root cause of your issues. I'd be tempted to put all tools back in the van until this was really sorted. Have the roof trusses been ordered? 1 Link to comment Share on other sites More sharing options...
ETC Posted October 11, 2022 Share Posted October 11, 2022 Keep both roof “cold” and put in ventilation. 1 1 Link to comment Share on other sites More sharing options...
Jilly Posted October 12, 2022 Author Share Posted October 12, 2022 14 hours ago, Iceverge said: You could rip them down to 150mm at the thin end removing the requirement for separate furring strips and making the most of your ceiling height. This is the root cause of your issues. I'd be tempted to put all tools back in the van until this was really sorted. Have the roof trusses been ordered? Yes, the problem is that I (stupidly) in hindsight changed to an architect's technician (from my original expensive, but good architect), who was a friend and made loads of mistakes and didn't work well the the structural engineer, so the builder and I are trying to make sense of it all. Currently some of the 220mm trusses are up, but the builder is away for a few days luckily, so I can breathe. He got very stressed when I suggested changing them... Link to comment Share on other sites More sharing options...
Iceverge Posted October 12, 2022 Share Posted October 12, 2022 He could cut them in situ. Assuming the roof falls from left to right in your drawings Alas these things happen. We learn as we go but from my own experience ploughing ahead without a clear plan leads to double time wasted later. Can you get a few photos of where the build is at at the minute? Can you post the full drawing of the section you have done above. One of the online PDF editors can be used to blank out your personal details. The 220mm timbers here are "joists" as they are just straight timbers that run parallel to the main span of the roof. They are off the shelf items and need to be trimmed on site to fit. The drawing you posted mentions trusses in orange writing in the bottom left. These are typically bespoke items made from much lighter guage timber manafactuered in a factory to size and are not adjustable on site. Once you order them you are stuck with what you've got. Do you know if these are ordered? Link to comment Share on other sites More sharing options...
Temp Posted October 12, 2022 Share Posted October 12, 2022 My understanding is that it _might_ be possible to put some insulation between and above on a warm roof construction. However doing this moves some of the rafter towards the cold side so a condensation risk analysis should be done. It would also be advisable/essential to install a vapour barrier below the insulation. I recall reading that about a 1/3rd of the insulation between is about the maximum you can do. 1 Link to comment Share on other sites More sharing options...
Jilly Posted October 12, 2022 Author Share Posted October 12, 2022 3 hours ago, Temp said: My understanding is that it _might_ be possible to put some insulation between and above on a warm roof construction. However doing this moves some of the rafter towards the cold side so a condensation risk analysis should be done. It would also be advisable/essential to install a vapour barrier below the insulation. I recall reading that about a 1/3rd of the insulation between is about the maximum you can do. It looks like we are going cold roof, so I'm checking ways to achieve good ventilation Link to comment Share on other sites More sharing options...
ETC Posted October 12, 2022 Share Posted October 12, 2022 (edited) https://glidevaleprotect.com/products/roofing-ventilation/ Edited October 12, 2022 by ETC 1 1 Link to comment Share on other sites More sharing options...
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