GlanMenai Posted March 17, 2023 Share Posted March 17, 2023 Hello everyone, Could someone guide me; I'm currently insulation our roof with a PIR, inbetween the rafters, and I'm just wondering what happens at the apex of the roof, or the ridge line? I've been searching it today, but haven't found much info. Our pitched roof has: - slates - counter batters - Tyvek breathable membrane - rafters when it comes to the ridge, we haven't got a 'false ceiling' or any horizontal rafters connecting one side of the roof to the other, everything just meets at the apex. My question is, should I add horizontal rafters to create that 'A' shape at the very top of the ridge so that one side of the roof ventilates over to the other side of the roof - or is it enough that it ventilates up through the Tyvek and the ridge tiles? I found a couple of links that shows the 'A' frame shape at the apex. Ours looks identical to the rood in the first link, but without that horizontal rafter. https://idostuff.co.uk/blogs/2013/attic-roof-insulation-and-plaster-boarding/ https://nottenergy.com/lets-go-eco/mini-guides/room-in-roof-insulation/ If I do need those horizontal beams, how much space do they need to down from the apex? I know I need to leave a 50mm gap under the Tyvek when I'm insulating, is it the same up there? Many thanks. Link to comment Share on other sites More sharing options...
Mr Punter Posted March 17, 2023 Share Posted March 17, 2023 It is fine to insulate right up to the apex. One benefit of having a flat area can be that lights are easier to place. It is good to have a small amount of PIR under the rafters as well to prevent cold bridging. 40mm helps a lot and it reduces the risk of condensation on the rafters. Link to comment Share on other sites More sharing options...
Redbeard Posted March 17, 2023 Share Posted March 17, 2023 @Mr Punter. You said "It is fine to insulate right up to the apex". True if there is no ridge-board but if it is like the roof in the OP's first link the between-rafter insulation will surely close off the cross-ventil'n. I always go for a small dropped ceiling. I am even doing so in my extension which does not have a ridge-board, 'just in case'!! Link to comment Share on other sites More sharing options...
nod Posted March 17, 2023 Share Posted March 17, 2023 2 hours ago, Mr Punter said: It is fine to insulate right up to the apex. One benefit of having a flat area can be that lights are easier to place. It is good to have a small amount of PIR under the rafters as well to prevent cold bridging. 40mm helps a lot and it reduces the risk of condensation on the rafters. Plus 1 Lighting is an issue when fully vaulted We have three vaulted ceilings and I’ll add a 600 mil wide flat to the top Link to comment Share on other sites More sharing options...
Mr Punter Posted March 18, 2023 Share Posted March 18, 2023 16 hours ago, Redbeard said: @Mr Punter. You said "It is fine to insulate right up to the apex". True if there is no ridge-board but if it is like the roof in the OP's first link the between-rafter insulation will surely close off the cross-ventil'n. I always go for a small dropped ceiling. I am even doing so in my extension which does not have a ridge-board, 'just in case'!! I guess it would still vent either side of the ridge board in any case. You don't need ventilation from one side to the other. You probably don't get much anyway, as the airflow will go up to the highest point then exit but I agree the flat ceiling area is worth doing. Link to comment Share on other sites More sharing options...
ProDave Posted March 18, 2023 Share Posted March 18, 2023 I am interested in how a roof with a ridge board works without collar ties or a lower level tie? i.e. what stops the roof spreading? Are you sure what you are describing as a ridge board, is not in fact a much heavier ridge beam? Link to comment Share on other sites More sharing options...
Redbeard Posted March 18, 2023 Share Posted March 18, 2023 (edited) @ProDave, what I have in mind as a ridge-board in a typical Victorian terrace roof (approx 125 x 25) is (a) definitely not a beam, (b) meant to have collars below. However I have on a number of occasions seen 'rusticated' attics where the collars have been removed and the soffits plasterboarded right up to ridge-board. Droooop! If you are lucky there are 2 purlins each side lower down and the rafters were spiked in (or some of 'em). @Mr Punter I may be wrong but I have always been of the view that, unless the ridge is vented (which most old ones aren't) you *do* need ventilation from one side to the other. You need an 'in' and an 'out'. Air is drawn through, AIUI, by differential pressure at each 'end', and if there is an 'in' but no 'out' the potentially incoming air has no incentive to travel. I am not as convinced as some that ridge vents are essential, but if there is no ridge vent (in my view) there must be eaves-to-eaves through-flow. One of the ventilation acoutrement manuf'rs had some good info on this but b*****ed if I can find which one. Edited March 18, 2023 by Redbeard + apostrophe Link to comment Share on other sites More sharing options...
ETC Posted March 18, 2023 Share Posted March 18, 2023 https://www.dupont.co.uk/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/BBA-DuPont-Tyvek-Supro-Roof-underlay-Warm-non-ventilated-roofs.pdf Link to comment Share on other sites More sharing options...
Mr Punter Posted March 18, 2023 Share Posted March 18, 2023 24 minutes ago, Redbeard said: unless the ridge is vented OK I get you. Yes I assumed vented because I normally focus on new build. Link to comment Share on other sites More sharing options...
ETC Posted March 18, 2023 Share Posted March 18, 2023 Read the BBA Certificate. Link to comment Share on other sites More sharing options...
GlanMenai Posted March 19, 2023 Author Share Posted March 19, 2023 Hello! Thanks everyone for the replies so far - and I'll just go into a bit more details onto what I'm dealing with. As @Redbeard commented, we do indeed have a ridge-board in a typical Victorian terrace roof (approx 125 x 25) with x2 Purlins either side of the roof - for a bit more clarity I've attached a photo looking down the length of the room. So it sound like I should put a small dropped ceiling in there, just to allow a little bit of air to go from one side to the other - and stop my insulation at the height of the false ceiling, to allow for air to travel from one side to the other freely - and not insulate all the way to the ridge board. Does anyone have any info on how much of a dropped ceiling to put in there? I plan on insulating under the rafters (as well as my current in-between) in here too. Any further help would be amazing. Thanks everyone. Link to comment Share on other sites More sharing options...
Redbeard Posted March 19, 2023 Share Posted March 19, 2023 Hi. Apols if you've said this before, but what is the existing eaves ventilation like? The staining on the membrane on the RHS looks like mould due to poor ventilation. Are you going to mess with the roof at all? For all I've said about not usually venting the ridge, if there's any Q over the 'quality' of the ventil'n at the eaves you might want the extra 'belt and braces' of the ridge vent. In terms of how much of a 'drop' to have at the apex, answer is enough to get a really good level of insulation at that point - say 300 mineral wool or other quilt - 'on the flat' . Make sure to get a really good 'hug' between the PIR and the quilt. Link to comment Share on other sites More sharing options...
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