Coffeepotclaire Posted January 30, 2021 Share Posted January 30, 2021 Hi everyone, Please help, getting conflicting advice from BCO, Celotex and my architect! In short, I am converting my loft space. The property is a detached bungalow with an up and over roof, hip ended at both sides with a full width dormer at the rear. My rafters are 6 x 2 and I want to use ideally, 120mm rigid insulation between the rafters and 25mm across the underside. This will obviously then give me an air gap between the top of the insulation and the roofing membrane of 25mm. For the record, I have used a Tyvek breathable membrane. The roof has been designed to be unvented apart from the dormer roof which is a cold deck and requires ventilation. This has been catered for by a vented ridge and vents at the front edge of the dormer. I should say that the dormer is the full width of the roof, hip to hip. BCO have said they would like the whole roof vented with a 50mm continuous air gap. My architect has said 25mm with the breathable membrane is more than sufficient. And Celotex have said 25mm is sufficient if the roof is not vented. However, 50mm if it is. I hope that all makes sense. Any help appreciated. Link to comment Share on other sites More sharing options...
ADLIan Posted January 30, 2021 Share Posted January 30, 2021 You need to check with Tyvek and their instructions & BBA certificate. This will show required gap any ventilation requirement. Compliance with a bba is deemed to comply with the Regs Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 41 minutes ago, ADLIan said: You need to check with Tyvek and their instructions & BBA certificate. This will show required gap any ventilation requirement. Compliance with a bba is deemed to comply with the Regs Thanks for your response. Tyvek have confirmed that their product will perform with either gap dependant upon roof design. Hence why I find BCO's stance frustrating! Link to comment Share on other sites More sharing options...
Jason L Posted January 30, 2021 Share Posted January 30, 2021 They are kinda all right, you don't have to ventilate it with a breather membrane, so a 25m air gap is ok, but if it is a ventilated roof, it should be a 50mm air gap. however i guess you can go with the 25mm air gap and add vents it as an extra precaution. Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 3 minutes ago, Jason L said: They are kinda all right, you don't have to ventilate it with a breather membrane, so a 25m air gap is ok, but if it is a ventilated roof, it should be a 50mm air gap. however i guess you can go with the 25mm air gap and add vents it as an extra precaution. Thanks for your reply Jason. Celotex are telling me that if I introduce vents I have effectively created a vented roof in which case the air gap would have to be a minimum of 50mm for the roof to vent efficiently. Hence why I'm finding this all so confusing! Link to comment Share on other sites More sharing options...
Jason L Posted January 30, 2021 Share Posted January 30, 2021 9 minutes ago, Coffeepotclaire said: Thanks for your reply Jason. Celotex are telling me that if I introduce vents I have effectively created a vented roof in which case the air gap would have to be a minimum of 50mm for the roof to vent efficiently. Hence why I'm finding this all so confusing! dont vent it then, but is BCO officer asking for it as the dormer isnt adequately ventilated? its best to get clarification from them exactly why he wants the whole roof done, .. your Architect should really be dealing with it for you. Link to comment Share on other sites More sharing options...
Temp Posted January 30, 2021 Share Posted January 30, 2021 (edited) If its vapour permeable 25mm is fine. As I understand it the 25mm is to allow the membrane to drape so its not pressed against the underside of the tile battens. If that happens it can stop any water blown under tiles from running down. If you don't allow that dirt can cause the water to pool above the batten causing the batten to rot. Edit: if counter battens are used the gap can be zero. However... as @Jason L said the BCO might be worried about the dormer roof not being adequately ventilated. The problem is the rafters in the dormer roof normally mean air cant move side to side so you need vents at eaves and ridge. Not sure you can fit a big enough vent at the ridge to get adequate air flow. What has been specified there? I think Celotex are just confused about what you are doing. Edited January 30, 2021 by Temp Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 6 minutes ago, Jason L said: dont vent it then, but is BCO officer asking for it as the dormer isnt adequately ventilated? its best to get clarification from them exactly why he wants the whole roof done, .. your Architect should really be dealing with it for you. Thanks Jason. The dormer is adequately vented. The rest of the roof was built effectively as a separate structure. 1 Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 2 minutes ago, Temp said: If its vapour permeable 25mm is fine. As I understand it the 25mm is to allow the membrane to drape so its not pressed against the underside of the tile battens. If that happens it can stop any water blown under tiles from running down. If you don't allow that dirt can cause the water to pool above the batten causing the batten to rot. However... as @Jason L said the BCO might be worried about the dormer roof not being adequately ventilated. The problem is the rafters in the dormer roof normally mean air cant move side to side so you need vents at eaves and ridge. Not sure you can fit a big enough vent at the ridge to get adequate air flow. What has been specified there? I think Celotex are just confused about what you are doing. Thanks Temp! The dormer roof has more than 80mm clear ventilation between the insulation and the roof deck from the front of the dormer out through the vented ridge. Link to comment Share on other sites More sharing options...
Temp Posted January 30, 2021 Share Posted January 30, 2021 Provided the free area of the ridge vents are ok that should be fine. Treated like any other flat roof they normally require 25,000 sqmm per meter. Eg a 25mm wide slot. Sounds like the BCO is just concerned about the mixture of techniques. Not sure why though. Link to comment Share on other sites More sharing options...
Temp Posted January 30, 2021 Share Posted January 30, 2021 I don't suppose you have enough headroom to make the dormer a warm roof? (Insulation above rafters rather than between). Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 58 minutes ago, Temp said: Provided the free area of the ridge vents are ok that should be fine. Treated like any other flat roof they normally require 25,000 sqmm per meter. Eg a 25mm wide slot. Sounds like the BCO is just concerned about the mixture of techniques. Not sure why though. Thanks Temp, Regarding the air ventilation, on the basis of your calculation it would appear I more than comply - thanks very much. Link to comment Share on other sites More sharing options...
Coffeepotclaire Posted January 30, 2021 Author Share Posted January 30, 2021 26 minutes ago, Temp said: I don't suppose you have enough headroom to make the dormer a warm roof? (Insulation above rafters rather than between). Unfortunately head room was tight for a warm roof. Link to comment Share on other sites More sharing options...
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