Post and beam Posted December 17, 2024 Share Posted December 17, 2024 Our build is a 'cold roof' according to the TF provider. The airtight boundary is at the first floor ceiling level between the plasterboard and the rafters ( not yet installed). With 400mm rockwool due to be placed on the loft decking. So the loft will be unheated, and presumably cold in the winter because the insulation should make this so. My confusion is therefore..... If the airtight membrane allows vapour to pass upwards into the loft will it not condense out any moisture contained within ? Have i totally misunderstood the role of the various membranes? Highly likely i think. Link to comment Share on other sites More sharing options...
ToughButterCup Posted December 17, 2024 Share Posted December 17, 2024 Cold roof design Cold vs Warm roof Link to comment Share on other sites More sharing options...
lookseehear Posted December 17, 2024 Share Posted December 17, 2024 Everything you say is correct other than the vapour won't condense because a cold roof space must be ventilated. Assuming this is a pitched roof the eaves of the roof typically have 50mm clear of any insulation to allow free air flow, and it's this ventilation which prevents moisture buildup. In terms of different types of membranes there are a lot! Typically you'll have a VCL on the warm side of the insulation which can also be an airtightness layer, but its primary purpose is to reduce moist air from living spaces moving into cold spaces. VCLs can permit a small amount of vapour through, but not enough to cause a build up of moisture in a properly ventilated loft. VCLs are not the same as breather membranes which are designed to allow vapour through but not liquid water, and are normally placed on the cold side of the insulation to allow any excess water vapour within insulation to pass through. Link to comment Share on other sites More sharing options...
Iceverge Posted December 17, 2024 Share Posted December 17, 2024 Forget about vapor diffusion. It's absolutely miniscule. Completely insignificant compared to moisture carried by air leaks. Like thousands of time less moisture gets transferred this way. It all gets carried by drafts. I would aim for a complete sealed airtight layer. "Reduced" or "good enough" is like "kinda" fixing your puncture on a car. If you do this you can implement a sealed attic too with ventilation above the rafters and no ventilation through the eaves. It'll prevent wind washing of your insulation. Link to comment Share on other sites More sharing options...
Post and beam Posted December 18, 2024 Author Share Posted December 18, 2024 Thanks for the responses yesterday guys, i feel more comfortable about this now. I intend to use the Pro Clima Intello Intelligent Airtightness Membrane at the ceiling ( warm side) Yes it is a pitched roof. I think there is a lot of work in making this Airtight boundary as good as it needs to be and still have access for the lights etc from the loft space cabling down through the membrane. Whilst also trying to make sure the plasterboard installers dont abuse my efforts. Link to comment Share on other sites More sharing options...
Iceverge Posted December 21, 2024 Share Posted December 21, 2024 On 18/12/2024 at 20:58, Post and beam said: the lights etc from the loft space cabling down through the membrane. Whilst also trying to make sure the plasterboard installers dont abuse my efforts. Install cross battens as a service batten and keep wires out of the roof! 2 Link to comment Share on other sites More sharing options...
Post and beam Posted December 30, 2024 Author Share Posted December 30, 2024 On 21/12/2024 at 10:13, Iceverge said: Install cross battens as a service batten and keep wires out of the roof! That sounds like it makes keeping the integrity of the airtight membrane even more complicated. Link to comment Share on other sites More sharing options...
Redbeard Posted December 30, 2024 Share Posted December 30, 2024 we----ll... It depends whether you follow the school of thought which says that a hole with a screw in it is not a hole. If you share my pessimism (that it might be; a bit) you could fix through little squares on EPDM or butyl tape which sort of 'self-heal' around the screw more than membrane does. Link to comment Share on other sites More sharing options...
Mr Punter Posted December 30, 2024 Share Posted December 30, 2024 A couple of coats of emulsion paint on the ceiling will make it sufficiently vapour resistant that it will not cause an issue. Make sure the loft hatch fits well and there are not loads of holes for downlighters. Link to comment Share on other sites More sharing options...
Iceverge Posted December 30, 2024 Share Posted December 30, 2024 7 hours ago, Post and beam said: That sounds like it makes keeping the integrity of the airtight membrane even more complicated. We put... ........digs out calculator.......... .......scratches chin....... ......285 screws through the airtight membrane to fit the service cavity battens. No extra sealants of any sort. Had a blower door score of 0.31. Link to comment Share on other sites More sharing options...
Iceverge Posted December 30, 2024 Share Posted December 30, 2024 7 hours ago, Mr Punter said: couple of coats of emulsion paint on the ceiling will make it sufficiently vapour resistant that it will not cause an issue. Vapour/diffusion and resistance is the greatest red herring of modern building. Compared to air leakage due to airtighess it makes almost zero difference. Get the airtightness right and you won't have any issues. Link to comment Share on other sites More sharing options...
Post and beam Posted December 30, 2024 Author Share Posted December 30, 2024 8 minutes ago, Iceverge said: Get the airtightness right and you won't have any issues. I totally agree. No argument there. Which... is why i am trying to visualise a regime that gives me exactly that. My 1st floor ceiling ( airtight boundary) is as follows and from higher to lower 400mm rockwool 22mm caberboard decking 100mm rafters Intello airtight membrane Ceiling plasterboard Regardless of whether i go with my plan of boxed in light/ MVHR fittings within the 100mm rafter space being fed from above, below or horizontally i need to preserve the membrane as well as i can. i will provide a drawing tomorrow to better show the idea. 1 Link to comment Share on other sites More sharing options...
Kelvin Posted December 31, 2024 Share Posted December 31, 2024 48 minutes ago, Iceverge said: We put... ........digs out calculator.......... .......scratches chin....... ......285 screws through the airtight membrane to fit the service cavity battens. No extra sealants of any sort. Had a blower door score of 0.31. I started sealing the screws then read you posting this before and stopped wasting my time. We got 0.44 on the final test. 1 Link to comment Share on other sites More sharing options...
Iceverge Posted December 31, 2024 Share Posted December 31, 2024 Great result, well done. Link to comment Share on other sites More sharing options...
lookseehear Posted December 31, 2024 Share Posted December 31, 2024 12 hours ago, Iceverge said: Vapour/diffusion and resistance is the greatest red herring of modern building. Does this mean that Intello type membranes are not worth the cost? Should we be looking at cheaper alternatives instead? I've always had a nagging doubt about gore-tex in outdoor clothing. Wearing it while cycling leaves me similarly sweaty to wearing a standard waterproof. Link to comment Share on other sites More sharing options...
joe90 Posted December 31, 2024 Share Posted December 31, 2024 1 hour ago, lookseehear said: I've always had a nagging doubt about gore-tex in outdoor clothing. +1, especially the shiny ones when it’s already covered in water, how can that breath? Link to comment Share on other sites More sharing options...
Mike Posted December 31, 2024 Share Posted December 31, 2024 2 hours ago, lookseehear said: 15 hours ago, Iceverge said: Vapour/diffusion and resistance is the greatest red herring of modern building. Does this mean that Intello type membranes are not worth the cost? Should we be looking at cheaper alternatives instead? The principle is to stop air moving through the structure, limit moisture entering the structure, while making it permeable to moisture vapour to allow trapped moisture to escape. Intello & other 'smart' membranes can play a big role in that (if applied with care and if junctions & penetrations are sealed), but do so by being resistant to moisture movement in the winter (when the risk of condensation in the structure is high) and more open to moisture movement in the summer (allowing moisture to escape). That's generally fine, but if moisture is entering the structure via other means - for example by absorbing external rainfall - then moisture levels can still build up to dangerously high levels. For example, I have a case from experimenting with WUFI (internal wall insulation on a north wall, with MVHR to help control internal humidity) where adding no VCL (but using a parge coat) is fine, but adding a regular moisture-permiable VCL leads to dangerous moisture levels, while adding a smart VCL is even worse. That's not to say that either product is bad, just that they need to be used with care - they aren't a magic wand; follow manufacturer's recommendations and ask them if unsure. 4 hours ago, lookseehear said: I've always had a nagging doubt about gore-tex in outdoor clothing. Wearing it while cycling leaves me similarly sweaty to wearing a standard waterproof. But if you were to keep wearing it for the next few hours it should dry - it won't act instantaneously. Link to comment Share on other sites More sharing options...
Iceverge Posted December 31, 2024 Share Posted December 31, 2024 9 hours ago, lookseehear said: Does this mean that Intello type membranes are not worth the cost? Should we be looking at cheaper alternatives instead? Whats your exact buildup? It's a good product, offer some drying capacity, hard to tear, offers very good airtightness but as Mike says it's not magic. It won't do much if not installed to an excellent standard and it's very expensive to boot. Link to comment Share on other sites More sharing options...
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