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The only job for the VCL in a warm flat roof is to prevent water vapour condensing inside the insulation layer isn't it? Maybe it would be better to say .... condensing on any suitable surface above the VCL. Should, therefore, the VCL extend up the sides of the PIR insulation (as well as underneath it) ?

I am in the situation where I am converting a garage with a pitched cold roof above. However I’ve also removed the ‘external’ wall that separated the garage and kitchen. I’m taking the opportunity to improve insulation with IWI in the garage and want to also improve airtightness. I’m happy with the floor-wall, wall-ceiling & wall-window details for VCL/airtightness layer with the garage itself. However I’m not sure what to do when I get to the original house. e.g. Crude line drawn on photo below showing continuous airtightness layer coming up external garage wall on left, under insulation in garage ceiling, under steels (to be insulated). then I arrive at kitchen ceiling with bedroom above. What happens when you have to terminate an airtightness layer in a refurb where not all rooms are being tackled? all I can think to do is apply liquid membrane between joists in GF/FF void to underside of FF chipboard & around joist entries into blockwork. Then tape/seal the membrane from the garage onto the liquid one. My air leakage path becomes up into the bedroom which sounds not great given kitchen has lots of moisture. Or carry the membrane right across kitchen ceiling and terminate midway into the house. Also seems wrong.

About to start fixing PIR insulation sheets across the inside of the timber frame. Need a method of ceiling the foil on the PIR to the concrete slab (will have no screed). The foil of the PIR will be taped to form the VCL. Timber frame company said they would normally prime the concrete slab with PVA, then use tape to seal this to the foil on the PIR - I'm assuming this is standard aluminium foil tape. Has anybody done this? Are there any alternative approaches for ensuring an airtight seal at this junction?

Hello, I would like some info/advice regarding a cold concrete flat roof. Sorry for the length of explanation.... But first a little background info...... The house we purchased was gutted, renovated and then sold on to us by a local building company... All certs for all upgrades and works were checked and passed.... Our problem.... 1 We moved in Sept 2107 all is fine until the first frost off this winter late 2018..when we started to notice a water stain in the rear utility ceiling (formerly downstairs rear loo and storeroom). These rooms were incorporated in to the house via the kitchen to form a utlity/cloakroom and extra kitchen space..... The roof itself is around 8 inches of concrete topped off with felt.. This was refelted just before we moved in... And the rooms below batterned, insulated, boarded, skimmed and redecorated. Oh and rads fitted. Since this watermark has appeared and got worse over the last month or two.. I cut a peice of board out, to be greated with a soaking wet inner roof, droplets everywhere.... Our problem 2.... The house is a semi formerly a council house, we have great neighbours who are council tennents... They have the same sort of rooms as us but still the original loo and store (no heat sourse).. And attached to ours So taking advise online of fitting a VCL then a thick insulation then watertight layer is not really an option without affecting the council properties roof.. So I am left with trying to sort the problem from inside.. Although there is much advice regarding how this is now not advisable.and insulating from outside is they way to go... Is there anyway I can fix this from within... I am happy to take down the internal ceiling and start again. But what would be the recommended materials etc etc.. To ensure no water ingress... You can see by the images that there is no VCL inside and the insulation is very thin and not sealed in anyway. Just placed in the roof void not a good job at all.. ... Please please any advice would be gratefully received.... Mark

Hi all, My Architect has specified I use DuPont AirGuard Control as my VCL. I'm aware it's a 'brand' and as such there may be more cost-effective alternative products. Can anyone recommend one, please? Con

I have not decided whether to use downlighters or the narrow panel Led lights that some have used but my main concern is that I have my VCL directly above the lights. I know the led lights do not produce as much heat as the halogen ones used to but it still concerns what the heat long term will do to the VCL. I know I could put some form of barrier in place but has anybody had to deal with this situation? TIA Distance from plasterboard to VCL is 120mm.

Hi, I'm purchasing my vapour control layer and wondering about tips for use of, and type used. I'm considering BarriAir, which says comes with integrated sealing laps. But is only 1.5m wide, is that a practical width? Although i'm only at panel erection stage, beautiful day for it today, i'm thinking about when i place my mid floor joists atop of the ground floor panels, and the need to have the VCL in place to be underneath the joists. As the VCL layer will go around the ends of the joists and be taped above and below. Does this make sense?

Now this may be a daft question and the answers may already be out there on the forum - I'm speedily working through all the posts and could have missed the relevant info. I've read all the roofing posts - currently on foundations at the minute then that sections done, but a thought keeps niggling at me about the warm roof VCL that I can't recall reading about. After reading @ProDave and others numerous plugs about sarking and warm roofs etc. I am sold that it is absolutely the way forward in my build. It seems a no brainer to me. So, as I understand it the roof makeup more or less is tiles, tile batten, counterbatten, breathable membrane, sarking (I'm sold on wood fibreboard), rafters, insulation fill between & below, VCL, plasterboard. Right as I understand it, the internal VCL stops warm moist air from the house permeating into the insulation where it could condense at dew point in the insulation. The external breathable membrane is there to stop moisture from rain etc. coming back into the structure but let moisture that may be in the insulation out - a one way system like a diode? If my thinking is correct about the above, my niggle is how do I continue the VCL down from under the rafters to the external wall of the rooms below to ensure a sealed box essentially for the whole house? Do I have to faff about bringing the VCL down between each joist at 600 centres and then taping/sealing around each one? A lot of the pictures I have seen show the VCL in the bedrooms running along the ceiling - but surely that creates two separate boxes - the roof and then the house. This would also mean every penetration through bedroom ceilings would need careful attention to ensure an air tight Seal? If I do then so be it? Just after a bit of clarity from those who have been there and have the t-shirt so to speak! TIA for any guidance. Jamie

Due to circumstances I have ended up with a cold roof for front hallway/porch. During renovation of the old house, I noticed mildew on underneath of the decking due to condensation. There was quite a bit of humidity at the time when the old internal slab of the house was being dug up. Roof didn't have any insulation or VCL at this time so I can see how condensation occured between cold/warm surface of the roof deck. I'm now at the stage to insulate and plasterboard the flat roof and want to minimise any future risk. I will have a 50mm ventilation gap between insulation layer and also a VCL beneath insulation layer as in this pic: However, still a bit concerned about condensation underneath the deck itself, so was either going to stick some VCL/polysheeting directly under deck or use some liquid DPM instead: Painting on the DPM would be a lot easier and a bit safer as over time the spray adhesive with the VCL could fail causing the sheeting to come unstuck and block any airflow. Any cons for me for using a liquid DPM instead? I'd end up with 2 VCLs - good if the main VCL is punctured at some point (electrician / downlights).