Thank you Tim. Delighted with your recommendations and prompt response. I will keep the group posted.
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With hindsight, the single most important checks we should have made on our loft would have been in winter during temperatures below 5 degreees and when the heating is cranked up.
This is when you really get to see whether you have a condensation problem through warm air getting into a cold loft.
My loft was fabulous in summer - it was under the above conditions that condensation truly showef itself.
Itās worth checking timbers for any water marks and lifting your insulation to feel underneath to see if it feels damp.
These mid winter checks would have been what enabled us to see we had a problem.
Hope this helps Jay 
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Hi Carla,
Brilliant thanks. We will check the loft timbers and under the insulation. Around the 21st November the forecast is 6 degrees centigrade. So hopefully we will have favourable conditions for a good stress test.
A huge thanks for your insights.
Best regards,
Jay
No need for a vapour barrier. Itās vented. Youāll probably do more harm than good.
But an air control layer is key. Typically, the attic ceiling plane isnāt good enough, including the attic hatch and all service - especially spotlights (Iād get rid of them, go for uplighting. If you must have them they need to be LEDs, and Iād put a little fire rated hood on top of them, taped from above).
Over rafter membrane systems take a long time to detail, the work really isnāt fun (itās impossible when itās baking hot up there, so itās nice to do it in the shoulder seasons when itās cool).
Wind washing, especially with fibreglass, can be an issue near the eves, so put in some nice baffles there.
You have to physically press down on tape, between two dry and dust free surfaces, for it to work well. I would get all the insulation out of there, and hoover the place thoroughly. Put good lighting up there, so you can see what youāre doing. And go around with a wet finger as a high-tech air movement sensor before putting the insulation back.
My little Henry hoover has been our friend while laying the vapour control layer on the loft floor.
We found we needed to hoover before and after laying a run of the VCL to tape it to the next piece.
In fact on the back of all the rubble our Little Henry hoover has removed from the loft these past few years - I just couldnāt recommend a Henry Hoover enoughā¦
No need for a vapour barrier. Itās vented. Youāll probably do more harm than good.
Iāve just reread this statement and suddenly realised that it suggests I might have put the wrong kind of layer on my attic floor.
How would it do more harm than good to put down a vapour control layer .
When you say itās vented - do you mean my loft is now vented?
The membrane I used is the pro clima intello plus.
When it is warm Intello is vapour permeable. When it is cold it is a vapour barrier. The idea is that vapour only passes through when there is no risk of condensation.
Edit: It is correct to fit it on the āhouseā side of the insulation, as per the instructions. I attach the instructions here for anyone contemplating the same route that you took:
Sorry, another edit: If the humidity on the inside (as your ceiling) approaches saturation then the membrane works like a pressure release valve and some humidity will escape
So to confirmā¦ I have installed the intello on the cold loft floor - the insulation will sit on top.
Is this as it should be ?
Just want to check this is correct, because we already made a lot of mistakes with our loft and understanding what is needed.
Thank you Frank, Carla and Tim. I will take on all your recommendations, cover the light fittings (and replace with LEDs for ones that have not been replaced so far) with hoods, seal and ensure adding baffles etc. Yes, I have a Henry vacuum cleaner so it can earn its keep. I am tempted to use the Passive Purple (liquid vapour control) as a paint and it can be worked into tricky joints.
I have also check for drafts.
Just received the Carbon Coop loan FLIR C2 thermal camera. It has already revealed several cold areas around the house and spotted a number of places on the ceiling that were unexpected. I will post some images and corresponding pictures once I am in the loft with the builder. Definitely recommend use of such a camera.
Thank you all for your excellent contributions.
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@pottyone72, First I should say that I am not an expert in vapour control membranes. I donāt have any. I control my humidity with heat and ventilation. I have a warm roof. My airtightness is addressed by 1/2ā plaster and Blowerproof, but is a candidate for improvement.
As I understand it you have installed the membrane and insulation, correctly. The only circumstances in which the system might leak moisture are if the heating were off for a cold snap and you were creating humidity which was not being extracted.
I have seen such circumstances in a private rental flat that was let out to benefit claimants with hardly any income. They couldnāt afford to run the woefully inefficient heating and the only heat they had was from cooking, laundry and bathing. They kept on to the heat for as long as they could by never opening windows or operating the cooker hood or bathroom extract. The walls were always damp and cold (and black) and Iām sure the uninsulated ceiling was even worse. The landlordās answer was not to insulate the loft and install efficient, cheap to run heating but to paint over the mould with gloss paint which he presumably believed was immune to new mould. It wasnāt.
Vapour barriers like polyethylene membranes donāt allow any moisture through. If you have two such barriers sandwiching a moisture sensitive material you can have problems.
Generally speaking, in our climate, in a ventilated roof you do not need to worry about vapour.
Worry about air tightness, especially in the bathroom, but not vapour.
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You have what is known as a unconditioned ventilated roof. Itās one of the most durable roof designs as it handles moisture very well.
Vapour from the inside will be negligible.
Itāll work just fine. Donāt worry. Itās just an expensive material to use in this context.
Intello works by being vapour open when thereās minor differences in humidity on either side of the membrane. If thereās a large difference it becomes a vapour throttle and slows down moisture transfer.
This property, originally developed to prevent bread from getting stale (I think), is useful when moving moisture against a temperature gradient. Which can be helpful with interior wall insulation systems and unvented conditioned roof systems as it reduces outward driven vapour from condensing in your walls/roof assemblies in winter, but letās that wall dry inwards when ambient temperatures increase. If you just put a vapour barrier there, it wouldnāt be able to dry inwards. Itās a minor effect that makes more sense in very cold climates.
Vapour from the inside will be negligible.
So the odds are, through good ventilation and heat recovery we mitigate against vapour from the inside.
Yeah, reducing the vapour generated from the inside is always a good idea, but your roof would still cope with it even if you didnāt do particularly good job.
My point is that your roof might not cope with a draft of moisture ladened air from a poorly sealed bathroom ceiling, as apposed to molecular vapour diffusion.
Basically, prioritize air sealing for durabilityās sake. Energy efficiency is a nice bonus.
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