I have a warm roof and 2 solar arrays. Fitting the arrays was no more difficult but it was up to me to make airtight after the teams had gone. The fitters basically push a big needle through the insulation and spray a token amount of foam into the hole afterwards. If I’d lined the insulation boards with anything it would have been more difficult.
Cold loft climate update 
I’m so pleased to report, how after changing the non breathable roof felt and a vapour control layer under the loft insulation we now have a dry and airy roof space, despite the damp cold conditions outside. 
It’s our belief, through changing the roof liner and adding ridge vents, this should have been enough to prevent the condensation we had dripping down the previous roof liner each winter.
However, the plaster board on the ceilings below has no foil back and is thin - warm air easily passes through and may cause moisture / mould on the underside of the sheepwool insulation we’d previously installed. To prevent this we decided to install the vapour barrier as well as the roof liner.
All in all, getting our roof dry and insulated has cost us in the region of £10,000.
The problems with the roof space were not picked up on the building survey owing to the loft being boarded out - the roof felt couldn’t be seen and we weren’t given any warnings from the surveyor.
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Congratulations on completing the job. The bill is horrendous.
If your surveyor didn’t specify that the state of the roof could not be assessed you may still be able to make a small claim. The question for you is whether the risks and potential gains are worth the potential expense.
The bill is horrendous!!
Indeed it’s a typical ‘buyer beware’ situation. We knew the surveyor couldn’t gain access to the roof because he advised us to get access to check the remaining part of the chimney was supported. He didn’t mention checking the roof liner 
.
There are a couple of consolations - we knew the ridge tiles, chimney and slates needed attention and that we’d need to insulate - we just didn’t think we’d have to strip the roof , refelt and reslate. It’s been frustrating , but at least we managed to get solar through the grant scheme - it all feels like quite an achievement and a big learning curve!!
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I would greatly value some thoughts on my plans for adding insulation and loft boards in my loft.
I have a detached house built around 1996.
Currently the loft has 190 mm joists about 700mm apart with about 100 mm of glass fibre insulation.
The plasterboard underneath is plastered and painted 2-3 with conventional vinyl paint on the living rooms side. Probably had about 3 layers of paint over time. The loft has very good vents and no condensation issues to date. Bathroom extractors are connected to tubes that are vented to the outside of the roof, exits near eves.
Plans: With the help of a builder, I am proposing to add 100 mm joists across the current joists, add in enough to create 270mm of insulation. Then we want to secure loft boards on top for storage of relatively light items. This would also give a 30 mm gap between the boards and the insulation. We are also adding in a loft ladder 1200mm by 700 mm and choosing an insulated version.
Questions: Should I include a vapour control/barrier on the plasterboard and on/around the current joists? Do I also need one under the loft boards that I planning to add on top. Some posts suggest it would be a good idea, others indicate that with a well-ventilated roof should not require the vapour control barrier installation. I also saw a link to a paint that would form a vapour control. May help avoid use of tapes to get at tricky joints and corners. Found this link via the AECB site.
Vapour Control Paint | Intelligent Membranes.
Does the current painted ceiling plasterboard act as an air tight seal? I have a thermal camera booked - can it help me assess any key requirements?
I would greatly value our coop teams insights, questions and thoughts.
Topping up the insulation is a good idea and fixing the additional joists across the existing ones will both spread the weight over the existing timber and reduce your thermal bridges at joists to just the crossover points.
Vinyl paint, particularly several layers, will pretty well act as a vapour control, particularly if the original coat is primed direct to the plaster and not onto a lining paper. You will, however, need to attend to all penetrations. They will normally be where wires and pipes pass through the ceiling. Foam spray could be used, followed by Blowerproof or, as you suggest, Passive Purple. I only have experience of the former.
As you are boarding the insulated space that reduces the need for a top membrane to stop wind washing but do make sure that the ends are sealed to the roof plate to prevent air blowing under the boards. See
When you next decorate rooms below the loft you could top up the insulation and remove those final thermal bridges by fixing insulated plasterboard below the current ceiling. There will also be a marginal reduction in room volume, which will reduce the amount of air to be heated.
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.
It’s 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. It’s vented. You’ll probably do more harm than good.
But an air control layer is key. Typically, the attic ceiling plane isn’t good enough, including the attic hatch and all service - especially spotlights (I’d get rid of them, go for uplighting. If you must have them they need to be LEDs, and I’d 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 isn’t fun (it’s impossible when it’s baking hot up there, so it’s nice to do it in the shoulder seasons when it’s 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 you’re 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 couldn’t recommend a Henry Hoover enough…
No need for a vapour barrier. It’s vented. You’ll probably do more harm than good.
I’ve 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 it’s 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 “house” 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 confirm… 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 don’t have any. I control my humidity with heat and ventilation. I have a warm roof. My airtightness is addressed by 1/2” 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 couldn’t 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 I’m sure the uninsulated ceiling was even worse. The landlord’s 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 wasn’t.
Vapour barriers like polyethylene membranes don’t 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. It’s one of the most durable roof designs as it handles moisture very well.
Vapour from the inside will be negligible.