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Thanks, I will have a look to see what options there are for discrete looking vents. I don't think they will be too noticeable from outside, unless you are looking for them. My only concern is water ingress. I have opted for Rockwool RWA45 for the cold roof space. Struggling to source either of the Knauf products. This will give me a minimum of 50mm of ventilation space. I will be cutting the slabs a little bigger than the space between the rafters and feeding them in through the pitched roof space. Hopefully it works! Due to condensation and ventilation issues inside the house I am planning to use a PIV (post trickle vent and bathroom extractor fan installations) to help draw fresh air in from the loft space. My thought is that this should draw air in from the eaves at a continuous rate. Just got a dehumidifier to use during the colder months to tackle drying clothes etc.

Thanks Tims, I'll pop my head up there in a bit to see if I can see anything. If there are no vents, is there a cost effective DIY solution to add vents into the fascia? I say fascia as the soffit is very narrow. I will also consider alternatives. Don't want any ventilation issues

So, bit of an issue when inspecting the existing ventilation on the flat roof. It doesn't appear to have any! The soffit most definitely does not have any, but there may be something where the top of the fascia board meets the roof overhang. Is there an easy way to confirm this? Possibly shining a light or something to see if it can be seen from inside the roof void?

For the flat roof, which one of the following would you use? https://www.knaufinsulation.co.uk/products/rocksilk-rs45 Or https://www.knaufinsulation.co.uk/products/omnifit-slab-35 After speaking to Knauf they confirmed that the RockSilk is water repellant (not waterproof) will this be a potential issue? Could this trap vapour (which has penetrated through the ceiling) between the plaster and insulation and prevent it from going through the insulation into the air gap for it to be ventilated out? The water vapour resistivity appears to be the same for both products.

Thanks, from inside the pitched roof I can only insulate up to the noggins without taking the roof or plasterboard off and I don't want to go down that route. I appreciate that there will be uninsulated spots if every other gap between the joists is only insulated up to the noggin, but surely some insulation is better than nothing? I will have to see what amount of clearance there is between the top of the noggin and the roof, hopefully it's sufficient. If not I won't insulate these areas, only insulating the voids where I can reach up to the eaves.

Looks like a good idea as it appears to be flexible so I should be able to put this into the hole from inside the room.

Thanks for the link to the article, has answered some of the questions I had. Also managed to find a good video on YouTube explaining cross/through ventilation and its importance. I think limited boarding and cargo netting will save me a lot of head ache down the line. I will try to seal gaps around extractor fans and down lighters where possible to reduce the amount of moisture entering the roof space. Fortunately not all of the rooms have down lighters! I will post a plan and some pictures later as a final before commencing the work incase there are any bits that I have missed and give some updates as I carry out the work. Thanks again.

Thanks, appreciate the advice. The same principle applies to the flat roof which is essentially 'boarded' by the ply above? This issue can be resolved in the pitched roof by not boarding the loft or limiting the amount of boarding/using cargo nets as advised and leaving as much gap as possible in the flat roof. Will any of the solutions I proposed in my initial post be suitable in this case?

Indeed, this is why I joined the forum as it's not my area of expertise in the slightest! To confirm my understanding, if I use PIR for a cold roof (in the pitched roof) I would have little issues with vapour (rising from the rooms upstairs and past the plasterboard on the ceiling) which would require less ventilation. Bear in mind I do not want to create a warm roof. If I go down the mineral wool route I should leave as much of an air gap as possible right?

So if I go down the mineral wool option and have it at 270mm rather than 300mm that will give me a total of 60mm between insulation and the bottom of the board (considering joist is 120mm and stilts are 210). Will that be enough? Or is it better to use PIR, not sure what depth I would need to be the same as 270mm of mineral wool, and leave a much bigger gap but still use stilts. For example: 140mm of PIR (assuming this is the same as 270mm of mineral wool). Total height to board of 330mm (120mm joist + 210mm stilt) Leaving a gap of 190mm.

Thanks for the pictures, the issue is I cannot afford (time wise) to do such an extensive amount of work by bringing the ceilings down. I can however install the PIR in the pitched roof area, similar to how you have done. When you say air barrier is that the first picture with the blue tape joining it? What is the purpose of this?

I would probably need a bit more than what's reachable from the hatch, although I should get rid of more than I keep! Im not sure what you mean when you say: 'Rigid boards are ok to transfer load (compressive strength), but too fragile to support. You'd need to board under (over the joists, filling between the joists), so the boards are what holds you standing on PIR.' If I decide to use PIR in the pitched roof instead the 300mm of Knauf will 100mm be sufficient? The joists are 120mm in height and I can board on top of that without the need for stilts or will I need to lay another layer of PIR on top (over the joists) to ensure the coverage and depth of insulation is sufficient? Regarding the flat roof, I was thinking of using something more flexible so that it fills the width between the joists incase the are not of an equal distance apart all the way along and use something to grip and push from the far end (closest to the eaves) to avoid compression instead of pushing from the closer end (closest to where the flat roof meets the pitched roof). If I used PIR here does it matter if it's close to spotlights and if there are small gaps near the joists? I'll try to cut the board a few mm more than the width though. If I use multiple PIR cuttings, do they need to be taped with aluminium tape? I will check the spotlights, the majority of them are LED with a few exceptions. How do I check if they are fire rated? Quick Google search of a model number? When you say sealing the gaps, what should I use? I will add some photos and diagrams later.

Thanks for confirming and explaining the purpose of an air gap. Will 30mm be sufficient? It's likely I will board the roof for storage as with the already limited height in the roof I can't imagine crawling underneath a suspended cargo net to store things. Although it is a thinking outside of the box solution.

No windows were left open. When checking for air leaks I assume you mean l should draught proof right?