Celotex between and under rafters - ventilation?

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Hi all, trying to get rid of glass fibre in roof as going to be spending a lot of time re wiring, comms centre, alarm installing as well as future diy projects and storage etc etc.

having previously considered putting celotex at ceiling level (over joists leaving existing fibreglass between), considering further I have quite a few downlighters cutting holes through the insulation, actually perhaps it would make more sense to insulate the roof level instead and just put some simple boarding over the ceiling level josts for storage.

I can see the advice on celotex.co.uk and understand I can use 50mm under sarking (leaving 50mm air gap) between + 55mm (+) underneath for regulation insulation. Then I can get rid of all the glass fibre in the roof.

What I am unsure about is what I then do about the existing eaves ventilation, do I simply block it up? If so do I need to do anything to provide ventilation elsewhere or is it assumed that the gaps between tiles and sarking is enough?

Thanks for any advice!
 
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Ok, so I think I've got a bit further. From here
http://www.diybanter.com/uk-diy/7796-celotex-kingspan-fire-fitting-question.html

It basically says to have bring ridge down to a flat "ceiling" section at the top with enough gap above to allow airflow along the roof and have vents each gable end.

1. I'm a little lost on how and what to use as gable end vents or whether to get ridge vents fitted. I've heard ridge vents are more efficient although I wouldn't do that myself but going up a very tall ladder equally doesn't sound much fun. How big do these vents need to be?

2. I have 95mm deep rafters and there are 3 sets of noggins at each end supporting gable straps. This reduces the gap to 20mm for these 2 channels at each end - does this cause a major problem and if so is there an easy solution? (e.g. drill a few 10mm holes through each one? - guess that wouldn't weaken too much?)

3. Each Truss has 2 long web members coming down from the apex which are obstacles all the way down the length of the building. I have given this a lot of thought (never dealt with anything like this before) and am guessing the only way to deal with this is to cut out sections of insulation and plasterboard with cutouts for each member and have one piece per gap between the trusses. I also assume each piece only has cutouts on one end to allow getting it in place. This sounds like fairly intricate work due to all the awkward angles, not to mention mitring the boards at each end as well as the cutouts having to be mitred also (as web members are 60deg angle or so). Any tips for making it easier or is it just measure twice, cut once, swear thrice? :D

4. Along with q.3 above, to fix the insulation/plasterboard I guess I just make twisted noggins out of 50x25s that I fix the edges of the top section to? Again, time consuming (I did the noggins for the gable straps and they take a long time!). Any tricks I'm missing?

5. How do I work out what the minimum size for the gaps down the length of the roof are? I want the "ceiling" section as high as possible but obviously need enough airflow along

Thanks for any suggestions
 

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