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Loft doesn't have dormers in the habitable rooms but there is one over stairs. Most of it's cross section looks like this;
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http://i239.photobucket.com/albums/ff167/LukeBurstow/oldstuff/loft_zps60b66c26.pn g"/> [/IMG][/URL]
red line = that bubble wrap silver foil stuff
blue line = 100mm rockwool type insulation between joists/studs
green line = 200mm rockwool type insulation laid at right angles and on top of joists
There is some insulation there already but it's 'tired' and pretty thin (should I remove it or just bung the new stuff on top - I'm worried if I do I'll end up with an air gap above each joist where the new stuff sits above their height and the 200mm stuff going over it won't work as well). The top loft and some of the side bits are partially boarded but I'd remove that as we dont; really use them for storage.
I guess with the angled roof bits I could shove 50mm celotex down the gap between the plasterbaord and roofing felt but it'll be a right pain, take a lot of time and be fairly expensive so I may leave that and look at using insulated board on the inside next time we redecorate.
There's some quite large voids around the dormer so the theory on insulating the underside of the roof and backs of the 'walls' that make up the rooms with the bubble wrap stuff is to try and keep those warmish - it'll also help hold the 100mm rockwool in place.
Does the above approach and materials sound reasonable and where's good to go to to buy this stuff? Wickes seem to have an offer on rockwool currently but I've not really worked out how much I'll need or shopped around.
If you use closed cell foam stuff eg Celotex, you need an air gap between the insulation and the felt for air flow to stop rot.
See install recommendations:
[url= https://farm8.staticflickr.com/7447/9914758565_3a9ac3ac80.jp g" target="_blank">https://farm8.staticflickr.com/7447/9914758565_3a9ac3ac80.jp g"/> [/img][/url][url= https://flic.kr/p/g78Jic ]Celotex between rafters[/url] by [url= https://www.flickr.com/people/75003318@N00/ ]brf[/url], on Flickr
thanks footflaps - it's why I'd use 50mm celotex as they're 100mm rafters and I could find a way to force the celotex against the back of the plasterboard I think. As I say, I'll probably line the inside of the existing plasterboard anyway rather than go down that route, for convenience as much as anything.
You can buy Celotex with PB bonded to it and fit over the rafters, I used both in the workshop. 75mm between rafters and 50mm over top.
between:
[url= https://farm9.staticflickr.com/8485/8258394476_842199275c.jp g" target="_blank">https://farm9.staticflickr.com/8485/8258394476_842199275c.jp g"/> [/img][/url][url= https://flic.kr/p/dzLqXE ]First insulation later complete[/url] by [url= https://www.flickr.com/people/75003318@N00/ ]brf[/url], on Flickr
over:
[url= https://farm9.staticflickr.com/8481/8278235633_35fcf778d4.jp g" target="_blank">https://farm9.staticflickr.com/8481/8278235633_35fcf778d4.jp g"/> [/img][/url][url= https://flic.kr/p/dBw83z ]2nd fit Celotex complete[/url] by [url= https://www.flickr.com/people/75003318@N00/ ]brf[/url], on Flickr
If you're not massively short of space or headroom, why bother with all the between rafter stuff, and just put it all below the rafter. Much quicker to install, and less likely to have little voids and gaps leading to cold spots. More room above too for air movement in the roof structure and for cabling.
that 2.3m was just on an image I found on the web - I couldn't lose 75mm+ head height on the ceilings, and don't want to bring the walls in by 100mm or so all the way round - I'm prepare to lose 25mm+the thickness of a sheet of plasterboard off the slopey bits as there's not a huge choice available.