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loft insulation guidance please

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redhand:
First don't use plastic sheeting to line the roof it will cause condensation. Think solar stills. I use foil backed bubblewrap (Screwfix) or celotex if I'm insulating roofs. But the previous post re increasing the depth of the joist is probably the best idea. fix them at right angles to the existing joists and  insulate between them with ordinary rockwool insulation.

winchman:
The problem here is there is a load of science to roof construction.
But most of us choose to ignore it.
Weight
The roof is designed to hold the weight of the tiles / slates I would imagine a couple of ton spread over the total area + a factor for a coat of heavy snow + a bit.
The other very important thing is the construction.
My 1960s roof is called a Cold roof and as such has tiles with a water proof membrain under it incase any tiles should leak ( just like roofing felt.
The problems start when you mess with the insulation, in 99% of cases its fine to just fill the gaps between the joists with any insullation you like, but the roof will still be vented as per the builder designed it so alls well.
If you insulate the slopy bit you will stop this natural ventillation and as I did get some condensation which is possibly the last thing you want as a nice dry loft is now damp and wet.
My advise would be insulate the gaps between the joists on top of the ceiling with rockwool as its cheap, if you are going to board the loft for storage get a set of calcs done just to prove the beams ar capable of taking the weight of the boards and any junk you put up there.
Draughts, the roof is desinged to have these as ventillation, you can get more benifit buy putting 6" of insulation above the ceiling as the draughts wont matter as they wont blow in to the house.

T30CDB:
With out be patronising or questioning your knowledge in any way , when you say trusses i take it you have one or two diagonal timbers coming from the roof timbers down to the ceiling timbers acting as braces. If this is the case and you have walls as you say supporting underneath you shouldnt have too much trouble boarding out a bit of floor to store light things on. no engines,axles or chassis though. As for insulating in between the trusses on the slope you would have to ensure you maintain an flow underneath the black felt otherwise this will form a cold bridge and cause condensation as previous posts mention. best bet is to insulate the ceiling and put an extra jumper on when you go up there

muddyjames:

--- Quote from: T30CDB on December 16, 2009, 07:55:22 ---With out be patronising or questioning your knowledge in any way , when you say trusses i take it you have one or two diagonal timbers coming from the roof timbers down to the ceiling timbers acting as braces.
--- End quote ---

my knowledge is VERY poor on roof vocabulary! I am meaning the timber that holds the roof up from front to house to middle and back down to back of house. Also where the felt touches and the tiles sit on.


--- Quote from: T30CDB on December 16, 2009, 07:55:22 --- If this is the case and you have walls as you say supporting underneath you shouldnt have too much trouble boarding out a bit of floor to store light things on. no engines,axles or chassis though. As for insulating in between the trusses on the slope you would have to ensure you maintain an flow underneath the black felt otherwise this will form a cold bridge and cause condensation as previous posts mention. best bet is to insulate the ceiling and put an extra jumper on when you go up there
--- End quote ---

This is why I was thinking about putting polystyrene in there half the thickness of the trusses so when I put hard board onto the trusses the polystyrene sheets fell onto it there for leaving a gap for air to flow and as I want to only insulate the top half of the roof the rest would be open to the loft.

redhand:
The foil backed bubblewrap is designed to be stapled across the roof spars. and thus still allowing proper ventilation of the roof. It is I believe equivalent to 4" of rockwool. 

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