Space Blanket

I am still in pondering mode about my loft insulation.
After a bit of looking around I see that knauf do a type of insulation that looks pretty neat. Its called space blanket (http://www.space-blanket.co.uk ).
The thing that I like about that is that I could lift up the insulation in the future for wiring etc, and it's all enclosed, so there shouldn't be so much dust flying around.
However I have 2 questions.
1) My joist height is 4 inches (100 mm), but their product only sells in 150mm and 200mm. If I installed the 150mm and later boarded on top of it, (effectively making 150mm into 100mm), would that simply mean that the insulation would act as if it were 100mm, or would it be less effective as there wouldn't be as much air to trap the cold.
2) The space _between_ my joists varies ever so slightly, it appears to be around 400mm. All of the loft insulation that I have seen sells in 370mm width (they must assume 400mm joist centres). Has anyone else seen this before and got any advice on what to do?
Thanks
Dean
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Sir
What I did was to put 2 layer down, one in the joists, one over the top, I have 400mm, which is OTT, but the ice don't melt off my roof. The normal fibre stuff has a bit of give in it with sizes, so 370mm and 400 are near enough to be OK.
The bit I boarded over has "cavity bats" as insulation, as these are more solid and can be cut to shape and size, using a bread knife. If I insulated under the boarded floor again, I would use celotex, which is expensve, but easy to cut to the right shape, and much nicer to work with than fibre.
Rick
On Sat, 29 Nov 2003 12:11:15 +0000, Dean Richard Benson

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Years ago I insulated my loft with polystyrene bats (non flammable type) then covered them with ply, I think you can get them in 8' x 4' x 3" sheets, you could then cut them to width and they would be easy to lift.
Mike P.
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