When is there too much insulation?

9 Dec 2009
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West Midlands
United Kingdom
Currently have 100mm celotex in between ceiling joists of tiled roof, which covers the kitchen. Now I want to insulate across the rafters before I plasterboard the ceiling, I still have 100mm celotex sheets left and was wondering whether using those would be fine, or is it overkill. As I would rather not spend any more money on say 50mm celotex.
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No problem, but be sure not to block anything that needs to remain vented. Any vents in the eaves for example.
its the law off diminishing returns
100mm will give you perhaps 90-95% saving a second 100mm will perhaps give you a 90-95% saving off the 5-10%

so in other words the first 100mm may save you £2 a week the next 100mm may save you 10-20p a week

if you have other areas that need attention like draughts single glazing no low energy bulbs
tvs /computers/ games machines that are on when not in use they will use more energy than you will save via the extra 100mm of insulation
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I have all the rest of the low energy things.
I appreciate the diminishing returns argument also. So now just need to that is 100mm in fact enough insulation for a roof with tiles?
If so great.
If not I will attempt to find those 140mm drywall screws lol
Thanks guys
Going across the rafters is a good idea to kill the cold-bridging. But having said that, 100mm is a bit OTT.

I recently found myself with spare sheets of 100mm left over after ordering too much, and needed some thinner stuff. So cut them into manageable pieces about 300mm wide, then cut them to approx 50mm thick with a reciprocating saw.

So long as you don't mind one side being a bit 'irregular', it works ok. Bit of faffing about, but didn't take too long.

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