Insulating roof space

Hi,
I'm using our converted attic a lot more now, and have noticed that it's very cold in winter, and heats up in a few seconds when the sun comes out. Clearly it's not very well insulated.
It's a slate roof with a layer of waterproof felt under the tiles (all in good condition) supported on rafters that are 100mm thick and 600mm apart. Ideally, I'd like to fit in as much insulation as possible, and then cover it all with insulation-backed plasterboard.
But the question is, how much insulation is "as much as possible"? As far as I can tell, the two options are:
1. 50mm celotex wedged between the rafters, but leaving a 50mm air gap behind for building regs as it won't be air-tight and condensation behind the insulation needs to be ventilated away.
2. 100mm sprayed closed-cell polyurethane foam, onto the back of the felt (NOT onto the tiles), thus removing the need for the air gap as condensation cannot penetrate from the inside.
To me, option 2 sounds better as that's twice as much insulation as option 1 (assuming it is not applied by cowboys and the roof is sound). Am I right that it removes the need for the ventilation gap and therefore can be sprayed to this thickness? So far I haven't been able to confirm either way, other than through material provided directly by foam contractors.
Thanks for your assistance,
Miles
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but rain will seep in through some slates and not be ventilated and rot the timbers..
[g]
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wrote:

Seconded. I;d be dubious about anything that removes ventilation and possible sccess to the inside of the roof.
I suggest you put 50mm of celotex between the rafters, and another 25mm over the top, then plasterboard - or, as you say, insulation- backed plasterboard. If you want deeper than that, attach 2 inch battens to the rafters, so you can get 100mm between, and 25mm over (which, by the way, is the depth that my local BC requires for a habitable room in the roof. I'm not sure where building regs would come in with your job - is the attic a habitable room already? If it is, I wouldn't involve BC, myself. I'd still have that 2 inch air gap, though.
Regards Richard
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Isn't the waterproof felt to prevent the water that gets through the slates getting further into the roof?
I can't see any water marks on the joists or the floor so the roof is definitely watertight.
Or am I missing something?
Miles
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On Sun, 17 May 2009 08:39:11 -0700 (PDT), a certain chimpanzee, snipped-for-privacy@googlemail.com randomly hit a keyboard and produced:

The felt is primarily a wind-barrier. It performs the secondary function of a waterproof layer.
Vapour from the inside of the building will condense on cold surfaces (and inside permeable elements such as joists). Normally this is not a problem as ventilation will remove most of the moisture, but if the rafters are encased in poly foam that prevent them breathing, the moisture can lead to dry rot.
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Precisely. And if you insulate between the joists, with foil-backed celotex on the warm side, you provide a vapour barrier to prevent condesnation on the rafters. If you have an air gap on the cold side, then any moisture that gets in from the outside can evaporate again. It's science, but it's not rocket science ;-)
Regards Richard
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On May 17, 12:21pm, snipped-for-privacy@googlemail.com wrote:

Which slate or tiles ?
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Both - it's all in good condition, just need to up the insulation.
Miles
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On Sun, 17 May 2009 04:21:45 -0700 (PDT), snipped-for-privacy@googlemail.com wrote:

Converted? how converted?

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