Roof space ventilation

If a natural slate roof is unfelted, does this provide any ventilation to the underside? I'd like to jam some celotex between the rafters, but won't be bothered if I need to install specific ventilation at eaves and ridge. The roof covering will probably be replaced in the next five years, when felt and specific ventilation can be added. I just want to still have the rafters in one piece when this happens.
The proposed layers would be:
----- --------------- ---- <-- SLATES ------------ ------------- +---+ no felt +---+ | | 50 mm | | | | air gap | |<------ RAFTERS | | | | ###| |###########| |## ###+---+###########+---+## <-- 25mm celotex ########################## ########################## <-- 25mm celotext x layed ************************** <-- 12.5mm foil backed p/b
The loft area will be used for light storage. Several pipes, including rising cold main pass through, so I'd prefer a warm loft situation.
Christian.
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On Mon, 1 Dec 2003 11:12:18 -0000, Christian McArdle wrote:

Ample, you ought to see the Tyvec flapping about under our slate roofs. Once you re-new the covering and fit a sarking layer then you need to worry about ventilation.
With what you propose I'd be more worried about driven rain/snow getting in and running down the back of the celotex. Where is that water going to go?
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
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Excellent, I'd hoped so. I seem to recall some make of felt that is breathable that can be used instead of the standard stuff. Am I imagining it, or does the stuff exist, and does it mean that when I replace the roof, I don't need to install ventilation? It is an Edwardian property and I'd rather have a totally invisible installation.

I'll think about it when I install it. In any case, the water have just landed on the lath and plaster below, so whatever I decide will probably be an improvement. A bit of guttering sending it to an overflow will probably do the trick.
Christian.
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Lots. And water as well.

If the insulation dries out quickly OK, but if its so dense that it dosent, I wouldnt. Unlined slate roofs do let in water, then the air comes along and dries them off again. They are by no means waterproof.
Regards, NT
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I thought that celotex was closed cell, so couldn't get soaked? Am I mistaken?
Also, would it break down over long periods? I have seen no sign of moisture ingress through the roof during these last weeks of rain, so it wouldn't be a deluge of water, just spots, at the most. I could always use foil backed stuff facing upwards. This could even reflect excess summer heat radiated from the tiles back up. (The insulation would be sufficient that no useful solar gain in winter would ever occur).
Christian.
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What's a Devon microdraft?
BTW, there will be two layers of 25mm celotex cross layed, to eliminate cold bridging and cracks.
Christian.
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