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.