Ventilation of pitched roof lined with Celotex?

[monkey magic]

Our loft space has tiles on battens with no felt/membrane underneath.

The underside of the rafters (which are not deep enough for between-rafter insulation) have a layer of foil/bubble-wrap stapled to them to keep dust/wind out and provide a bit of insulation.

I was thinking about adding some 120mm Celotex to the underside of the rafters/foil layer. Would I need to leave a ventilation gap at the eaves and apex? (I think a 100mm gap at the eaves/apex was recommended with the bubble wrap).

If so, wouldn't the convection much reduce the effectiveness of the insulation?

Are there any other options available?

 

[Static]

Put more insulation between the ceiling joists. You dont want to trap moisture between the tiles and the "bubble wrap" material.

 

[monkey magic]

Unfortunately the joists aren't deep enough for insulation, which is why I was going for under rafter Celotex.

I don't think moisture build up will be an issue between bubble wrap (under joists) and outside tiles as that space is well ventilated (there is no felt or membrane under the tiles and therefore lots of air movement.