Questions about insulating a flat garage roof (cold roof)?

26 Feb 2011
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West Midlands
United Kingdom
Our garage ceiling was made of 8x4 cement boards which contained asbestos so we have had this removed. It is a flat roof contruction.

The joists are 9" x 2" with a 4.5m span. 19 of them spaced at 15". They run from left to right across the garage.

On the right hand end there is no soffit just fascia with a gutter on.
The left hand end is against the gable end of the bungalow.

The plan is to fit 100mm kingspan insulation between the joists which leaves apx a 5" void above which I am aware will need ventilating.

As there is no soffit on the right hand side at all I have been advised to drill 70mm round vents into the facia and fit them behind the guttering but rather than drilling out all 19 vents in the facia between each joist instead do it every 3 and drill 50mm holes through the joists themselves to veltilate the ones inbetween.

Im ok with doing that but what about the left hand end where the joists go into the gable wall?

Most have gaps around the joists on this end of upto 1" or so in places and some with large holes where cables have been put through. The gaps around the joists go through into the cavity of the gable.

What are peoples thoughts on what to do here? Do I foam up the gaps to stop it venting through into the cavity or leave it as it is for additional ventilation?
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