Insulating and cladding a summerhouse roof

10 Oct 2010
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United Kingdom
I am planning to insulate a 6M x 3M wooden pitched roof summerhouse in my garden. My plan was to use 80mm celotex between the (100mm) rafters and hold up with tongue and groove cladding.
The issue I can see though is that the 10 rafters run from the front of the summerhouse to the back, parallel with the eaves and the ridge along the top, and therefore the 20mm air gap between the top of the celotex and the cladding on the roof will not have any airflow to prevent moisture building up
The roof is merely good quality shed felt.
The only way a can think of venting, what will become 10 enclosed spaces, would be to make vents at the front and back of each of the runs in the 20mm
Do I even need to and if so any ideas on the best method?
See below a picture:
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