On 28/01/2020 23:10, alan_m wrote:
On 28/01/2020 22:31, polygonum_on_google wrote:


You could try a cowl vent or flap vent as shown he
https://www.screwfix.com/c/heating-plumbing/air-vents/cat840496

Some cowls also have the inbuilt flap that only opens with airflow from
one direction

Single flap hinged at the top

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