Cold area from a small roof space. How to make air tight and insulate

[Tim_Gilbert]

Referring to @lloydham’s earlier post, you definitely need to read @pottyone72’s topic,

Installing a vapour control layer on a (cold) loft floor

Our (cold) loft had moisture problems - through multiple causes. a non-breathable plastic roof liner a prior poor renovation of the loft space for a make-shift office had blocked ventilation at the eaves -stack effect from pipes in a stud wall open at the top in the loft was billowing in hot air in the winter -poor U values of plaster board ceilings in the rooms below were enabling the passage of warm air into the cold loft (another example of stack effect). more uninsulated hot water pipes …

As an extra note you need the extract duct to rise vertically and then descend without interruption to the exit vent. That way moisture will either leave the building or fall back into the bathroom. You definitely don’t want water collecting in it. The whole structure needs to be insulated, of course.