Ducting for in line extractor fan

9 May 2010
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United Kingdom
My axial extractor fan in ensuite has given up the ghost and looking to replace it with a more powerful inline fan.
My husband's friend who is fitting it is a retired bathroom fitter and he was going to use the existing flexi ducting which isn't insulated however everything I read online suggests ducting should be insulated. The existing ducting currently runs between layers of existing insulation so is technically insulated but the inline fan will be sat on a joist so I think will be a bit higher and unable to be covered properly. I was thinking he should fit proper insulated flexi ducting.
It's a short run - 1-2 metres max & will exit out the soffit.
He seems to think there shouldn't be any problems using existing ducting and says he has never had to fit insulated ducting previously.
I don't want to offend him/feel like I'm telling him how to do his job but am I right? Mainly concerned about potential condensation building up (he says it shouldn't be an issue as the ducting will slope down to soffit).
Can condensation build up on exterior of flexi ducting and drip on ceiling?
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Ducting does not need to be insulated it depends on the material and location.

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