Extractor fan ducting

27 Dec 2017
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United Kingdom
My bathroom extractor started leaking today. I removed the extractor fan and the ducting. So the ducting on the inside is dry. However the insulation (middle sectoin) is soaking and dripping wet.

Any idea how this could have happened? Should I replace?

As a point the ducting is about 1.5 m long and connects to a roof tile.

A picture is included. If you look to the left you will see some of the insulation which is soaking wet.


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this is ducting which is connected to a roof tile and got wet after a spell of heavy rain?

I'd guess rain is getting in.

Unlikely to be caused by the duct.

Show us the route of the duct please. If it is vertical any condensation inside (rare if it is insulated) will tend to run straight out, not to accumulate.
Thanks for replying. Appreciate it.

The ducting is lying down and then bends up.

If rain is coming in from the tile. Would that need to be replaced? It is only 5 years old.
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Johns more than likely right, but is there a hole in the bottom of the ducting it by any chance - even though you say it's dry inside. As the damp air flows though the ducting, water will condense out, and if there's a hole, then that'll be the problem. Flexible ducting isn't great to use in a loft as water can then get held in the crevices, plus you should have a drip extractor fitted above the ceiling fan to stop and water that condenses damaging the fan. Channel ducting with a slight fall and sealant in the joins is better, but as your ducting seems to raise up at the end (which is a no no) then that would be difficult.
I initially thought a hole in the ducting could be the problem.

Should the outter ducting be taped tight to the inner. As it is not. It has been cut and then fitted.
Yes, it's insulated duct. My initial thought was that the inner part had a hole and that was causing the leak on to the middle part, which is the insulation.

It lies flat on its journey to the roof tile and then is angled up to meet it. In terms of size it is longer on than journey than it needs to be. Quite a bit lies flat. You could cut some away.

The water is all in that middle, insulated section. You can ring it out and water pours out.

The finish at both ends is like in the picture. I am sure the outer part should be sealed/ taped tight to the inner??? This would have stopped the water leaking at least. However, the fact water is getting in there is still a concern.

Also, the whole thing has loft insulation laid on top. So it is well protected.

When I disconnected the ducking the tile it is connected to was not dripping wet. Also if it did drip, it should still not get to that inner insulation of the ducking. Hope that makes sense.

Thanks for the advice. This site is quality for when these things pop up.
You’ll have to determine if it’s water ingress or condensation. You could try disconnecting the duct and running it into a bucket for a couple of weeks - with the middle hooked up high (like a washing machine hose). And don’t forget to block up the roof tile vent. See how much water collects in the bucket.
To be honest given the recent temperatures it's surely condensation. Maybe now the insulation it's wet is no longer effective at insulating.
The best solution might be as the other John's suggestion of use rigid duct connected properly and insulate.
Also make sure the fan runs on once the humidity is dropping. If you only run it during a shower then off after, the wet will remain in the duct
Thought I would postpcture of how the ducting leaves the bathroom to roof tile.

I took the whole thing off today. It is dry on the inner wall. Just the middle insulation which is wet and that is where the dripping came from.

Does the setup look ok?


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IT doesn’t matter how it looks... it’s not working, so something is not right and some trial and error is required.

I’d try shortening and straightening the route and use a wooden splint with long cable ties to support the duct on its way up to the vent... the warm moist air needs to be pushed out before it can condense in the duct.

Edit: the roof tile vent also needs to be able to run any condensation out and away from the duct so it doesn’t run back down it! Check to see if that “dew point” is clean and not covered in dead skin dust :eek:

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