Insulating Extractor Duct

Hi all!
I have fitted a galvanised duct for a bathroom extraction system and find that high levels of condensation are forming and running back to the ceiling grill. The duct and fan are located in the loft space and discharge through a gable end wall. How much (what thickness/type/supplier) insulation is needed to remedy this problem? Has anyone experienced similar problems - and solved them?
TIA
Phil
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Anywhere where warm moist air is coming into contact with cold dry air, you'll get condensation. The best ducting designs allow for this, and ways of draining it off. The best solution is to allow the duct to have dip or sag where the water collects and can be drained out to somewhere safe.
http://www.tombling.com/ducting/index.htm
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Thanks BigWallop
My installation was guided by the fan requirement to ensure that ducting sloped away from the fan! It therefore now falls away from the fan in two directions: towards the external grill in the gable end and towards the grill in the bathroom.
I was hoping that, if I lag the duct sufficiently, then the moisture will remain air born until it reaches the outside world.
I have heard of condensate trapping, but wouldn't have expected a serious issue in just 3m of 5" duct. In any case, the only place I could see to drain condensate would be via hose or similar through the outside wall. I reckon that, the time when this would be most needed ie winter, would see this arrangement frozen up.
So my question revolves around: How effective would lagging the duct be? How much difference would a switch to plastic duct make?
Phil
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thescullster wrote:

That's correct. Works fine or me.

What you need is to heat the duct below the fan where it slopes back into the room. You might get away with lagging that bit. Don't worry about lagging beyond it - let it drip outside.

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BigWallop wrote:

Totally wrong. Take the duct to a high point as near to the room (warm) as possible so the condensation runs away outside.
You really want to drain down a bathoom fan every day?
Mad!

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thescullster wrote:

You probably didn't follow instructions, which on my fans clearly stated that the duct MUST go up, then down to the outside so condensation runs away from the unit.
Insulatiion won't help a duct that is open to the cold outside air.

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On 28/12/2003 "The Natural Philosopher" a wrote :

But the insulation will help keep the heat in that is flowing up from the bathroom and at least help prevent it condensing. Some sort of flap valve on the outside will also help stop cold air entering when the fan is not actually running.
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Harry Bloomfield wrote:

Flap valve is good. Bu the biggets problem is the metallic nature of teh duct. It will cool the inboard end of the duct like crazy.
Best is to insulate the inboard end, and put a thermal isolator - some kind of plastic coupler? - at the high point of the duct, which is where the fan should be anyway.
Then cold bits lie outboard and dwonhill of the fan, and with flap valve the inbopard end of the duct should fill with warm air from the room and stay warm.
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Thanks guys, but what about the plastic/galv question? Does anyone have enough experience to know what the improvement would be if I changed material?
I have a backdraught shutter in line just downstream of the fan. I also have a louvred shutter on the outlet, although the way these things clatter in the wind, I'm not convinced that these are overly effective.
Also, the condensation so far experienced was down to plastering of the ceiling and some skimming in an unheated bathroom, so the volume of air born water may have been rather greater than normal.
My only other thought was trace heating to come on with the lights and fan. Not sure whether this is over the top, or what the install/operating costs would be.
Phil
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thescullster wrote:

Plastoc will help as its less heat conductive. Won't rust either.

They help a little.

Oh wll - there you are. Wet plaster and no heating is a massive dollop of saturated air just waiting to condense. That is no indication of what will happen in practice assuming you heat the bathroom.

Just heat the bathroom up to 25C or so, so its pleasant to bathe in. That warm air will in any case heat up the duct a bit. Ive got a similar arrangement in one shower/toilet and there is no dripping - not even outside. Thats plastic spiral type duct tho. Very low mass and heat conduction.

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Thanks NP
With the problems I've already experienced, I think paranoia was getting the better of me! Now the plasterwork is drying out, the condensation levels (obviously) are dropping.
To be on the safe side, I will take the loft insulation over the top of the duct to get some house warmth into it.
I was tempted to go for concertina type plastic duct, but as I have a 3.5m run including 2 elbows, the penalty on flow would have been high.
Phil
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