Retro-fit backdraft shutter?

23 Dec 2007
Reaction score
United Kingdom
This is only kinda plumbing, and very basic - I do apologise for that.
I have a wall-mounted 10cm timer extractor fan in an upstairs bathroom. In the trio of storms we had a couple of years ago the driving rain must have come inside as water marks have appeared all over the wall, and remained - I'm talking a good year or so ago since noticed - I've kept an eye on it and they've not gotten worse, so I don't think it's any other issue (we've had an easier time of it weather-wise since then).

So I have used CoverStain and redone the wall so it looks nice, that meant I took the fan off - it's a straight run of duct to the outside where there is a flat cover with a fly-guard on it.

I see no reason why driving rain could not come in through this entry point again and ruin all my good work. I think I have a few options and I wanted to run it by folk with opinions on what's the best way to make me future less worrisome...

1) Fit an external duct cover that has one of those cowls, that aim downwards and are designed to stop rain ingress (access is difficult from the outside)
2) Get a replacement fan with some kind of integrated shuttering system (but is that mainly for air / odour and not water?)
3) Get some kind of retro-fit backdraft shutter (same question as above, but a very cheap 'fix' if it would work)

For 3) I've seen these flaps for sale ( but they do appear to be designed to sit inbetween two runs of duct (hence "in-line" I suppose), and join them, there's a ridge mid-way especially for this. My question is - does a product exist that just sits inside an existing run of duct? Or do you just buy it and sand the ridge off so it slides down the duct? The obvious advantage here - if it works - is that I can do the work from the inside and access won't be a problem. But there's no point doing it if it won't work.

So... at the end of the day, is 1) the best long-term option?

Or can I just shove a piece of plastic or metal down there, shaped as a shallow semi-circle to act as a kind of dam if any small amount of water makes its way in there? Guess it'll just go back out down the external wall then... not something you desire... but the rain was there anyway and better than the internal wall.
Sponsored Links
Maybe something like this 100mm with built-in backdraft shutter is my best bet?

I have no idea if the brand is any good. I've only previously heard of Manrose, Xpelair and um, that's about it.
Cowl vent outside.

Most builders fit cheap 100mm fans with a throughflow of about 80 cu m/hr

You can get better, quieter ones, such as Soler & Palau, or Envirovent, which have ball-bearing motors, inbuilt backflow flaps, and rubber mountings; but if you have steamy showers, and can put a duct in the loft, you can fit a ducted inline fan with about 3 times the power.

If you ever fit a duct yourself, core the wall so it slopes to the outside for water to run out, and fit a cowl vent.
Thanks. Cowl it is. It's where my mind was heading. I'll just have to find someone who has the capability to get up 12m high.

At least by keeping the current extractor I'll not have to worry about wire length / position - the ones connecting to this seem to be cut to almost the exact length needed (for this fan's terminals).

The people who renovated this building properly never thought water coming along this duct would be an issue - I've stuffed a measure down and it touches the grille at the other end at 575mm.
Sponsored Links
Are you sure its rain water ?.
The hot air being pushed out by the fan could condensate inside the cold duct and flow back into the fan .
Get a long cord, and pull it up from inside! :)

I figured that was a joke... but I know nothing about these things, really, so I'm gonna ask... is that actually a serious comment that has legs? I can imagine I can get it up there, dangling around, but doesn't it then need screwing into the wall from the outside anyway?
Are you sure its rain water ?.
The hot air being pushed out by the fan could condensate inside the cold duct and flow back into the fan .

Am I sure? Well... no, I'll admit that. It's just I pinned it on when we had Storms Dudley, Eunice and Franklin - I thought that was in 2021... but I must be a little mixed-up as Google tells me they were early in 2022. (?) Anyway, we had water ingress throughout the building and the stains on this particular wall were all coming from the bottom of the fan.

I've fixed the external issues with the roof and the windows and projecting window ledges last year, and then left the insides to sort themselves out - as I say, it's not got any worse in that long period of time - so I assumed it was a one-time event. I'll put money on it anyway... but I can't be 100% sure, no.
Cowls were always on my to-do list, having seen them on other buildings. It's just having no knowledge and problematic accessibility. I'll progress the cowl angle.
. is that actually a serious comment that has legs?
Yes, it can be a bit awkward, but it's actually doable!
If you have a lightweight cowl, you can glue it to the duct from the inside, with a band of silicone. The angle of the cowl can be adjusted by putting your arm through the hole and twisting it.
Also use a slip knot on the cord to allow its release.

Maybe something like this 100mm with built-in backdraft shutter is my best bet?
Just to reiterate, the built-in backdraft shutter are usually flimsy bits of plastic film, that can be noisy in the wind. An external cowl/shutter is far superior IMO. :)
Thanks, I wasn't sure you weren't pulling my leg... but I am sure that my arms aren't that long to be able to reach the cowl at the other end and manipulate it (never mind remove the existing cover).

Everything (everyone) is saying cowl. I concur.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links