What to use to fill large gap around extractor vent pipe?

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Krome10

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Hi folks

Another general DIY query if you don't mind.

I'm in the middle of putting up a kitchen extractor. The cooker is on an internal wall, so the ducting takes a right hand turn above the hood and runs for a meter and a half or so to the nearest outside wall. The previous hood had small rectangular ducting that exited bang in the corner there the two walls and ceiling meet. The new ducting is round and much larger at 6"/150mm diameter.

I've made the hole in the wall, but because of the previous rectangular hole I'm now left with three quarters of a nice round hole, and a quarter of a large gap where the previous hole was. I feel reasonably sure none of that make much sense, so there's plenty of pics below from inside and out. As you'll see, it's a cavity wall.

I'm guessing/thinking I'll need to fill the large gap with the ducting in place, but let me know if you think otherwise.

But more to the point what's best to use? Mortar? Expanding foam? Or???

And does it matter if I bridge the cavity seeing as it is right in the corner and the ducting bridges it anyhow?

Many thanks


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Woody2Shoes

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The ducting should slope downwards from inside to outside so that you don't bridge the cavity from outside to inside - moisture wise. I would mix some strong mortar say 4 cement to one sharp sand and seal round the tubing from the outside - then put a weatherproof terminal on the outside wall. The gap on the inside could be filled with expanding foam.
 

Stigmorgan

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+4 for Woody, expanding foam inside and cut off the excess once its set, sharp sand and cement at a 4:1 mix with a consistency of toothpaste on the outside.
 

Craig22

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A vent louvre is essential. Without, a short period away from home at the right time of year will end up with birds nests in there. Apart from the fact they make a hell of a mess (experience!), you wont be able to use your extractor until they fledge.

But in general +4!
 

Krome10

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Many thanks for the replies.

As recommended, I'm going to go for expanding foam in the central parts of the hole. It would have been very difficult to reach them with sand/cement or anything else, so that's another plus point RE expanding foam. I'll then finish on the outside with sand/cement and perhaps level off inside with filler.

I've got one of these, so all good on the vent grill front:

https://luxairhoods.com/150mm-Louvred-W ... ght%20Grey

Once that's fitted, I'll run a sealant where it meets the wall to stop any water getting in behind. I might also run a bead where it slides onto the pipe as the fitting isn't all that tight.

Any ideas what sealant/silicone (or whatever) would be best for that?

Cheers
 

Richard_C

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I fitted similar 10+ years ago, no problems. The vent louvre should be a snug fit to the pipe, I wouldn't seal it because you may need to pull it apart for cleaning from time to time unless you can reach in from a close-by joint inside.
 

Krome10

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Thanks @Richard_C

I'd say it's a quite snug fit. I've got other fittings, such as a connector and an elbow. Those are very snug, and with either of them fitted I can lift up a 1m section of pipe holding just the connector. Can't do that with the vent louvre, it just pulls straight out. So there's a tiny bit of wiggle room with that.

Tbh, I'd prefer to keep the vent louvre free as you suggest, so that I can access it for cleaning etc. And perhaps I'm overthinking it. I just wondered if the fact that it's not a mega snug fit would mean that the extracted air and moisture would works it way into places it shouldn't be, and that a sealing it in some way would prevent that. But if it's an over thought-non-issue then I'll just slot it in and leave it be.
 

Krome10

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Another thought too...

Does it matter if the mortar and/or expanding foam bridges the cavity? I guess I thought that a small amount of bridging in the top corner wouldn't do any harm but am I wrong? Do I need to avoid it totally and at all costs?

Someone has also suggested wedging in a piece of rockwool to stop mortar etc dropping in the cavity. Would it be ok if the insulation stayed in place permanently? It would only be a small piece. I know there should be an air gap and it shouldn't bridge, but it would be a lot easier if I could just plonk it in and leave it there...

Many thanks
 

flying haggis

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If you want the mortar to stick better to the through wall pipe , coat the bit in the outside wall in solvent cement and then whilst its wet sprinkle sand on it. It will give the mortar a key to hold the pipe in.
 

Krome10

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If you want the mortar to stick better to the through wall pipe , coat the bit in the outside wall in solvent cement and then whilst its wet sprinkle sand on it. It will give the mortar a key to hold the pipe in.

Good thinking - hadn't thought of that... Any thing else anyone can think of that I could do to the pipe to help the mortar stick better? It's tough getting out anywhere at the moment and the pressure's on to get the job finished. I've not got any solvent cement unfortunately.

Would roughing up the outside of the pipe with sandpaper help or would it not be enough? Heck, could I glue a piece of sandpaper to the outside of the pipe, or would it get too soggy when the mortar is added?

Those are my stupid suggestions; anyone got anymore sensible ones?

Cheers
 

Sachakins

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Roughing up well will do fine. Also you could use electricians insulation tape around the pipe to get a snugger fit of vent onto pipe, will still be able to get it off then if needed.
 

Krome10

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Bit of an aside, but relevant to the post, when using mortar for repairs such as these should the substrate get a coat of PVA first? I was hoping to do it in the morning, and I don't have any PVA, so that will slow things up. But if it makes a lot of difference and is recommended, then I'd rather get it right.

Cheers
 

thetyreman

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I would do it by stuffing the gap full of rockwool, then expanding foam tape on the inside and outside possibly some backer rod as well if the gap is even enough, leaving it slightly inset so it can then be filled in with silicone on the inside and mortar on the outside.
 

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