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

[DIYNotIan]

Hi folks

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

 

[CBW]

Personally I’d fill the gap with brick/expanding foam and then a small sand and cement mix over the top. Others may have a more professional/experienced approach.

 

[foxhole]

Several options but solution above is good.

 

[DIYNotIan]

Many thanks for the replies.

I'll go with that suggestion then and use expanding foam in the central parts of the hole. That will be easiest too because it will be very difficult to reach those area with sand/cement or anything else. I'll then finish on the outside with sand/cement and on the inside with filler.

Once that's all done and I fit the vent grille, 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

 

[opps]

I would use a MS ploymer such as CT1 or sikaflex. Unlike silicone, they can be painted over.

 

[BlueLoo]

Dunno what your actual final make up will be, but, normally, you have a 6" pipe through the wall, flush to the outside but proud to the inside. You then fit the ducting over this pipe with flexible hose, zip clips, ties, etc etc.

Point being, you've left very little room on the wall side of the pipe to make good or hook up the extract duct to the pipe.
I'd move it over 50mm to allow you to get a slim trowel in to make up the wall and a brush to finish it off. You also need to think about how the interface is finished betwixt wall/duct/pipe. Normally, these are covered up by the extract fan covers (Vertical).
Granted, you might not see this if its going to be above the cupboard height but even so, having it so close to the wall needlessly so is going to be a bit of a PITA.

 

[JohnD]

As it is a cavity wall, you want to prevent mortar or other material falling down inside.

You can do it by stuffing mineral wool loft insulation into the gap, followed by a piece of cardboard (it is easier to pick off fallen material from the smooth cardboard)

If you are doing a neat job you can build up the wall with layers of mortar, packed against the pipe, and you can rotate and pull out the plastic pipe after each has stiffened. At least the outer wall should be mortared for weather resistance and reasonable appearance. Do the outer wall first. With a 6-inch pipe you can put your hand through and smudge it with your fingers and a filling knife if you do not have ladder access. A trowel and brush will be rather awkward.

if you set the duct with a slight outward fall, rain and condensation will trickle outwards.

 

[DIYNotIan]

I would use a MS ploymer such as CT1 or sikaflex. Unlike silicone, they can be painted over.

Thanks. I've used Geocel TheWorks in the past and that's always been good too. So I'll go for that.

Dunno what your actual final make up will be, but, normally, you have a 6" pipe through the wall, flush to the outside but proud to the inside. You then fit the ducting over this pipe with flexible hose, zip clips, ties, etc etc.

Point being, you've left very little room on the wall side of the pipe to make good or hook up the extract duct to the pipe.
I'd move it over 50mm to allow you to get a slim trowel in to make up the wall and a brush to finish it off. You also need to think about how the interface is finished betwixt wall/duct/pipe. Normally, these are covered up by the extract fan covers (Vertical).
Granted, you might not see this if its going to be above the cupboard height but even so, having it so close to the wall needlessly so is going to be a bit of a PITA.

Thanks for sharing your thoughts. The hood is around 1.5m away from the hole. There's a very short piece of semi rigid aluminium going up from the hood and that's connected to a plastic elbow. From there to the wall (and through the wall to outside) is all in 6" pipe. There are no cabinets; it's all being boxed in.

The bit of pipe in the photo was just an offcut that I put in the hole to demonstrate the amount of space that needs filling.

Some good points you make, but ultimately we wanted the boxing in to be as small as possible which is why I've gone as close to the ceiling and wall as possible.

Thanks for all the help

 

[DIYNotIan]

Thanks @JohnD - wasn't ignoring your post, just that our posts crossed to I hadn't seen it.

That's some good points RE 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?

RE the insulation 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...

 

[JohnD]

I try not to bridge the cavity at all.

My house has CWI so I roll up the quilt into a sort of collar, larger than the pipe, and stuff it into the cavity. it is then tight enough not to get dislodged by bits of rubble or mortar, and will hold them so they can be picked out. But it is easier to clean a bit of cardboard.

the blown fibre loose fill cannot be relied on to hold rubble or dust, but the collar can. I leave it in place. While you are working you can wriggle the duct free to clean out dropped mortar and tidy up the inside. If you have a bit spare I suppose you could make a half-pipe which would be easier and would collect its own scraps. Occasionally I have twisted and pulled the duct out from the mortar afterwards, if the jointing spigot is too tight. It seems to me that the outside brickwork leaf needs to be filled best, to keep the weather out.

the insulation will also prevent a cold spot, which might attract condensation.

 

[DIYNotIan]

Thanks

One other thing... I'm thinking the mortar might not stick so well to the shiny surface of the pipe. 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 to help the mortar stick?

Cheers

 

[JohnD]

it doesn't have to stick to the pipe. If you pack it well into the brickwork, it will be a tight fit. If you turn it before it sets, you will (may) be able to screw it out from the wall when hardened, if you ever need to. But it will be like a cork in a bottle, not draughty. I do all mine like that.

 

[BlueLoo]

Thanks

One other thing... I'm thinking the mortar might not stick so well to the shiny surface of the pipe. 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 to help the mortar stick?

Cheers

What goes in usually has to come out at some point so don't worry about it.

 

[DIYNotIan]

If you turn it before it sets, you will (may) be able to screw it out from the wall when hardened, if you ever need to. But it will be like a cork in a bottle, not draughty.

Sorry, but I don't quite follow that. Am interested to know what you mean though.

Thanks both for the responses

 

[JohnD]

the pipe is surrounded by wet mortar, which starts to set. It has no strength at that point.

if you rotate the pipe, like turning an arm in a sleeve, it will break any slight adhesion. This means that although the smooth pipe is a perfect fit, and the mortar is moulded against it, you will be able to turn it again once the mortar is fully hard. If you turn it and pull once the mortar is hard, it will come out, like a cork from a bottle. Don't pull it while the mortar is soft, or it might pull out.

This enables you to look in the cavity, if you want, maybe pack in some more insulation, check for damp, patch any gaps in the mortar, remove any snots, get at the vent. In my case, I had to saw a bit off the duct, then slide it back into the wall, because the spigot on the fan did not quite fit the duct in the wall. I have had that a couple of times recently (and not with duct from the same batch or the same supplier). One one, the duct would not come out, and I had a terrible job with a Dremel and a multicutter.