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

[Oxbow16]

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

 

 

 

 

 

 

 

[Oxbow16]

 

 

[PeterW]

Foam them then trim back and fill with a couple of layers of mortar / render / plaster where appropriate. 

[Oxbow16]

Many thanks for the reply.

 

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

[Oxbow16]

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

[saveasteading]

23 minutes ago, Oxbow16 said:

wedging in a piece of rockwool

If you use  cavity batt mineral wool it is designed to be waterproof so it can be left in place.

ROCKWOOL Full Fill Cavity Batt Thermal Insulation 75mm -

or Knauf

 

Otherwise no, as it may get soggy then fall some where and wick water through.

[Oxbow16]

Thanks @saveasteading

 

Is this the right stuff or is it for internal only?  

 

https://www.knaufinsulation.co.uk/products/rocksilk-rs60

 

Getting it into position is what might be the tricky part, but it could work... 

 

So just to be clear, if the cavity was bridged with expanding foam (albeit it just a small amount) would that be a problem?  

 

Ta

 

 

[saveasteading]

2 hours ago, Oxbow16 said:

Is this the right stuff

No, it needs to be this, or another that mentions water repellent.

https://www.knaufinsulation.co.uk/products/dritherm-cavity-slab-32

2 hours ago, Oxbow16 said:

would that be a problem?  

Could be. The pipe already bridges the cavity, so make sure it is level or sloping down a tad (outwards).

 

Expanding spray foam is difficult to control and you have no idea what is happening out of sight, So put in the barrier the best you can and spray from both sides. If  there are any tiny gaps the foam will probably start to penetrate and give up and expand the easy way...outwards.

 

You will probably end up stuffing bits of mineral wool in as best you can until it looks about complete.

[Oxbow16]

That's a great help, thank you.  

 

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

[saveasteading]

2 hours ago, Oxbow16 said:

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

 

Loving the new idea. But you are right it would get soggy, rot and disappear. On the same lines you could glue sand to the pipe, but it would be easier to roughen it with the sandpaper as your original suggestion.

Whether it matters I don't know, but would do no harm. If you keep the mortar as dry as you can that it is still manageable, and push bits of brick into it when there is enough room it wil reduce shrinkage and o gap should appear.

 

I think we all deserve a report and photo of the finished project.

 

 

[ETC]

Lift it up and build underneath it in block.

[Oxbow16]

1 hour ago, saveasteading said:

I think we all deserve a report and photo of the finished project.

 

 

That all depends on whether the result is DIY success or more Frank Spencer Christmas Special   

 

I've gone with the roughen with a bit of sandpaper option.  Everything's taped up and ready to go now, but with work tomorrow the final fitting will be tomorrow evening and over the weekend.    

 

 

1 hour ago, ETC said:

Lift it up and build underneath it in block.

 

Do you mean the pipe?  If so, it can't go any high as the ceiling is in the way.  But perhaps I misunderstood?  

 

Thanks for all the great help.  

 

 

[Oxbow16]

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

[saveasteading]

I wouldn't. But try to get rid of most dust on the surface, and wet it. 

Then try to force some mortar into all the surface and corners you can, before mass filling.

If you keep the mortar on the dry side at first to see how it handles, it will then stay in place. but if too wet it will run and slump.

You can either fill it flush and trowel smooth ot leave it shy, and later add a finish.

Don't over worry.

[Mr Punter]

It is good to have a slight fall to the outside so any moisture or condensation runs away.