Cooker hood ducting - Update

[M85]

Is it a cavity wall?

House built circa 1911 so double brick and being in Scotland, harling on the outside.
Not sure how to finish the bigger hole on the outside to make it water tight round the new vent? I'll need to take harling off to get it to sit flush.
Screw it on obviously then silicon round the outside?

Geez, this kitchen has been a learning curve

 

[JohnD]

I'd press mortar all round the inside and outside where the ends of the duct are. If the gap is more than about a quarter inch, you can butter mortar onto the brick and push the duct through, if its very small, I just press it in with fingertips. As it is not a cavity wall there's no problem of excess mortar falling into the gap.

Spray the brick with water first.

Use a stiff brush to remove smears or excess from the face of the wall after the mortar has started to dry. After a few days hardening, you can remove the duct by pushing straight, the mortar doesn't stick to plastic much. If you rotate the duct while the mortar is fresh it will break the grip. This may enable you to pack mortar into any hollows or roughness. It is much easier to saw or trim the duct if you can slide it out of the wall to work on.

The cowl vent will cover the ragged edge of the hole unless it is very clumsily done. The duct should be sealed into the wall with mortar to keep draughts and insects out, the vent should be close to the wall but need not be sealed.

I believe harling is lime render with stone chips, I don't think we have it round here. Try not to take much off as I think it will be difficult to patch and look good. Silicone sealant should not be necessary.

 

[M85]

I'd press mortar all round the inside and outside where the ends of the duct are. If the gap is more than about a quarter inch, you can butter mortar onto the brick and push the duct through, if its very small, I just press it in with fingertips. As it is not a cavity wall there's no problem of excess mortar falling into the gap.

Spray the brick with water first.

Use a stiff brush to remove smears or excess from the face of the wall after the mortar has started to dry. After a few days hardening, you can remove the duct by pushing straight, the mortar doesn't stick to plastic much. If you rotate the duct while the mortar is fresh it will break the grip. This may enable you to pack mortar into any hollows or roughness. It is much easier to saw or trim the duct if you can slide it out of the wall to work on.

The cowl vent will cover the ragged edge of the hole unless it is very clumsily done. The duct should be sealed into the wall with mortar to keep draughts and insects out, the vent should be close to the wall but need not be sealed.

I believe harling is lime render with stone chips, I don't think we have it round here. Try not to take much off as I think it will be difficult to patch and look good. Silicone sealant should not be necessary.

Many thanks again John, very helpful.
Of course, it doesn't help that we are one floor up and the vent on the wall is a bit inaccessible.

 

[JohnD]

with a 6-inch hole you can put your arm through to press mortar into the joint, if you don't mind using your fingers. You'll need a ladder though to finish off neatly. Warn people not to park underneath due to droppings.

There are some vents that can be pushed through from inside, but I've never used them. The cowl vent is very good for keeping the weather out.

 

[M85]

Yep, got a cowl vent. What's best to fix the vent to the inside of the pipe?

 

[JohnD]

It just fits on, and you screw the surround to the wall. Doesn't need to be glued and I never have done. I suppose you could use plumbers silicone if you felt the need, but try not to get it on the wall. Stainless screws won't rust.

 

[M85]

Brilliant, cheers

 

[M85]

One other thing, the last 30cm that goes through the brick wall will be cold. Do I need to insulate it? A few wraps of aluminium foil maybe enough?

 

[JohnD]

Don't see any reason to. If the duct slopes down slightly to the outside, any condensation will drip out. The vent flap will prevent cold air blowing in.