best way to seal kitchen extractor vent

[Robbox]

Old stone house, so not very flat walls on the outside. Where the kitchen fan bendy pipe is coupled to the square flap thing on the outside, the original owners have sealed the plastic flap thing to the stonework with just huge gobs of silicone. It's all coming away from the wall and I want to ask what would be a more satisfactory solution instead of more silicone.

 

[JohnD]

possibly pack mortar to make a flat surface. It may be easier if you can take the plastic terminal off the wall and (having first packed the sides and let it harden) use a flat trowel or float to fill in the centre smooth.

You would be effectively rendering a small section of the wall. If the mix is a fair colour match with the stone or mortar it may not look bad.

Otherwise you could use expanding foam, but it is very sticky, so mask off the wall with tape and newspaper to prevent it sticking anywhere else, and trim off excess with a serrated knife after it has fully set. It degrades with sunlight but you can paint the exposed edge or run a silicon ribbon to protect it.

 

[^woody^]

Use the screw holes provided