Can I run ducting inside a cavity wall?

[Hooo]

I've got an awkward route to get the ducting from my cooker hood to an outside wall. If I run above the ceiling it would emerge level with a flat roof, so I need to run it vertically up the wall of the room above for about 500mm and then vent out through the wall. I would rather not have a boxed in section of ducting in this room, so I was thinking about cutting a hole 500mmm high by 150mm wide through the inner layer of breezeblocks and running the duct inside the wall, mostly in the cavity itself. The bottom of the hole would be below the floor of the upstairs room and there would be a 90deg bend to a vent at the top.
Is there a reason why I shouldn't do this? I was going to use the original blocks to fill the gap afterwards, but would I need to reinforce it?
The cavity is insulated with rockwool, if that makes any difference.

 

[Steve]

I dont understand. What situation is the cooker fan in? I dont see how running it up 50cm will be any different to going stright out through the wall???

In any case you cant put it in the cavity, its too small.

 

[Hooo]

Sorry, I should have made it a bit clearer.
There is a single storey extension next to the kitchen, so there is no outside wall within a reasonable distance on the ground floor. If I run the duct to the original outside wall, then up the inside for about 500mm into the floor above, I can fit a vent in the wall (above the flat roof of the extension). The trouble is that this means the extractor duct runs through the bedroom above the kitchen - not exactly ideal. I was hoping to chase it into the cavity wall, by cutting a hole right through the inner wall and making good over the duct afterwards. If the duct is too fat to fit in the cavity, I'll make the inner wall thinner when I replace it over the duct.
I just wanted to know if there are issues with cutting right through the inner wall, or with fitting ducting in the middle of the wall.

Thanks

H

 

[chimbs]

H it was a long time ago but I find myself in the same situation - I was wondering if you went ahead and did it this way and was it successful? I have a 75mm cavity but was planning on using 220 x 90mm rectangular duct. Cheers chimbs