Is it a 65 x 215 airbrick? Is there a clay liner built into the inner skin which crosses the cavity too?

The periscope in place. With hindsight I should have removed more bricks and had the opening lower, but too late now.

Cheers Matt,

Thanks for all the detail you’ve shared, it’s helped me work out what I’m actually looking at!

After a closer inspection I realised the “mystery fluff” in my opening is just gutter-brushes that someone has shoved into the cavity (I had no idea people did that). I’ll post a few photos when the light’s better, but here’s what I’ve got:

Outer leaf: brick

Inner leaf: lightweight block, so the hole on the room side is much larger, about 320 mm Ă— 100 mm Ă— 100 mm deep.

Everything sits below the DPC, which keeps things simpler.

I tried a generic plastic telescopic sleeve from Screwfix, but it’s nowhere near deep enough to reach the air-brick. Plan B is to pick up a sleeve with the right cavity depth and then reduce the block-side opening with a bit of brickwork so the whole lot fits snugly.

appreciate the pointers so far!

Thanks Tony, thanks for your previous question (missed it earlier) and to answer your question no, there is no clay liner.

The airbrick seems 3"x9", which seems to be reasonably standard (so used to thinking in mm that It never occurred to look at the other edge of the tape) but I need a 160mm to clear the cavity and the inner leaf of blocks, which seems unusual but eminently doable.

I may have a solution for you that I used myself in a similar situation:
https://timloc.co.uk/products/1201-telescopic-underfloor-vent-accessories/

The only problem are the minimum order quantities in which the parts are usually available.
So I have some 10 of "1201ABGR - Air brick front in grey" left over, currently offered here on eBay:
https://www.ebay.co.uk/itm/266736345216

Depending on how I proceed with my retrofit, I may also have some 20 of "1203 – Horizontal Front Extension Sleeve" left over that I meant to use but may not need now, though it may be a wee while before I can make a decision about this.

Just get in touch if any of this is of interest. No need to buy from me through eBay, I can send you what you need and I can spare.

By the way, I knocked out the existing air bricks in the outer wall leaf and replaced them with the above air brick front as the cross-sectional area available for air flow was much greater than in the original air brick. It won't spoil the looks of the facade as the air bricks will eventually end up behind a rain screen.
If you don't want to do this, I am sure you could just secure the horizontal vent section against the inner face of the existing air brick. Just need to check sizes match reasonably well.

I have only got as far as installing the new vents just after I had the failed blowing wool (glass wool beads from about 35+ years ago) replaced with new cavity wall insulation (Knauf SupaFil40 blowing wool again but installed properly this time). I will add external wall insulation (wood fibre) and rain screen. For this I need to extend the air vents, thus have only spray-foamed them in for now so that I can pull them out and extend once the EWI is installed. See a couple of photos below.
I will write more about this in a new thread as I hope to get some insights from some of the experts on this forum.

Below: the vent brick temporarily sealed with spray foam.