Does this calc sound right (as it sounds like a lot of air bricks):

 

 

If I'm using these air bricks which say they are 1,290 mm^2?

 

List No. 350 Square Hole Air Brick 215 x 65mm - Forterra

 

I'm only ventilating a few cm behind a brick plinth which doesn't do any structural support btw. Can I still use the floor area calc even though I'm not ventilating the floor area?

 

Would peeps use, say, some stainless mesh masticed behind? 

 

So is this an insulated masonry cavity wall?

Do you want for looks? We've gone for weep holes, which are much less visible, even though there are lots of them, and vermin proof! (Uninsulated cavity between GF block skin round timber frame).

@Mr Punter - an 11-course brick plinth around the base of our Suffolk-barn-like structure, not load bearing, just for aesthetics - both for us and the planners.

 

@G and J - that's interesting, do you need to put mesh behind them to stop creepy crawlies?

@G and J - this kind of thing?

 

We used these. Didn't mesh as will "deter" larger insects, not sure would stop determined ants but 🤞

 

 

The details supplied by @G and J quite rightly say 'For discharging water from external walls', which is what I have always understood. How many weep-holes would you need to equal the cross-sectional area of just one 'ordinary' air-brick? AI says: For most standard brick veneer or cavity walls, building codes and masonry councils recommend a weep hole cross-sectional area of at least 100 mm² to 250 mm². [1]

 

A search for 'equivalent area of 225 x 150 air-brick yields the (AI) response:

"A standard 225 × 150mm (9× 6in) double air brick provides an equivalent ventilation area (free area) ranging from 4,300mm² to 18,000mm², depending on the material and the design".

 

We appear to be comparing apples and... something else.

If the cavity does not need ventilation you could fit a DPC cavity tray around the base of the plinth bricks linked to the inner leaf and fit the mini weep vents at 900mm centres.