"These let a great deal more air through than clay - something like three or four times more if memory serves. They have downward angled slats to prevent rain getting in, so well designed too. And cheaper!!"
"The Terracotta Air Bricks 9 x 3" are such value for money @ £1.69 it's a no brainer. They're easy to fit too as they are made of plastic they are able to flex slightly if the hole isn't exactly the right size."
"I does exactly what you'd expect a product called an air brick to do, unless you were expecting to use it in combination with invisible mortar and build a see through house. It's a plastic brick that allows air through and did the job of providing ventilation to a bricked up chimney perfectly at a perfectly reasonable price."