"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!!"
"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."