Technically, it's "mousse expansive", or in English "expanding foam" but around these parts it's just goo.
Squirt enough of it into a hole and it will expand to fill all the spaces: a brilliant idea when you're dealing with a stone wall where much of the mortar has disappeared, or the gap between a stone wall and a brick wall where no serious attempt has been made to join the two.
Our kitchen wall has these sorts of issues, so for the past week I have been squirting goo like it's going out of style. It takes a week, because after you fill one gap you have to wait for the goo to harden before you can fill the gap next to it. I think we are getting close to having the problem sorted, because now I dont feel a breeze anywhere when I pass the back of my hand in front of the wall.
at least one hole the wind blew through