Hi, I will be installing air vents into the exterior walls around my house. The problem is the walls are 3ft thick mixed stone and rubble. There are a few places where I can find spots to dig through and I can keep the air flowing around the sub floor o.k. but the hire places just give me the old slow head shake with the tut tut tut when I ask about cutting or drilling through. Any advice? Thanks, Jonny.