I hope someone can give me some advice if any of you have done this.

Before I have always had a cat flap in a door but now I have a new house and
the back door is a large double glazed door which would just cost me to much
to have a flap put in. A builder friend has offered to put one in the wall
for me. Has anyone any experience of doing this? I have a couple
questions:

1. As it's a cavity wall do cats have a problem going through a tunnel?

2. The actual door seems to be on the inside leaving the tunnel exposed to
the elements (or have I got that wrong?). What's stopping it filling with
rain and allowing water to seep into the cavity wall?

Knocking holes in walls is quite a drastic thing to do so I don't really
want to do it unless it really is the right thing............though I'm not
sure what else I will do if it isn't!!

Thanks in advance

Angela