I hope someone can give me some advice.

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............I'm not sure 
what else I will do though if it isn't!!

Thanks in advance

Angela