Susan from Calgary, Alberta has mice that just keep coming into her house. She knows where they get in and would like to block the entrance. She wonders if the foam in a can would stop them.
No. They will eat right through it. But if you mix the foam in a can, or caulking, with steel wool, they won't eat through that -- it is too tough on their little noses. If you want it to be a permanent job, you should use Stainless Steel Wool or Brass wool, both available for kitchen cleaning.
If you have brick siding you will find weeping holes all along the bottom of the wall. These holes are there to let water weep out but also to let air in. In fact these holes are part of what we call a Rain Screen siding. Yet, that little space is wide enough for small field mice to sneek into the wall and then climb up into the attic. You cannot use the foam and steel wool solution shown above as that would block the air from getting in and the water from getting out. If you put in just the steel wool, they will quickly simply pull it all out to open up the doorway they have gotten used to using. So you need a grill that cannot be easily removed.
Rid-Of-Mice is like a stainless steel spring. You squeeze it shut and push it into the weeping hole. It springs open and totally blocks any rodents from getting in -- but with its open grill it lets air and water continue to pass. You can remove it with a pair of needle nose plyers.
So if field mice in your attic is your problem and you have blocked up all the other holes -- try this gadget on a brick wall.
If you have regular ventilated siding, you will probably need to install a screen at the bottom of the wall to let the wall drain again, but to stop the rodents from this tiny space that you did not imagine would let them into the attic.