I have a mid-60's ranch on a slab in eastern MA. I have a large mouse problem in my walls and attic. I am 99.9% that their prefered path in is via openings and gaps near the sill and under the exterior sheathing. I have had extensive termite damage, so I am sure there are access points from termite damage that we have not yet uncovered. (we have worked our way 60% around the house,fixing as we go).
The sill is only a fir 2x4 on the slab, with the 2x4 wall foot plate on top. The slab is only 1 - 6" above ground level depending on what part of the house you look at. Ground slopes away. Most of house is under 2' roof overhang, except for two side walls.
My concern - If I stuff the voids under the exterior sheathing with copper mesh and expanding foam to block access to the sill, am I risking any kind of water - moisture damage - as I would now be blocking any run-off escape or 'breathing room'?