Late summer is the cheapest moment of the year to seal a building. Do it before the autumn push and you are excluding an empty structure. Do it after and you have to remove first.
Here is what to walk around and check.
The roof line
Soffit returns, where the roof edge meets the wall. This is the single most common squirrel and rat entry on residential buildings.
Gable vents and ridge vents: screened from behind with hardware cloth, not just louvred.
Fascia boards and drip edge: look for gaps, rot and lifted sections.
Roof-to-wall intersections and any point where two roof planes meet at different heights.
The chimney: capped, with the cap secured and the screen intact.
Ground level and the foundation
Crawl space vents: screened and intact, not broken or missing.
Foundation cracks and the gap where siding meets the foundation.
Utility penetrations: water, gas, electrical, cable, AC line sets. The gap around a line set is a classic mouse entry.
Dryer and bathroom vent terminations, which should have working dampers and guards.
Garage door seals, especially the bottom corners, which wear out first.
Door sweeps on any exterior door, including the one into the garage.
The yard
Trim limbs back from the roof. Six to eight feet is a reasonable target; squirrels jump further than people expect.
Move woodpiles and stored materials away from the building. Anything stacked against a wall is cover and a ladder at the same time.
Clear dense ground cover from the foundation line and cut the perimeter grass short.
Secure bins, remove fallen fruit, and stop feeding birds directly against the house if rodents are already an issue.
Inside
Store pantry goods in sealed containers. Check under and behind appliances, which is where entry gaps are usually hidden.
Look for grease marks along baseboards and pipe runs, which mark established routes.
Check the attic before insulation is disturbed, and the crawl space before it gets cold enough that nobody wants to.
What to seal with
Hardware cloth, metal flashing, sheet metal and appropriate sealant. Expanding foam alone is not a rodent barrier; it is chewed through easily and should only be used behind metal.
Steel wool packed into a gap is a stopgap, not a repair. It rusts and it works loose.