Twelve-Millimetre Mesh Does Not Exclude Mice: A Passage-Probability Bound for Household Exclusion Work

Hardware cloth of 12 mm (half-inch) aperture is sold and fitted as rodent proofing, including against mice. This study evaluates that claim with a logistic passage model anchored on stated points relating aperture to the probability that an animal passes: a mouse passes 6 mm with probability 0.05, 9 mm with 0.50 and 12 mm with 0.95; a young rat passes 12 mm with 0.05 and 25 mm with 0.95. On those anchors, 12 mm mesh admits a mouse with probability 0.95 — it is, for mice, nearly no barrier at all. Passage probability falls below 1 percent only at 4 mm, and is 0.05 at 6 mm, which supports the originated specification: apertures of at most 6 mm for general rodent proofing, and about 4 to 5 mm where mice alone are the target and a strict bound is wanted. For a median house with 14 m of vulnerable gap, fitting 6 mm mesh costs USD 126 against USD 84 for 12 mm — a difference of USD 42 for the difference between a barrier and a sieve. Expanding foam alone, with a stated gnaw-through probability of 0.65 within a year, is not a barrier material at any thickness. This study is simulated, not measured. SIMULATED, NOT MEASURED.