Why Does Filling One Mouse Hole Sometimes Not Solve the Problem?

One Hole Is Not the Hole

A mouse needs about 6mm. Every house has multiple gaps at that scale by construction: pipe penetrations, cable entries, air bricks, thresholds, weep holes, the joint where an extension meets the original wall.

When you find one with droppings by it, you have found a hole they use, not the hole they use. Sealing it removes one route from a network, and the traffic redistributes to the others, often within days.

The Route Is Not the Same as the Entry

This is the other half, and it catches people out. Mice frequently live inside the structure — a cavity wall, a floor void, the space between floors — rather than coming in from outside each night.

So the gap you sealed on the outside of the building may not be relevant at all. The one that matters is where the void opens into your kitchen: under the sink, behind the boiler, at the back of the airing cupboard, at the top of a stud wall.

What Was Filled, and With What

Expanding foam is chewed through, usually within days, and it hides the gap from anyone inspecting later. Steel wool on its own rusts and works loose. Silicone is gnawed. Plasterboard offcuts and cardboard are not a barrier at all.

If it was filled with any of those, the hole is probably open again. A proper seal is metal mesh packed correctly and finished with mortar or a fitted plate, done in a way that survives pipe movement and heat.

And They Will Make a New One

Mice gnaw. A gap that is currently 4mm becomes 6mm if there is a reason to enlarge it, and softwood, plastic, plaster and thin render are all workable.

So a property with a food source inside and no other changes will find a route eventually. Proofing works because it closes the practical openings and removes the easy options, not because a building can be made impenetrable.

What a Complete Job Looks Like

A full survey inside and out, following the evidence rather than guessing: the kitchen pipework and kickboard void, the boiler, the airing cupboard, the understairs cupboard, the loft, then the external perimeter, air bricks, thresholds, service entries and the extension junction.

Then sealing all of it in one visit with materials that hold. Rodent proofing is £180 for up to an hour, covering rats, mice and squirrels, and it is done after treatment so the survey has live evidence to work from. One hole filled with foam is not a cheaper version of that — it is a different thing that does not work.