What should be used to fill the space between the wall and drywall to keep mice out?

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What should be used to fill the space between the wall and drywall to keep mice out? - briefly
Insert stainless‑steel wool (or copper mesh) into the gap and seal it with a durable silicone or low‑expansion polyurethane foam to block rodent entry.

What should be used to fill the space between the wall and drywall to keep mice out? - in detail
To prevent rodents from entering through the cavity between a structural wall and the interior drywall, the gap must be sealed with a material that resists gnawing, adheres securely, and remains effective despite temperature fluctuations.
A suitable sealant should possess the following properties:

Rodent resistance: Hard enough that mice cannot chew through it.
Adhesion: Bonds to both masonry or wood studs and gypsum board.
Durability: Maintains integrity when exposed to moisture, heat, or cold.
Flexibility: Accommodates slight movement of the wall without cracking.

Materials that meet these criteria include:

Steel wool combined with expanding foam.

Insert a dense steel wool pack into the opening to create a physical barrier.
Apply a low‑expansion polyurethane foam over the wool to lock it in place and fill remaining voids.
The steel wool deters chewing, while the foam prevents air and dust infiltration.

Copper mesh (hardware cloth) with sealant.

Cut a piece of ¼‑inch gauge hardware cloth to cover the gap.
Fasten it with construction adhesive or screws, then seal the edges with silicone or acrylic caulk.
Copper’s hardness and the mesh’s small openings stop mice while allowing the sealant to create an airtight finish.

Rodent‑proof caulk.

Use a silicone‑based or polyurethane caulk specifically marketed for pest control.
Apply a continuous bead along the perimeter of the cavity, pressing it firmly into the joint.
This method works best for gaps under ¼ inch; larger openings require a filler material first.

Rigid foam board with sealant.

Cut a thin piece of extruded polystyrene (XPS) or polyisocyanurate to fit the gap.
Secure it with construction adhesive, then seal all seams with a rodent‑resistant caulk.
The foam’s density discourages gnawing, and the caulk ensures a complete seal.

Application steps common to all methods:

Clean the gap of debris, dust, and loose material.
Measure the opening to determine the size of the barrier needed.
Insert the chosen barrier material, ensuring it fills the entire depth of the cavity.
Apply sealant or foam over the barrier, smoothing it to eliminate air pockets.
Allow the product to cure according to the manufacturer’s instructions before reinstalling trim or baseboards.

By selecting a combination of a hard physical barrier (steel wool, copper mesh, or rigid foam) and a sealing agent (expanding foam, caulk, or adhesive), the space between the wall and drywall becomes effectively rodent‑proof, reducing the likelihood of mouse intrusion.