Q: An exterior wall in a client’s finished bathroom has a mouse infestation. The wall contains plumbing (for a pedestal sink) as well as electrical wires. I need to gut the wall carefully from the exterior without disturbing the interior tiled finish. How can I insulate the wall properly and keep the critters out?

A: Steven Baczek, a residential architect from Reading, Mass., who specializes in designing durable, low-energy homes, responds: Aside from being careful that you don’t dislodge or damage any tile, mirrors, or other delicate interior surfaces, you need to keep the mice from returning, insulate the wall, and provide a good level of airtightness to prevent moisture problems from occurring in a vulnerable wall in what is a notoriously damp room in most houses.

After you’ve removed the exterior finish and sheathing, it should be obvious where the mice have been living. Mice can be carriers of many diseases, so protect yourself with a good respirator and gloves when you handle the contaminated insulation. My guess is that the mice have been getting into the walls through the spaces around the plumbing pipes and electrical wires, and with any luck, those bays are the only ones that have been contaminated. After removing all of the insulation from the wall, inspect the studs and the back of the wallboard. If there is a lot of staining, clean affected areas with a cleaner specifically designed to neutralize rodent urine, such as Nature’s Miracle.

Keeping the mice from returning is a challenge given how little room they need to gain access. I worked on an old home a few years ago with a mouse problem. We lined the joist bays with ¼-inch hardware cloth that we custom bent for each bay. For that project, we sprayed foam over the hardware cloth. The mice have not returned after more than six years. In your case, I would first mold hardware cloth, turning up the four sides at least 3 inches, to fit the bottom of each stud bay, like a small cage. Where the plumbing comes up through the bottom of the bay, carefully cut out the hardware cloth to the exact size of the pipe. For the larger exit plumbing pipe, I’d fashion an L-shaped collar out of the hardware cloth to fit around the pipe as added protection.

This strategy assumes that the mice are coming in at the bottom of the stud bay. Check the tops of the bays as well. Mice are resourceful and can gain access from the top, especially if electrical wires are fed from above. If there is any doubt at all, I’d do the hardware-cloth treatment at the tops as well. And speaking of the electrical cables, inspect all of them thoroughly—mice are famous for gnawing on the insulation around wires. Have your electrician replace any damaged wires that you find.

To insulate the wall, custom-rip unfaced rigid foam-board insulation for each stud bay and pack the entire depth of the bay solid. Either EPS or XPS products would work fine. Cut the insulation to fit tightly around the pipes and the wires in the bays.

Before installing the first layer of foam board, run a healthy bead of an acoustical sealant, such as Tremco’s Acoustical/Curtain Wall Sealant, around the perimeter of the stud bay. After setting the first layer of insulation into the sealant, bed each additional insulation layer in sealant along the studs and plates. The acoustical sealant provides a continuous air barrier between the rigid foam and the wood frame, and it stays flexible over time, so the air barrier will not be affected by any future movement in the framing. Fill the spaces around pipes with pest-resistant expanding foam, such as Great Stuff Pestblock.

Finally, as you sheathe and finish the wall, be sure to blend the new work into the existing assemblies. The drainage plane and areas of water management need to be integrated with the existing work and sealed properly to ensure the weather-tight integrity of the new wall.