When narrowing a recessed hole in your bathroom wall to accommodate a new cabinet, you should use lumber that is suitable for wet or damp environments. Bathrooms are prone to moisture and humidity, so it's important to choose a type of lumber that can withstand these conditions. Here are a few options:
Pressure-treated lumber: Pressure-treated lumber is treated with chemicals that make it resistant to rot, decay, and insect damage. It is commonly used for outdoor applications but can also be suitable for bathroom projects where moisture resistance is crucial. However, keep in mind that pressure-treated lumber can have a greenish tint, so it may not be the most aesthetically pleasing choice.
Moisture-resistant plywood: Plywood that is specifically designed for moisture resistance, such as marine-grade plywood or exterior-grade plywood, can be a good choice. These types of plywood are made with waterproof adhesives and are better able to withstand moisture and humidity compared to standard plywood.
Cement board: Cement board is a highly durable and moisture-resistant material often used in wet areas like bathrooms. It is made of cement and fiberglass mesh, providing excellent resistance to moisture, mold, and mildew. Cement board is typically used as a backer material for tile installations, but it can also be used as a structural material for narrowing a recessed hole.
When selecting lumber, consider the specific dimensions and requirements of your cabinet, as well as any local building codes or regulations that may apply. It's also important to properly seal and finish the lumber to provide additional protection against moisture. Consult with a professional or local hardware store for specific recommendations based on your project's requirements and local conditions.