What type of insulation makes it most challenging to locate a missed electrical box in a wall?

Foam-in insulation presents unique challenges when trying to locate a missed electrical box in a wall. This type of insulation is sprayed in and expands to fill the space, creating a solid, continuous barrier that adheres closely to the wall structure. As a result, it forms a tight, impenetrable layer around any electrical boxes that may be present.

Unlike other types of insulation, such as loose-fill or fiberglass batts, which allow for some visibility and may settle or compress in a way that could expose the location of the electrical boxes, foam-in insulation integrates fully with the surrounding surfaces. This makes it difficult to visually or physically locate boxes that have not been installed properly within the wall.

Moreover, the density and rigidity of foam-in insulation provide additional obstacles, as it can obscure the positioning of electrical wiring and boxes, making any manual searching or probing more complicated. Consequently, when diagnosing issues or making changes within walls insulated with foam, a more extensive and careful process is often required to identify existing electrical components.