Wall Insulation Guide
Wall insulation fills the cavities between studs in exterior walls. The insulation type and R-value are determined by the stud size: 2x4 walls have 3.5-inch cavities (R-13 to R-15), and 2x6 walls have 5.5-inch cavities (R-19 to R-21). Never compress insulation to fit a smaller cavity -- compression reduces R-value.
Insulating finished walls from the inside requires removing drywall, which makes it a major renovation project. The alternative is blown-in insulation injected through small holes from the outside -- a professional service that avoids tearing out walls. New construction and gut renovations are the ideal time to insulate walls properly.