In some constructions, it will be necessary to install part of the insulation before the sheathing is applied or before the mechanical or electrical work is roughed-in. These are areas that will be impossible or difficult to insulate later, such as where plumbing fixtures are next to outside walls and at soffits or overhangs.

Where attics are accessible, ceiling insulation may be installed from the attic space after the ceiling finish is in place. Many roofs, however, are of shallow pitch, making it difficult to insulate the outer edges of the ceiling from above. The work, therefore, is best accomplished before the ceiling finish is installed.

Floors over unheated spaces that will not receive a bottom surface may be insulated at any time after the subflooring and mechanical and electrical work are complete.