Building Shell, Insulation and Air Sealing
Shell basics
The shell includes walls, roof, floor, windows, and doors. Understand insulation types and where each belongs: fiberglass batts, cellulose, spray foam (open and closed cell), rigid board, and mineral wool. Know R-values per inch roughly: fiberglass about 3.1-3.7, cellulose about 3.5, open-cell foam about 3.5-3.8, closed-cell foam about 6-7, and polyiso about 6-6.5 per inch. Closed-cell foam is an air barrier and vapor retarder; open-cell is an air barrier but vapor-permeable.
Air sealing priorities
Air sealing follows the pressure boundary. Priority locations: attic floor penetrations, top plates, recessed lights, chimney chases, rim joists, and basement penetrations. The attic is usually the biggest opportunity because stack effect pushes conditioned air up. Seal before you insulate so the insulation performs as intended.
Traps
A frequent trap is placing a vapor barrier on both sides of a wall, which can trap moisture. Another is assuming batts alone stop air; they do not. Watch for questions implying that adding R-value fixes air leakage or that recessed lights can be buried without an IC rating and airtight cover.
Worked example
A 1,200 sq ft attic with R-11 existing insulation needs to reach R-49. If you add blown cellulose at R-3.5 per inch, you need (49 − 11) ÷ 3.5 ≈ 11 inches added, but only after air sealing and installing baffles at the eaves to preserve ventilation.
Windows and doors
Understand U-factor (lower is better) and SHGC (solar heat gain). In cooling climates, low SHGC matters; in heating climates, U-factor matters more.