Building Enclosure, Insulation, and Air Sealing
Control layers
Every assembly should manage four things, in rough order of consequence: bulk water (drainage and flashing), air, vapor, and heat. Roofs, walls, and foundations each need a water control layer, a continuous air barrier, a vapor retarder appropriate to the climate, and insulation. An air barrier only performs if it is continuous and aligned with the insulation.
Insulation
Approximate R per inch: fiberglass batts 3.1-3.7, cellulose 3.5-3.8, open-cell foam about 3.7, closed-cell foam about 6.5, XPS 5, EPS 4, polyiso about 6 (declining in cold). Batts must fill the cavity and touch all six sides; compressing a batt or leaving gaps sharply reduces effective R. Radiant barriers need a facing air space and pay off mainly in hot, sunny climates.
Air sealing priorities
Attic floor plane (top plates, wire and plumbing penetrations, chases, recessed fixtures), rim joists and sills, band joists, window and door rough openings, duct boots, and the garage-to-house separation. Never seal combustion air openings or obstruct the combustion appliance zone. Blower door testing quantifies leakage before and after.
Traps
Quoting cavity R as whole-wall R. Ignoring bridging. Installing vapor retarders on both sides of an assembly. Using spray foam instead of fixing a bulk water leak. Insulating an attic without air sealing the attic floor first.
Worked example
A 2x4 wall with R-13 batts at 16 in. on-center is about 25% framing at roughly R-4.2. U = 0.75/13 + 0.25/4.2 = 0.058 + 0.060 = 0.118, so whole-wall R is about 8.5, not 13. Adding 1 in. of continuous R-5 foam in series gives about 8.5 + 5 = 13.5 and also eliminates the bridging.