Insulation is one of the cheapest upgrades for energy savings, and one of the easiest to under-order.
R-value targets
R-value measures resistance to heat flow: the higher, the better. DOE guidance for attics is roughly R-30 to R-49 in warm zones and R-49 to R-60 in cold ones. Walls are usually R-13 to R-21 depending on stud depth. Your local energy code sets the legal minimum.
Batts vs blown-in
Batts are easy to fit between regular framing and to count: area ÷ coverage per bag. Blown cellulose or fiberglass suits attics with obstructions and irregular joists. Count blown insulation from the settled depth needed for your target R-value and the bag coverage chart.
Settled depth
Blown insulation settles. Cellulose gives roughly R-3.6 per inch and fiberglass about R-2.5. For R-49, that is around 13½" of cellulose or nearly 20" of fiberglass. Install to the installed depth on the maker's chart, not the settled depth.
Things that go wrong
- Blocking eaves ventilation. Fit baffles first.
- Squashing insulation under boards. Compressed insulation loses much of its value.
- Covering recessed lights not rated for contact with insulation.
- Forgetting the attic hatch: insulate and draught-seal it too.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.