Batts and blown insulation reach similar R-values per inch. Where they differ is how well each one fills an awkward space, and awkward spaces are where insulation actually lives.
| Material | R per inch | Notes |
|---|---|---|
| Fiberglass batt | 3.1–3.4 | R-13 in 2×4, R-19–21 in 2×6 |
| High-density fiberglass batt | 3.7–4.3 | R-15 in a 2×4 |
| Blown fiberglass | 2.2–2.7 | Loose, low density |
| Blown cellulose | 3.2–3.8 | Denser, better air resistance |
| Mineral wool batt | 3.7–4.3 | Fire and sound performance |
Attics: blown wins on installation quality
An attic floor is full of obstructions — joists at irregular spacing, wiring, bracing, ducts, the ceiling boxes. Batts have to be cut around every one, and every cut is a chance to leave a gap.
Blown insulation flows around them. A study-grade batt install in a clean attic beats blown material; a real attic almost never permits one.
Blown also lets you add depth cheaply, which is exactly what an attic wants.
Walls: batts win, unless the wall is closed
An open stud bay is a rectangle. Batts fit rectangles, and they are far cheaper and cleaner to install than setting up a blowing rig.
In a closed wall — retrofit, existing plaster — dense-pack cellulose blown through drilled holes is the only practical option, and it is a good one. Dense-packed at 3.5 lb/ft³ it resists settling and substantially reduces air movement inside the cavity.
Settling is the objection, and it is mostly historical
Loose-fill cellulose settles about 20% from its blown depth; manufacturers account for this and the coverage chart states settled depth. Follow the bag's coverage table, not the depth you happen to see on install day.
Dense-pack does not settle meaningfully. Blown fiberglass settles very little.
Coverage charts are the specification
Every bag states: how many square feet it covers at a given R-value, the minimum bag count per 1,000 ft², and the settled depth. Bag count is what makes the R-value, not depth on the day.
Installers short on bags produce fluffed material at the right depth and the wrong density, which measures fine with a ruler and underperforms for the life of the house. Count the empty bags.
Mineral wool, briefly
Higher R per inch than fiberglass, does not burn, does not absorb water, and much better at sound. It costs more and it is stiffer to work. Where it earns its place: party walls, mechanical rooms, and rim joists.
Whichever you choose
The insulation is second. Air sealing is first, and the return on sealing a leaky ceiling is several times the return on the last R-10 above it.
Work out quantities either way on the attic calculator — it takes bag counts from the coverage chart rather than from depth.