Blown-in vs batt insulation for an attic

R-value per inch and depth for R-38 and R-49

Batts and loose fill reach the same R-value at different depths. Cellulose packs the most R into each inch, so it is the shallowest. Loose-fill fiberglass is the fluffiest, so it is the deepest. A batt sits between the two and has a fixed thickness from the factory.

The blown fiberglass depths use R-2.5 per inch, the conservative end of the published range. The fiberglass bag chart we checked reaches R-38 at 13 in and R-49 at 16-1/2 in, so a current product may need less depth than the table shows. Under the 2021 IECC, R-49 ceilings apply in zones 2 and 3 and R-60 from zone 4 up; R-38 can stand in for R-49 where the full depth covers the whole ceiling and runs over the wall top plates at the eaves (R402.2.1).

Bags and batts per 1,000 ft² of attic

Loose fill is bought by the bag, and the bag chart sets the coverage at each R-value. Batts are counted one per joist bay, so the count depends on the joist spacing and the batt length, not the R-value.

A single R-49 batt is thicker and heavier than an R-38 batt, so fewer come in a package. The bag count above uses 10 per package; divide the batt count by the number printed on the package you buy. Attic batts also come as long rolls cut to length; count those by the square feet printed on the package. R-60 has no common single batt; it is two layers laid crosswise, which doubles the count.

Settling, depth markers and the bag chart

Settling is the main practical difference between the loose fills. The cellulose bag chart we checked lists 11-9/16 in installed settling to 10-7/16 in at R-38, and 14-3/4 in to 13-5/16 in at R-49: about 10% at both. The R-value is rated at the settled depth, so blow cellulose to the installed depth. The loose-fill fiberglass chart lists the same figure for both and calls settling negligible. A batt has a fixed thickness, but one squeezed under wiring loses R-value, and gaps at its edges let heat bypass it.

Depth alone is not enough with loose fill. Both bag charts list a minimum number of bags per 1,000 ft² as well as a minimum thickness, and the R-value holds only when both are met. Fluffing the fill to reach the depth with fewer bags leaves it short.

Air sealing and where each insulation fits best

Neither batts nor loose fiberglass stop air. Warm air leaks up through top plates, wire and pipe holes, light boxes, chases and the attic hatch, carrying heat and moisture straight through the insulation. Seal those first, from the attic side, before any insulation goes down. Cellulose is not an air seal either.

• Blown-in, either type: open attics, joists at uneven spacing, attics crowded with wires, blocking and truss webs, and topping up existing insulation. It needs a blowing machine and baffles at the eaves to keep the soffit vents open.
• Cellulose: where depth is limited, such as near low eaves, since it reaches R-49 in about 13.6 in settled.
• Batts: a clear attic with regular joist spacing, floors over crawlspaces and garages, knee walls and walls, and small jobs without a blower. They are easy to inspect and do not drift, but must be cut tightly around every obstruction.

Keep any insulation back from recessed lights that are not rated for insulation contact, and from flues and chimneys by the clearances the manufacturers set. Vermiculite, knob-and-tube wiring or signs of moisture are a reason to stop and call a professional before adding anything.