For an attic floor or a closed wall, blown fiberglass and blown cellulose are both standard choices. Compare them at the same installed R-value rather than picking a material first. For open studs during a remodel, the usual comparison is fiberglass batts against sprayed or blown fill. Either material only performs as well as the installation and the air sealing done before it.
Compare equivalent jobs
| Job | Fiberglass | Cellulose |
|---|---|---|
| Vented attic floor | Blown loose-fill | Blown loose-fill |
| Closed wall retrofit | Blown into cavities | Blown into cavities |
| Open wall cavities | Batts, or fibers sprayed with an adhesive | Sprayed with an adhesive, or blown behind netting |
| Basement walls left exposed | Batts with a special flame-resistant facing | Check the product listing and local code |
DOE’s insulation fact sheet says blown loose-fill cellulose and fiberglass, usually installed by professionals, can go into wall cavities, attic floors, irregular spaces and around obstructions. In open wall cavities of a new house, either fiber can be sprayed after mixing it with an adhesive or foam so it resists settling. For blown cellulose in walls, netting holds the material until the drywall goes up. DOE’s existing-attic guidance covers both blown cellulose and blown fiberglass at the ceiling plane of a vented attic.
Air seal first
The same DOE fact sheet warns not to confuse insulation with air sealing. Insulation reduces air movement only within the space it fills. It won’t stop air leaking through gaps between building parts. Seal attic bypasses, top plates and penetrations before adding either material. See attic air bypasses .
DOE also says loose fill must be kept out of vents and away from heat-producing equipment such as recessed lights, using baffles or retainers.
Adding to existing attic insulation
New insulation compresses the old layer slightly, which lowers its R-value. DOE suggests adding about one extra inch when the existing layer is fiberglass, or about half an inch when it’s rock wool or cellulose, to reach the target total.
For blown material, use the bag’s coverage chart for the target R-value and minimum settled depth, and install depth markers. Ask the installer to record the product, number of bags, area covered and depth. A deep-looking layer in a photo doesn’t prove the R-value. See blown-in insulation settlement .
Check these first
- Vermiculite: EPA advises assuming vermiculite insulation may contain asbestos and leaving it undisturbed. Get professional advice before any work that could disturb it.
- Moisture and wiring: fix roof leaks, damp materials and unsafe wiring before you cover them.
- Access and ventilation: keep soffit vents clear, and don’t compress insulation under storage boards.