Blown-in insulation is a loose material that keeps heat in or out of a home’s structure. Also called loose-fill insulation, it improves a home’s R-value, the industry standard for measuring how well a material resists heat flow. Unlike batts or rolls, which line walls in sheets, blown-in material uses fine fibers that a machine forces into place. The filler looks like cotton candy and packs into every gap around wires, pipes, and framing.
For existing construction, loose-fill fiberglass and cellulose insulation is widely considered the best way to add R-value to attic floors and enclosed wall cavities without opening finished surfaces. This article compares blown-in insulation with rolled fiberglass batts so you can match the method to the space, the budget, and the labor you can commit. The two approaches differ in material cost, equipment needs, coverage of irregular spaces, and the skills required to install them well.
How Blown-In Insulation Works
An insulation blower is a machine that forces loose material into wall cavities, attics, and other spaces. It has a hopper that holds the bags of insulation, an engine that powers the feed, a blower that aerates the material, and a hose that carries it to the cavity. The operator feeds material into the hopper at a steady rate, and the machine shreds and fluffs it before pushing it through the hose. Density matters: too little material leaves voids, and too much can bow drywall or block airflow.
Renting a blower typically costs $60 to $100 per day plus the price of the material, and one bag of cellulose covers roughly 40 to 50 square feet at a 10-inch depth. Most rental yards include the hose, a setup guide, and a quick demo. Two people make the job faster: one feeds the hopper while the other works the hose in the attic, keeping the depth even and the coverage consistent.
What R-Value Measures
R-value is the industry standard for measuring the resistance value of a specific insulating material; the higher the number, the better the material insulates. Loose-fill products list an R-value per inch, so total performance depends on installed depth and settling. Cellulose settles 10 to 20 percent over the first year, so installers either overfill or return to top up. A 12-inch layer of cellulose at R-3.7 per inch delivers about R-44, while the same depth of low-density fiberglass lands lower, near R-26.
Where Loose-Fill Performs Best
Blown-in material works best in existing construction because it flows around obstructions that would force a batt installer to cut dozens of pieces. It is the standard retrofit choice for open attic floors and for wall cavities reached through small access holes. A full building insulation comparison for energy efficiency shows how fiberglass, cellulose, spray foam, and rigid foam boards differ in cost, R-value, and moisture behavior, which helps you position loose-fill against the alternatives before you rent a blower.
Materials Used for Blow-In Insulation
Several loose materials work in a blower, and each has a different R-value per inch, density, and cost profile. The choice changes thermal performance and how the material behaves over time, so the comparison below is a useful starting point before you price a job.
| Material | R-Value per Inch | Source | Key Characteristic |
|---|---|---|---|
| Cellulose | R-3.2 to R-3.8 | Recycled newspaper and cardboard | Treated with boric acid; settles over time |
| Loose fiberglass | R-2.2 to R-4.3 | Glass fibers | Lightweight; resists moisture; little settling |
| Mineral wool | R-3.0 to R-3.3 | Rock or slag | Fire resistant; heavier; higher cost |
| Cotton or denim | R-3.0 to R-3.7 | Recycled fabric | Less common; treated for pests and mold |
Cellulose: The Most Common Loose-Fill
Cellulose is the most common material used for blow-in applications. It is made from recycled newspaper and cardboard treated with boric acid, which makes it resistant to insects and mold. The dense packing of cellulose slows air movement through the insulation, which raises its effective performance in cold climates. The trade-offs are weight and settling: cellulose is heavier than fiberglass, and it compacts over time, so installers account for shrinkage when they set the final depth.
Loose Fiberglass and Mineral Wool
Loose fiberglass is lighter than cellulose and settles far less, but at the same depth it allows more air movement, which lowers its effective R-value in winter. Mineral wool adds fire resistance and handles moisture better than either option, at a higher price per bag. Insulation decisions also reach below the floor line: specifying thermal insulation below grade and under slabs follows a different set of moisture and compressive-strength rules than attic work, because buried insulation carries structural load and sits in constant contact with damp soil.
Rolled Fiberglass Batts and Rolls
Fiberglass batts and rolls are the alternative method. Batts are pre-cut panels sized for standard stud and joist spacing, while rolls are continuous sheets that you cut to length. Both install by friction fit between framing members, which makes them the cheapest insulation per square foot and the easiest material for a homeowner to handle. That keeps them popular for new construction and for open-wall remodels where the framing is fully exposed.
Where Batts Work Best
Batts work best in open cavities with standard spacing, where one piece fills the bay without gaps. They perform poorly in irregular spaces full of pipes, wires, and blocking, and their installed performance depends heavily on the person doing the work. A batt that is cut slightly short leaves an air gap at the edge, and a batt compressed to fit a narrow bay loses R-value because the trapped air space is what does the insulating.
Common Batt Installation Mistakes
The most common mistakes are compressing the batt, leaving gaps at the edges and corners, doubling material over itself, and covering recessed light fixtures or soffit vents. Air leaks around and through the insulation often cost more energy than the insulation type itself, so air sealing and insulation should be planned as one project, not two.
Contractors who want to lift attic performance without switching materials follow professional techniques for blowing fiberglass insulation, which address density, coverage depth, and air sealing in a single pass. The same guidance applies to anyone renting a blower for a weekend attic job.
Blow-In vs Rolled: Cost, Coverage, and Performance
The practical choice comes down to four factors: material cost, labor, coverage of irregular spaces, and consistency of the final layer. The table below summarizes how the two methods compare in typical retrofit conditions.
| Factor | Blow-In Loose-Fill | Rolled Batts |
|---|---|---|
| Material cost per square foot | About $0.60 to $1.20 | About $0.40 to $0.80 |
| DIY difficulty | Moderate; blower rental | Easy; no machine needed |
| Irregular and retrofit spaces | Excellent | Poor |
| Coverage consistency | Depends on operator | Depends on fit |
| R-value per inch | Up to R-3.8 (cellulose) | About R-3.0 to R-3.5 |
Cost and Labor Considerations
Batt material is cheaper, but cutting and fitting it into a finished attic costs time and patience, and wasted off-cuts add to the total. Blow-in requires a machine rental for the day, yet it covers a large attic in hours and reaches wall cavities that batts cannot touch without removing drywall. For retrofit walls, loose-fill is often the only practical option short of a full tear-out.
Air Sealing Comes First
Either method performs poorly over an unsealed ceiling. Seal penetrations, top plates, and chases before adding insulation, and confirm attic ventilation so moist interior air can escape. Placement matters as much as quantity: proper insulation placement in roofs and walls explains how too much insulation in the wrong spot can trap moisture and create the very problems the insulation was meant to prevent.
- Choose blow-in when the attic has many obstructions, wires, or irregular joist bays.
- Choose blow-in when you need to insulate enclosed wall cavities from the outside.
- Choose blow-in when you can rent a blower and want the job done in hours.
- Choose blow-in when you want the densest air-sealing performance per inch.
Planning an Insulation Upgrade
Start with an inspection. Measure the existing insulation depth, look for settling and gaps, and decide whether the goal is comfort, lower energy bills, or both. Most building codes publish target R-values by climate zone, and your local utility may offer an energy audit that identifies the biggest losses first.
- Inspect the attic floor and wall cavities for existing depth and condition.
- Air-seal penetrations, chases, and top plates before adding material.
- Choose the method by space: loose-fill for retrofit cavities, batts for open framing.
- Verify attic ventilation and keep insulation clear of vents and fixtures.
- Document installed depth and R-value for future reference.
Hiring a Professional or Doing It Yourself
Renting a blower and buying bags of cellulose costs less than a contractor for a straightforward attic, but a professional handles access, coverage, cleanup, and any air-sealing work in the same visit. A complete upgrade also looks below the floor line: perimeter versus full under-slab insulation strategies change foundation energy loss and floor comfort, and they are planned at the same time as the rest of the envelope.
Where extreme R-values, moisture resistance, or a continuous thermal layer is required, rigid foam board systems for sheathing and foundations work alongside loose-fill to close the gaps that batts and blown material cannot reach. Matching the material to each part of the building envelope is the difference between insulation that looks good on paper and a home that stays comfortable through every season.