Blown-in insulation, also known as loose-fill or spray-on insulation, offers significant advantages over batt insulation in many applications. The material is pneumatically conveyed through hoses and deposited into building cavities, conforming to irregular shapes, filling around obstructions, and providing seamless coverage that minimizes gaps and voids. Blown-in insulation is particularly well-suited for attic applications, retrofit wall insulation, and any situation where the cavity geometry is complex or access is limited. This comprehensive technical guide examines the properties, equipment, installation methods, and performance characteristics of blown-in fiberglass and cellulose insulation.
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Types of Blown-In Insulation Materials
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Blown-in fiberglass insulation is manufactured using the same glass fiber production process as fiberglass batts, but the fibers are processed into a loose, fluffy material that can be pneumatically conveyed through hoses. The fibers are typically coated with a binder that activates when the material is installed, providing some cohesion and reducing settling over time. Blown-in fiberglass has an R-value of R-2.2 to R-2.7 per inch at the standard installed density of 0.5-0.8 lb/ft³. The primary advantages of blown fiberglass are its non-combustibility (it is made from glass, which does not burn), its resistance to moisture absorption (glass fibers do not absorb water), and its low cost. The primary disadvantages are the relatively low R-value per inch, requiring greater depth to achieve the target R-value, and the tendency of the material to settle by 5-15% over time.
Blown-in cellulose insulation is manufactured from recycled paper products—typically 80-85% post-consumer recycled newsprint and cardboard—that are processed into small fiber particles and treated with fire-retardant chemicals (primarily boric acid and ammonium sulfate). The fire retardants give cellulose a Class I fire rating (flame spread index of 25 or less) and provide resistance to mold, fungus, and insect infestation. Cellulose has an R-value of R-3.2 to R-3.8 per inch at the standard installed density of 1.5-2.5 lb/ft³ for attic applications and 3.0-4.5 lb/ft³ for dense-pack wall applications. The higher R-value per inch compared to fiberglass means that less depth is required to achieve the target R-value, and the higher density provides better air flow resistance, reducing convective heat loss through the insulation layer.
| Property | Blown Fiberglass | Blown Cellulose |
|---|---|---|
| R-Value per inch (attic) | R-2.2 to R-2.7 | R-3.2 to R-3.8 |
| Installed density (attic) | 0.5-0.8 lb/ft³ | 1.5-2.5 lb/ft³ |
| Installed density (wall dense-pack) | 2.0-3.0 lb/ft³ | 3.5-4.5 lb/ft³ |
| Settling factor | 5-15% | 5-20% (attic), minimal (dense-pack wall) |
| R-Value at R-49 depth | 18-22 inches | 13-15 inches |
| Air flow resistance | Low (permeable) | Moderate-high (dense-pack is air barrier) |
| Moisture absorption | Low (< 1% by weight) | Moderate (up to 30% by weight) |
| Fire resistance | Non-combustible (Class A) | Class I (treated, smolders under heat) |
| Recycled content | 20-40% | 80-85% |
| Cost per sq ft (R-49) | $1.00-1.50 | $1.50-2.00 |
Blowing Equipment and Installation Techniques
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Blown-in insulation requires specialized pneumatic equipment designed specifically for the material being installed. The equipment consists of a hopper that holds the insulation material, a feeding mechanism that meters the material into the airstream, and a blower that provides the air pressure and volume required to convey the material through the hose to the point of application. For attic installations, the hose is typically 3-4 inches in diameter and 50-200 feet in length, with the blower set to deliver the material at the required density. The operator controls the material flow rate and distribution pattern by manipulating the hose nozzle, ensuring uniform coverage across the entire attic area.
Attic insulation installation requires careful attention to depth marking, coverage uniformity, and ventilation pathway protection. Depth markers must be installed at regular intervals throughout the attic to ensure consistent coverage depth. The markers should be visible from the attic access and placed on a grid pattern of approximately 10-foot spacing, with additional markers in areas with obstructions or irregular geometry. The insulation should be blown in a consistent pattern, maintaining a uniform depth across the entire attic floor. Areas near the eaves require special attention because the available depth is reduced by the roof slope, and insulation baffles must be installed at each rafter bay to maintain ventilation airflow from the soffit vents to the ridge.
Dense-pack wall cavity insulation requires a fundamentally different installation technique than attic loose-fill. The material is blown into enclosed wall cavities through access holes drilled in the interior or exterior sheathing. For wood-frame walls, access holes are typically drilled at the top of each cavity and at intermediate points as needed (typically every 4-6 vertical feet for two-story walls). The insulation is blown at high pressure (8-12 psi at the hose nozzle) to achieve the required density, which forces the material to fill all cavity spaces completely, including around wiring, plumbing, and obstructions. The fill tube is inserted to the full depth of the cavity and gradually withdrawn as the cavity fills, with the operator monitoring the sound and resistance of the material flow to determine when the cavity is full.
Retrofit Applications and Existing Construction
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Blown-in insulation is the most practical and cost-effective method for adding insulation to existing buildings without removing interior or exterior finishes. For existing attics, loose-fill insulation can be blown directly over existing insulation (provided the existing insulation is dry, undamaged, and free of vermiculite or asbestos-containing materials). Before adding insulation, the attic should be thoroughly air-sealed as described in the attic insulation guide. The depth of the added insulation should be calculated to achieve the target total R-value, accounting for the R-value of any existing insulation and any settling that may occur in the new material.
For existing walls, dense-pack cellulose or fiberglass can be installed through small access holes (typically 1.5-2.5 inches in diameter) drilled in the interior wall surface (between studs) or the exterior sheathing. The holes are located at the top of each stud cavity, and the fill tube is inserted to the full cavity height before the material is introduced. The high-pressure injection ensures that the cavity is completely filled from top to bottom without voids. After installation, the access holes are plugged with appropriately sized plugs that can be painted or concealed. The thermal improvement from adding dense-pack insulation to uninsulated existing walls is dramatic—adding R-13 to R-15 to an uninsulated wall reduces heat loss through the wall by approximately 75-80%.
The performance verification of blown-in insulation installations is critical for quality assurance. For attic installations, the primary verification method is visual inspection of depth uniformity, supplemented by density checks using a pre-weighed container of known volume. The installed density should be within 10% of the manufacturer’s specified target density. For dense-pack wall installations, infrared thermography is the standard verification method—the thermal image reveals voids, gaps, and incomplete fills by showing temperature differences on the wall surface between insulated and uninsulated areas. Blower door testing provides an overall measure of building airtightness that reflects the quality of the insulation and air sealing installation.
Performance Characteristics and Long-Term Considerations
Settling of blown-in insulation over time is a normal phenomenon that must be accounted for in the initial installation. All loose-fill insulation settles after installation due to the effects of gravity, vibration, and air movement through the material. The settling factor varies by material and installation density: standard fiberglass attic insulation settles 5-15% over the first year, while cellulose settles 10-20% unless it is installed at higher densities. The settled R-value, not the initial installed R-value, is the relevant performance metric for the life of the building. To account for settling, the initial installed depth should be increased by the settling factor—for example, if the target R-49 requires 15 inches of settled cellulose and the expected settling is 15%, the initial installed depth should be 17.25 inches.
Moisture management is a critical consideration for blown-in insulation, particularly cellulose. Cellulose can absorb up to 30% of its weight in water, which reduces the R-value by 50-80% when wet and creates conditions for mold growth. Although the boric acid treatment provides some mold resistance, prolonged wetting will allow mold to develop on the cellulose fibers. Cellulose must be protected from moisture sources during and after installation—the building must be watertight before cellulose is installed, and the insulation must not be used in applications where it will be in contact with soil or exposed to groundwater. In areas with high humidity or moisture exposure risk, blown fiberglass or mineral wool may be preferred over cellulose.
Blown-in insulation offers an excellent combination of thermal performance, cost-effectiveness, and installation versatility for both new construction and retrofit applications. The key to successful installation is proper equipment setup, correct density control, attention to air sealing before installation, and thorough quality verification after installation. By following the material-specific installation guidelines and addressing the unique requirements of each application, building professionals can achieve the full performance potential of blown-in insulation systems and deliver comfortable, energy-efficient buildings.