Which Has Better R-Value Per Inch: Blown-in Cellulose or Fiberglass?

Blown-in cellulose delivers a better R-value per inch than blown-in fiberglass. Cellulose settles in at R-3.2 to R-3.8 per inch, while blown-in fiberglass comes in at R-2.2 to R-2.7 per inch according to the Department of Energy’s Building America Solution Center. That means cellulose provides roughly 30 to 40 percent more thermal resistance per inch of thickness. In practical terms, cellulose achieves R-38 in about 10 to 12 inches, whereas fiberglass needs 14 to 17 inches to hit the same target. However, the comparison is not that simple. Fiberglass is lighter, resists moisture damage better, and does not settle as aggressively over time. Cellulose is denser, fills irregular cavities more effectively, and provides better resistance to air movement. Both materials are widely used in residential and commercial insulation projects, and the right choice depends on the specific project conditions, climate zone, and installation method.

TLDR / Key Takeaways

• Blown-in cellulose delivers R-3.2 to R-3.8 per inch, beating blown-in fiberglass at R-2.2 to R-2.7 per inch
• To reach R-38, cellulose needs 10 to 12 inches of depth while fiberglass requires 14 to 17 inches
• Cellulose settles 10 to 20 percent over time; fiberglass settles only 2 to 4 percent
• Cellulose is denser and better at reducing airflow through wall and attic cavities
• Fiberglass is lighter, will not be permanently damaged by water saturation, and is safe for mobile home ceilings
• In dense-pack wall applications, fiberglass can achieve R-4.2 per inch, surpassing cellulose at R-3.2 to R-3.5
• Oak Ridge National Laboratory testing found loose-fill fiberglass lost up to 50 percent of its R-value in extreme cold, while cellulose showed no convection loss
• The EPA identifies greater thermal resistance and recycled content as key sustainability criteria when choosing insulation

Head-to-Head R-Value Comparison

The numbers from the Department of Energy’s Building America Solution Center tell a clear story when it comes to loose-fill applications in attics. Here is how the two materials stack up side by side:

The R-value advantage for cellulose is meaningful. In a standard attic floor application where depth is limited by code or structural constraints, cellulose delivers more thermal resistance in less space. This matters in retrofit situations where the existing framing or roof pitch restricts how much insulation can be added.

Dense-Pack Performance: A Different Picture

The comparison shifts when we look at dense-pack wall installations rather than loose-fill attic applications. Dense-pack methods involve forcing insulation into closed wall cavities at higher pressures and densities, preventing settling and filling gaps around wires, pipes, and obstructions.

According to the Building America Solution Center’s Dense-Pack Insulation resource, dense-packed cellulose delivers R-3.2 to R-3.5 per inch at an installed density of 3.5 to 4.5 lb/ft3. Dense-packed fiberglass, by contrast, achieves R-4.2 per inch at a density of 1.8 to 2.6 lb/ft3.

In dense-pack applications, fiberglass actually pulls ahead on a per-inch basis. The higher density eliminates the air voids that reduce fiberglass performance in loose-fill applications, allowing the material to reach its full thermal potential. However, cellulose at dense-pack density still fills cavities more completely and provides better resistance to air infiltration through the wall assembly.

Cold Climate Performance: The Convection Factor

One of the most frequently cited differences between these two materials comes from research conducted by Oak Ridge National Laboratory (ORNL). The ORNL study tested both materials in a simulated attic assembly under winter conditions, keeping the indoor side at 70 degrees Fahrenheit while dropping the outdoor side to minus 18 degrees.

The results were striking. Loose-fill fiberglass showed thermal resistance reductions of 35 to 50 percent at the largest temperature differences, a phenomenon attributed to natural convection currents within the insulation. Cold, dense air from the attic was penetrating the loose fiberglass, circulating through the material, and carrying heat away from the ceiling below. Cellulose showed no such convection problem due to its higher density and lower air permeability.

It is worth noting that this ORNL study was conducted on early 1990s loose-fill fiberglass products. As Energy Vanguard has documented, fiberglass manufacturers subsequently changed their production methods, using smaller, unbonded chunks that nest together and reduce the air permeability that caused convection loops. Modern fiberglass products may not experience the same R-value drop, though independent testing on current products remains limited.

Real-World Scenarios: Which Material Fits

Every insulation project comes with its own set of constraints. Here are five realistic scenarios showing how the R-value comparison plays out in actual jobs:

Factors That Influence the Decision

The best insulation choice depends on more than just the R-value per inch. Several variables affect real-world performance and long-term value:

• Climate zone: In colder zones (4 through 8), the Department of Energy recommends R-49 to R-60 for uninsulated attics. Cellulose reaches these targets with less depth, which matters when space is limited
• Cavity depth: In standard 2×4 walls with 3.5 inch cavities, cellulose delivers a higher total R-value than fiberglass at loose-fill density
• Moisture exposure: Fiberglass recovers fully after water exposure; cellulose can break down and lose its insulating properties when saturated
• Structural capacity: Cellulose weighs two to four times more than fiberglass at the same R-value level, which matters for ceiling drywall on 24-inch-on-center framing
• Air sealing needs: Cellulose naturally reduces airflow through insulated assemblies better than fiberglass, providing an air-retarding benefit that fiberglass does not match at standard densities
• Installation quality: Both materials require proper density and even coverage. Under-installed fiberglass is particularly problematic because it is easier to “fluff” below the manufacturer’s minimum density
• Settling over time: Loose-fill cellulose settles significantly more than fiberglass, which can reduce effective R-value unless stabilized or dense-packed products are used

Environmental and Sustainability Considerations

The EPA’s Identifying Greener Insulation guidelines highlight several factors for evaluating insulation sustainability: thermal resistance, recycled content, embodied energy, and greenhouse gas emissions. Cellulose has a clear advantage on most of these measures. It is manufactured primarily from recycled newspaper with 75 to 85 percent post-consumer recycled content. Fiberglass typically contains 20 to 40 percent recycled glass, though some products have reached higher levels.

Manufacturing cellulose requires far less energy than fiberglass. The production process involves shredding paper and adding borate fire retardants, with no need for the high-temperature furnaces required to spin glass fibers. BuildingGreen’s analysis estimates that fiberglass has roughly seven to ten times the embodied energy of cellulose on a per-R-value basis. Cellulose also biodegrades at end of life, while fiberglass does not.

Who Blown-in Cellulose Is For and Is NOT For

Blown-in cellulose is the right choice when:

• The project has limited cavity depth and maximum thermal resistance per inch is the priority
• Air infiltration reduction is needed alongside thermal insulation
• The building is in a cold climate zone where convection through loose-fill fiberglass is a concern
• Environmental goals include maximizing recycled content and minimizing embodied energy
• Existing walls need to be insulated through retrofit drilling and blowing

Blown-in cellulose is NOT the right choice when:

• The structure cannot support the added weight, such as mobile home ceilings or lightweight framing on wide spacing
• The installation area has a history of moisture problems or roof leaks
• Budget constraints favor the lowest material cost regardless of depth

Who Blown-in Fiberglass Is For and Is NOT For

Blown-in fiberglass is the right choice when:

• Weight is a primary constraint, such as ceiling assemblies with drywall on 24-inch-on-center framing
• The project involves deep attic spaces where depth is not a limiting factor
• Moisture exposure is likely and the insulation needs to recover after getting wet
• Dense-pack wall installation is planned and the higher dense-pack R-value can be fully utilized

Blown-in fiberglass is NOT the right choice when:

• Shallow cavities demand maximum thermal resistance per inch
• Air sealing is a secondary priority but the budget does not include separate air barrier work
• Installation quality is uncertain, as under-density fiberglass severely underperforms

Get Expert Insulation Guidance for Your Project

Choosing between blown-in cellulose and fiberglass is not just about the R-value per inch on a label. It is about matching the right material to the right application, accounting for cavity depth, climate, moisture conditions, structural capacity, and long-term performance. Our team at Lamothe Insulation and Contracting has the experience to evaluate your specific situation and recommend the material and installation method that delivers the best results for your building.

Call us at (508) 847-0119 or email [email protected] to discuss your project. We serve homeowners and contractors throughout the area with professional blown-in insulation installations backed by real-world expertise.

Request a Free Quote | Schedule an Insulation Assessment

Frequently Asked Questions

Does blown-in cellulose really have a higher R-value per inch than fiberglass?

Yes, in standard loose-fill attic applications. Cellulose delivers R-3.2 to R-3.8 per inch while blown-in fiberglass provides R-2.2 to R-2.7 per inch. In dense-pack wall installations, fiberglass can reach R-4.2 per inch, surpassing cellulose.

How much does cellulose settle, and does that R-value loss matter?

Loose-fill cellulose settles 10 to 20 percent over time, which can reduce effective R-value. Dense-pack and stabilized cellulose products eliminate most settling. Fiberglass settles only 2 to 4 percent, so it retains its installed R-value better in standard attic applications.

Can fiberglass insulation lose R-value in cold weather?

ORNL research showed early loose-fill fiberglass lost up to 50 percent of its R-value at extreme temperature differences due to convection within the insulation. Modern fiberglass products use improved formulations to reduce this effect, though independent testing on current products remains limited.

Which insulation material is better for retrofitting existing wall cavities?

Dense-pack cellulose is generally preferred for wall retrofits because its higher density fills cavities completely around wires and pipes, provides better air infiltration reduction, and does not require precise cavity measurements the way fiberglass batts do.

Which material holds up better if the roof leaks or moisture gets in?

Fiberglass is not permanently damaged by water saturation and dries out with its R-value intact. Cellulose can break down and lose its insulating properties when saturated, and may require replacement if severely water-damaged.

Sources

• Building America Solution Center – Loose-Fill Insulation – DOE-funded resource providing settled R-values, densities, settling rates, and installation specifications for cellulose, fiberglass, and rock wool insulation.
• Building America Solution Center – Dense-Pack Insulation – Technical guidance on dense-pack installation methods including recommended densities and R-values for both cellulose and fiberglass in closed wall cavities.
• U.S. Department of Energy – Insulation – Comprehensive overview of how insulation works, R-value definitions, climate zone recommendations, and factors affecting insulation performance.
• Oak Ridge National Laboratory – Thermal Performance of Fiberglass and Cellulose Attic Insulations – Peer-reviewed research paper documenting convective R-value losses in loose-fill fiberglass under cold conditions, with comparison to cellulose and batt insulation.
• EPA – Identifying Greener Insulation – Federal guidance on sustainability criteria for insulation materials, including thermal resistance, recycled content, and embodied energy considerations.