Attic insulation plays a critical role in regulating temperature, reducing energy consumption, and protecting structural components from moisture-related damage. While many homeowners understand that insulation matters, fewer understand that not all insulation materials perform the same under real-world conditions.

Choosing the correct insulation type requires evaluating thermal resistance (R-value), air sealing capability, moisture behavior, installation method, and long-term durability.

This article provides a technical comparison of the three most common attic insulation materials: fiberglass, cellulose, and spray foam.

Understanding R-Value and Thermal Performance

R-value measures a material’s resistance to heat flow. Higher R-values indicate better insulating performance.

However, R-value alone does not determine overall system efficiency. Air leakage, compression, and installation quality significantly influence real-world results.

Typical attic R-value recommendations vary by climate zone but often range between R-30 and R-60.

Fiberglass Insulation

Overview

Fiberglass insulation is composed of fine glass fibers and is typically installed as batts or blown-in loose fill.

Thermal Performance

• R-value per inch: approximately R-2.2 to R-3.8

• Performs best when installed without compression

• Sensitive to air movement

Fiberglass does not inherently stop airflow. If attic air sealing is not completed first, thermal performance decreases significantly.

Advantages

• Cost-effective

• Widely available

• Non-combustible

• Easy to install in open attic spaces

Limitations

• Reduced performance when compressed

• Can trap moisture if ventilation is poor

• Requires separate air sealing

Fiberglass is often a suitable choice when budgets are limited and air sealing is handled independently.

Cellulose Insulation

Overview

Cellulose insulation is typically made from recycled paper products treated with fire retardants and installed as dense-pack or blown-in material.

Thermal Performance

• R-value per inch: approximately R-3.2 to R-3.8

• Performs better than fiberglass in reducing air movement when densely packed

Cellulose offers improved resistance to air infiltration compared to fiberglass due to its density.

Advantages

• Higher density improves air resistance

• Better sound dampening

• More environmentally sustainable

• Fills irregular cavities effectively

Limitations

• Can settle over time if not installed properly

• More susceptible to moisture absorption

• Requires proper ventilation management

Cellulose often performs well in retrofits where irregular spaces must be filled thoroughly.

Spray Foam Insulation

Overview

Spray foam insulation is a polyurethane-based material that expands upon application. It is available in two primary types:

• Open-cell

• Closed-cell

Thermal Performance

• Open-cell: R-3.5 to R-3.7 per inch

• Closed-cell: R-6 to R-7 per inch

Closed-cell spray foam offers the highest R-value per inch among common attic insulation materials.

More importantly, spray foam provides both insulation and air sealing in one system.

Advantages

• Superior air barrier

• High R-value per inch

• Reduces moisture infiltration

• Strengthens structural framing (closed-cell)

Limitations

• Higher installation cost

• Professional installation required

• More difficult to remove once applied

Spray foam is often selected for high-performance homes where air sealing and moisture control are top priorities.

Air Sealing Performance Comparison

Air movement significantly impacts insulation effectiveness. In many cases, air leakage accounts for more energy loss than conductive heat transfer.

Moisture and Vapor Behavior

Moisture management is critical in attic assemblies.

Fiberglass allows moisture to pass through but does not prevent air-driven moisture transport.
Cellulose can absorb and release moisture but may degrade if chronically wet.
Closed-cell spray foam resists water intrusion and acts as a vapor retarder.

Improper moisture management can lead to:

• Mold growth

• Wood rot

• Insulation degradation

• Reduced R-value

Insulation selection should align with regional humidity conditions.

Cost Considerations

While pricing varies by region, relative cost comparisons typically follow this pattern:

• Fiberglass: lowest upfront cost

• Cellulose: moderate cost

• Spray foam: highest upfront cost

However, lifecycle cost must also be considered. Higher initial investment may result in lower energy expenses over time.

Homeowners should evaluate:

• Existing insulation condition

• Air leakage severity

• Climate zone

• Long-term occupancy plans

Installation Variables That Affect Performance

Regardless of material type, improper installation reduces effectiveness.

Common issues include:

• Gaps in coverage

• Compression of batts

• Inadequate depth

• Lack of air sealing before installation

• Blocking soffit ventilation

Even high-performance materials underperform when installation standards are not followed.

When to Choose Each Type

Fiberglass May Be Appropriate If:

• Budget is limited

• Attic air sealing is completed separately

• Access is easy

• Climate is moderate

Cellulose May Be Appropriate If:

• Filling irregular cavities

• Improving sound reduction

• Retrofitting older homes

• Seeking improved air resistance over fiberglass

Spray Foam May Be Appropriate If:

• Maximizing air sealing

• Converting attic to conditioned space

• Reducing humidity infiltration

• Seeking highest R-value per inch

Final Evaluation

There is no universally “best” attic insulation material. The optimal choice depends on performance goals, climate conditions, installation quality, and budget.

In many cases, performance differences are less about material selection and more about air sealing, ventilation strategy, and proper installation practices.

Homeowners seeking maximum energy efficiency should evaluate insulation as part of a comprehensive building envelope strategy rather than a standalone upgrade.