Cellulose vs Fiberglass Insulation: Performance, Cost & Comfort

Composition and Manufacturing

Cellulose insulation is primarily composed of recycled paper, mostly ground-up newspapers treated with fire retardants like borates. This gives it its characteristic resistance to fire despite being inherently combustible. It's considered an eco-friendly option due to its high recycled content.

Fiberglass insulation is made from melted recycled glass and sand spun into fine fibers. These fibers create a lightweight, non-combustible material with inherent fire resistance, requiring no additional chemicals. The manufacturing process results in products with precise fiber density for consistent thermal performance.

Both materials emphasize sustainability, but fiberglass is notable for certifications like GREENGUARD Gold, which highlight low emissions and indoor air quality benefits.

Common Forms and Applications

Cellulose insulation is almost exclusively installed as blown-in cellulose, ideal for attics and retrofit sidewalls using "drill and fill" methods. It can be applied wet or dry, but due to its weight and dustiness, it requires specialized equipment and care during installation.

Fiberglass insulation comes in several forms: batts, rolls, loose-fill ("blown insulation"), and rigid boards. Batts and rolls are popular for new construction walls and floors because they're easy to handle. Blown-in fiberglass is effective in attics and irregular spaces, offering a cleaner and lighter alternative to cellulose.

We often find fiberglass favored for its ease of installation and less dusty application, while cellulose's superior air sealing in retrofits can be a deciding factor in older homes.

R-Value Comparison

R-value measures thermal resistance; higher values indicate better insulation. Cellulose typically offers an R-value between 3.6 to 3.8 per inch, while fiberglass ranges from 2.2 to 2.7 per inch. This means cellulose can provide up to 30-40% better thermal resistance in the same thickness.

This difference is critical in colder climates or homes with irregular spacing where efficient heat retention is essential. While fiberglass provides adequate R-values for many new builds, cellulose's denser structure contributes to more consistent thermal performance over time.

Impact on Energy Savings

Choosing insulation affects how much we save on heating and cooling bills. Cellulose's higher R-value and density reduce heat loss more effectively, leading to an average energy savings improvement of 18-28% compared to fiberglass with similar nominal R-values.

These savings come from less air infiltration and a more stable thermal barrier, which is especially important in climates with wide temperature swings. Although fiberglass often costs less upfront, the long-term energy cost benefits of cellulose usually offset the initial investment within 3-5 years.

Air Sealing Properties

Air sealing greatly influences insulation performance beyond R-value alone. Cellulose's dense, blown-in application fills gaps and voids, reducing air infiltration by approximately 35-40%. Fiberglass reduces infiltration by only about 5-15%.

Lower air leakage minimizes drafts and temperature fluctuations, improving comfort and lowering HVAC system workload. Proper installation of cellulose is key here; it creates a nearly continuous thermal barrier, whereas fiberglass batts can have gaps, reducing overall effectiveness. For better home energy efficiency, air sealing is a top priority.

Blown-In and Loose-Fill Techniques

Blown-in insulation is commonly used with cellulose and loose-fill fiberglass. Cellulose is typically dense-packed or loosely blown into cavities, filling gaps and irregular spaces effectively. This method creates an airtight layer that reduces drafts and increases thermal resistance.

Loose-fill fiberglass uses a similar blown-in approach but tends to be lighter and less dense. It can be easier to install in open spaces like attics but risks leaving gaps if not applied carefully. Both materials require specialized equipment for blowing and are generally installed by professionals to ensure consistent coverage and avoid settling issues.

Batts and Rolls: Suitability and Use Cases

Fiberglass is widely available in pre-cut batts and rolls, which fit standard stud spacing in walls and ceilings. This format allows for straightforward installation, often suitable for DIY projects. Batts offer predictable thickness and R-values but require careful fitting to avoid compression or gaps.

Cellulose does not come in batt form and is rarely used as rolls. Batts work best in new construction or areas with uniform cavity sizes. Fiberglass's roll and batt options provide versatility and convenience for builders and renovators, but they offer less effective air sealing compared to blown-in cellulose.

Professional vs. DIY Installation

DIY installation is feasible with fiberglass batts due to their manageable size and simpler handling. However, precision is critical. Poorly installed batts with gaps or compression reduce insulation effectiveness significantly.

Blown-in insulation, particularly cellulose, generally demands professional installation because it requires specialized blowing machines and trained operators. Professionals ensure dense, uniform coverage and reduce settling-related performance degradation. Hiring a professional may increase upfront costs but improves long-term insulation reliability and energy savings.

Cost Considerations

Material and labor contribute to overall installation costs, with fiberglass typically less expensive upfront. Loose-fill fiberglass runs about $0.30 to $1.10 per square foot, while cellulose ranges from $0.30 to $1.80 or higher per square foot.

Professional installation costs vary based on project size and complexity. Fiberglass batts offer cost savings for DIYers but may lead to performance issues if not done correctly. Cellulose's higher installation cost reflects its energy efficiency benefits and superior air sealing. Evaluating immediate expenses against expected energy savings helps us choose the best option for each project.

Fire Safety and Retardants

Fiberglass is naturally non-combustible because it is made from glass fibers, which do not burn or support combustion. This inherent property means fiberglass does not require chemical fire retardants to meet fire safety standards. It can withstand high temperatures without melting quickly, maintaining its structure during a fire.

Cellulose, on the other hand, is made from recycled paper treated with borate-based fire retardants. These chemicals provide a Class A fire rating. In real-world tests, cellulose tends to outperform fiberglass by up to 50% in fire resistance. Its dense structure limits oxygen flow and forms a protective char layer under fire conditions, slowing spread and increasing evacuation time.

Indoor Air Quality and Handling

Fiberglass insulation can irritate the skin, eyes, and respiratory system during installation due to tiny glass fibers. Using masks, gloves, and protective clothing is necessary for safe handling. Once installed, fiberglass does not typically impact indoor air quality if undisturbed.

Cellulose insulation poses less irritation risk during installation but contains borates as pest and fire retardants, which are considered safe in properly installed amounts. Because cellulose is dense, it can reduce air infiltration better than fiberglass if installed correctly, potentially improving indoor comfort and energy efficiency.

Pest and Moisture Resistance

Fiberglass itself is resistant to pests and will not support mold growth if it remains dry. However, if fiberglass insulation becomes wet, it can lose its thermal effectiveness until fully dried, and moisture can encourage mold growth on surrounding materials.

Cellulose can absorb and slowly release moisture, which allows it to moderate humidity levels somewhat. However, prolonged moisture exposure can cause settling or microbial growth. The borate treatment in cellulose also provides resistance to pests, including insects and rodents, helping protect the home from infestations. Proper ventilation and moisture control are essential for maintaining cellulose's performance over time.

Which is Best for Attics and Why?

In attics, cellulose insulation often outperforms fiberglass due to its dense-pack installation method. Its higher density reduces air infiltration and limits heat loss better than loose fiberglass batts. Cellulose's ability to conform around irregular attic structures creates a continuous thermal barrier.

Moisture resistance is crucial for attics. Cellulose can absorb some moisture and release it when dry, which helps prevent mold growth. Fiberglass resists moisture but tends to trap it, potentially causing degradation if not properly ventilated.

Fiberglass batts are quicker to install and less expensive upfront, which can be advantageous in large attic spaces where budget and speed are priorities. However, cellulose delivers superior long-term performance in managing temperature swings and air sealing. For more detailed guidance on choosing attic insulation based on your climate, see our guide on best attic insulation for different climates.

Which is Best for Walls and Why?

For wall cavities, cellulose insulation provides several advantages. Its blown-in form fills gaps around wiring and plumbing more effectively than rigid fiberglass batts, limiting drafts and air leaks.

Cellulose's higher R-value per inch makes it a better choice in retrofit and irregular-framed walls, ensuring consistent thermal performance. It also offers better sound dampening, which is beneficial for interior walls.

Fiberglass batts can be effective in new construction walls with standard framing because of their simple installation. They are less labor-intensive but require precise fitment to avoid compression and thermal bridging that reduce efficiency.

Which is Best for Basements and Why?

Basements demand insulation that handles moisture well. Fiberglass insulation often works better here due to its lower moisture absorption. It resists mold and mildew, making it suitable for damp basement walls if paired with vapor barriers and proper drainage.

Cellulose can be used if moisture levels are controlled because it absorbs moisture and releases it gradually; however, this variability means more risk of settling and performance loss in poorly ventilated or wet basements.

Because basements are often cooler and damper, the installation method and moisture management strategy are more important than just R-value. Fiberglass's resistance to mold and ease of replacement make it a common choice.

Which is Best for Crawl Spaces and Why?

Crawl spaces benefit from insulation with good air sealing and moisture resistance. Cellulose insulation dense-pack applications effectively reduce air infiltration and help regulate humidity when installed properly with a vapor barrier.

Cellulose's ability to fill irregular cavities and limit pest access is an advantage in crawl spaces, which are often irregular and exposed to varying moisture levels. However, correct moisture management is critical; poor installation can lead to settling and mold risks.

Fiberglass may be simpler to install in crawl spaces, especially if used with a sealed vapor barrier, but it offers less effective air sealing. It tends to compress or degrade faster in humid environments, which limits long-term performance.

Which is Best for Garages and Why?

Garages are often less climate-controlled and may not require the highest R-value. Fiberglass insulation is typically preferred because of its lower cost and faster installation, suitable for garage walls and ceilings where moderate temperature control is acceptable.

Fiberglass resists moisture and mold better in areas prone to condensation, which is beneficial in garages. Its lighter weight and batts shape fit well between framing members.

Cellulose could be used but may be less cost-effective given typical garage use. Its settling over time can reduce effectiveness, and the denser material might not justify the extra expense where insulation demands are lower.

How long does cellulose insulation last?

Cellulose insulation typically lasts around 20 to 30 years. Its longevity depends on proper installation and moisture control to prevent settling and degradation over time.

How long does fiberglass insulation last?

Fiberglass insulation can last 80 years or more when kept dry and undisturbed. It resists settling better than cellulose, maintaining effectiveness for a longer period under optimal conditions.

What are the pros and cons of cellulose insulation compared to fiberglass?

Cellulose offers better thermal resistance per inch and superior soundproofing. It's eco-friendly due to high recycled content but may settle over time and requires special equipment for installation.

Fiberglass is widely available, affordable, easy to install, and naturally fire-resistant. However, it may need thicker layers for similar thermal performance and can irritate skin and lungs during installation.

How does cellulose insulation R-value compare to fiberglass insulation?

Cellulose typically has a higher R-value per inch, around 3.5 to 3.8, compared to fiberglass's 2.9 to 3.3. This means cellulose provides better heat resistance in the same amount of space.

Which is more cost-effective: blown cellulose or blown fiberglass insulation?

Fiberglass generally has a lower upfront cost. However, the improved thermal efficiency of cellulose can reduce energy bills over time, possibly offsetting its higher initial price.

What are the benefits of using cellulose instead of fiberglass insulation in an attic?

Cellulose's density allows better air sealing, reducing drafts and improving energy efficiency in attics. It also dampens sound more effectively and has a smaller environmental footprint from recycled materials.

How do cellulose, fiberglass, and foam insulation materials differ in performance?

Spray foam offers the highest insulation effectiveness by sealing gaps and providing a continuous barrier. Cellulose and fiberglass trap air but differ in density and soundproofing, with cellulose outperforming fiberglass in both.

In terms of installation in attics, how do cellulose and fiberglass insulations compare?

Fiberglass batts are easier for DIY installation, while blown-in cellulose usually requires professional equipment. Cellulose's installation process is more involved but offers better coverage and fewer gaps when done correctly.