Cellulose or Fiberglass Insulation, Which is Better for Your Attic?

Fiberglass insulation has long been a popular choice due to its wide availability and low upfront cost. However, its limitations often become clear in attic environments, especially in terms of performance, comfort, and air quality.

Today, most home insulation installers, including those in the Dr. Energy Saver network, recommend alternatives like cellulose or spray foam for attic insulation upgrades. These options offer better real-world results and long-term value for homeowners.

In this article, we’ll explore the drawbacks of fiberglass insulation and explain why cellulose is quickly becoming the go-to choice for improving energy efficiency and home comfort.

 

What is the Biggest Problem with Fiberglass Insulation?

Fiberglass insulation is known for losing its effectiveness over time. Major issues include:

• Reduced R-value in extreme temperatures
• Air leakage through gaps and seams
• Setting, sagging, and wind washing
• Poor moisture and mold resistance

While it’s still widely used in new construction due to its low upfront cost, fiberglass doesn’t offer the same long-term performance as more modern insulation materials. Its inability to provide consistent thermal resistance, combined with poor air sealing and mold vulnerability, makes it an outdated option for homeowners seeking lasting comfort and efficiency.

R-Value and Thermal Performance: Cellulose Has the Edge

R-value measures how well insulation resists heat flow. The higher the R-value, the better the performance.

Since cellulose is blown in, it fills all the gaps, crevices, nooks and crannies in your attic, unlike batts that can leave gaps. Compared to fiberglass, cellulose is a superb air-blocker.

• Blown-in cellulose has an R-value of about 3.5 per inch, making it approximately 23% more effective per inch than fiberglass batts.
• In cold climates, fiberglass can lose up to 50% of its R-value, according to the Oak Ridge National Lab.
• In hot attic conditions, fiberglass loses up to 3x more R-value than cellulose (Brookhaven National Lab)

Dr. Energy Saver typically installs blown-in cellulose insulation to a depth of 17 inches, achieving an R-60 value. This method covers attic joints entirely, eliminating thermal bridges and creating a continuous layer of protection.

Air Sealing: A Major Advantage for Cellulose

Insulation only works well when air can’t move through or around it. Unfortunately, fiberglass batts are responsible for one-third of an average home’s heat loss, leaving gaps between joists, pipes, and wiring. Blown-in cellulose fills every nook and cranny, reducing air leakage and boosting efficiency. Cellulose also helps stop convection currents that rob heat in winter and increase heat in the summer.

Moisture Resistance and Mold Prevention

Moisture can ruin any type of insulation, but cellulose manages moisture better than fiberglass. Fiberglass tends to hold moisture in localized areas, which can lead to mold growth, rot, and structural damage. Once wet, it’s very slow to dry and may need to be replaced entirely.

Cellulose, on the other hand, is hygroscopic. It absorbs and distributes moisture evenly across its surface. This characteristic prevents liquid from pooling and allows the material to dry more quickly, reducing the risk of mold and mildew. Additionally, cellulose is treated with borates to avoid decay, fungal growth, and pest infestations.

Fire Resistance and Sustainability: Surprising but True

Many homeowners are surprised to learn that cellulose insulation is safer in a fire than fiberglass. Although it’s made from recycled newspaper, you might think a paper-based product would be a fire risk, but that’s not the case, and here’s why:

• Cellulose is treated with fire-retardant borates, making it fire-resistant and code-compliant.
• It’s more tightly packed than fiberglass, choking off airflow and limiting fire spread.
• Fiberglass batts often have flammable paper backing, and open cavities allow fire to move more freely.

Cellulose is made of 80% post-consumer recycled newsprint. The Cellulose Insulation Manufacturers Association (CIMA) claims that insulating a 1500-square-foot house with cellulose will recycle as much newspaper as an individual will consume in 40 years. It’s treated with borate minerals that prevent insects and rodents from eating the material. It also will not rot, decay, or mildew; cellulose does not support fungus or mold growth.

People Also Ask: Common Insulation Questions Answered

1. What is the biggest problem with fiberglass insulation?

   The biggest issue with fiberglass insulation is that it doesn’t hold up under real-world conditions. While fiberglass batts may carry a certain R-value on paper, their performance drops quickly once exposed to the extreme temperatures of an attic. Because fiberglass is highly temperature-sensitive, it struggles to provide consistent insulation. In cold climates, air moves easily through and around the batts, leading to a noticeable loss of R-value. In hot attic environments, fiberglass fibers cannot block radiant heat effectively, further reducing performance. Over time, these weaknesses result in lower energy efficiency, higher utility bills, and less consistent comfort throughout the home.

2. Is fiberglass insulation safe?

   While fiberglass is generally considered safe when properly installed and left undisturbed, it does come with notable drawbacks. Fiberglass batts are made of spun glass filaments that are lightweight and brittle. When the insulation is cut, moved, or compressed, tiny shards can break off and become airborne. This can occur during installation, routine maintenance, or even from air movement within the attic. Once in the air, these microscopic fibers may be inhaled or come into contact with skin and eyes, often causing irritation, coughing, or discomfort—particularly for individuals with allergies or respiratory sensitivities.

   In contrast, cellulose insulation is made from recycled materials and treated for fire, mold, and pest resistance. It is dense, stable, and does not release irritating particles into the home, making it a safer and healthier choice for indoor air quality.

3. Should I remove old fiberglass insulation?

   In many cases, yes. Old fiberglass insulation may be compacted, dirty, or contaminated with dust, mold, or pests. It may no longer provide reliable thermal protection. Dr. Energy Saver often removes aging fiberglass insulation during attic upgrades and replaces it with safer, high-performing materials like cellulose or spray foam.

4. What type of insulation is best for an attic?

   Blown-in cellulose is considered the best all-around insulation for attics, particularly when installed to the recommended R-60 depth. In some situations, spray foam may air seal and insulate roof decks. Dr. Energy Saver helps homeowners determine the best solution for their unique attic and climate.

5. Is batt insulation good for attics?

   Batt insulation, such as fiberglass batts, is not ideal for attics. It leaves gaps between framing, settles over time, and doesn’t effectively air seal. This results in energy loss and uneven home temperatures. More modern approaches, like blown-in cellulose or spray foam, offer superior results.

What Dr. Energy Saver Can Do for You

If your attic still contains outdated fiberglass insulation, Dr. Energy Saver can help replace it with safer, more effective materials. We offer:

• Professional removal of old fiberglass insulation
• Expert installation of blown-in cellulose or spray foam
• Comprehensive air sealing and ventilation upgrades
• Customized energy-saving solutions for your attic and entire home

Stop Wasting Energy, Get a Free Attic Insulation Inspection

Are you unsure if your attic insulation costs you energy and comfort? Let us help! Our Dr. Energy Saver contractors offer no-obligation attic inspections to help homeowners identify insulation problems, air leaks, and energy inefficiencies that could drive up utility bills and make their homes less comfortable.

Don’t wait until the next heat wave or cold snap—schedule your free attic evaluation today and take the first step towards a healthier, more comfortable, and more energy-efficient home.

Updated on August 26, 2025