Cellulose vs. Fiberglass in Attics: Which Performs Better in Extreme Heat?

Choosing insulation for your attic is more than just a home improvement decision. In Arizona, it can shape how livable your home feels during the summer and how much money you spend on keeping it comfortable. 

With attic temperatures climbing well over 140°F during peak season, the type of insulation you choose matters a lot. Two of the most common options are cellulose and fiberglass, and each has strengths worth knowing. Let’s explore how they stack up when the desert climate is at its worst.

Why Your Insulation Choice Matters

Attics in Arizona take a beating from the sun. And without proper protection, that heat builds up and seeps into living areas below. The constant pressure forces your air conditioner to run longer, which can quickly show up on your utility bills.

Insulation acts like a shield, slowing down the flow of heat. The right material makes your home easier to cool, reduces strain on your HVAC system, and helps you avoid wasting energy. In other words, it’s one of the simplest ways to get relief during the hottest months.

What Is Cellulose Insulation?

Cellulose insulation is made mostly from recycled paper, such as old newsprint, that has been treated with safe fire-retardant materials. This makes it environmentally friendly and naturally resistant to flames and pests. 

In Arizona, Green Fiber blown-in cellulose is a popular choice because it covers well and doesn’t rely on harsh chemicals. Cellulose insulation is often measured and described by these key factors that matter most in Arizona attics:

• R-value per inch: Typically ranges from 3.2 to 3.8. To meet Arizona’s recommended R-38, installers usually apply 10 to 12 inches.

• Blown-in application: The material is spread evenly across the attic floor, filling in cracks and odd-shaped spaces.

What Is Fiberglass Insulation?

Fiberglass is made from tiny spun glass fibers and comes in two main forms: batts or loose-fill. Knauf is one of the top brands used in Arizona, offering fiberglass with recycled glass content and binders free of formaldehyde.

Fiberglass insulation is also explained best by looking at its main performance details and installation methods:

• R-value per inch: Usually between 2.7 and 3.4. Installers must apply it at the correct depth to reach energy-efficient standards.

• Installation options: Batts are cut to fit between joists. Blown-in fiberglass fills spaces more evenly, especially in older or irregular attics.

Head-to-Head Comparison in Arizona Attics

When it comes to slowing down the heat, cellulose often has the upper hand. Its dense coverage helps seal small gaps, making it harder for air to sneak in. Fiberglass also performs well, especially in loose-fill form, but it tends to allow more air movement than tightly packed cellulose.

Both materials can last for decades if installed correctly. Cellulose may settle slightly, while fiberglass is more likely to move if walked on or disturbed. From a sustainability standpoint, cellulose wins points for being made from recycled paper, though fiberglass also includes recycled glass.

Each material has its own strengths beyond insulation value. Cellulose provides added fire safety and pest resistance, while fiberglass is better at resisting moisture, though it’s less effective at deterring insects and rodents.

Which One Saves the Most Energy?

The truth is, both cellulose and fiberglass can help Arizona homeowners cut cooling costs — but their effectiveness comes down to the quality of installation.

• Cellulose is great at sealing gaps, which may lead to noticeable drops in AC use.

• Fiberglass also performs well, but it relies on even coverage and proper sealing to reach its potential.

• Rebates and incentives are available through APS and SRP for both types of insulation when installed to recommended standards.

The biggest risk with either material is poor workmanship. Gaps, uneven coverage, or compressed areas will reduce performance, no matter how good the material is on paper. That’s why choosing an experienced installer is just as important as picking the proper insulation.

Other Things to Keep in Mind

Insulation type isn’t the only factor that affects how your attic functions. A professional energy checkup can reveal leaks, gaps, or duct issues that should be addressed before new materials are installed. And with FOR Energy, we don’t believe in one-size-fits-all solutions. Every project begins with a full inspection to understand how your attic is currently performing.

If your attic contains old or damaged insulation, removal may be necessary to avoid dust, pests, or moisture problems. Once new insulation is installed, the benefits go beyond lower bills. A well-sealed and insulated attic can make your home less dusty, healthier, and more consistent in temperature year-round.

Time to Beat the Heat

Getting the right insulation for your attic is one of the smartest upgrades you can make in Arizona. With a free evaluation from FOR Energy, you can learn whether cellulose, fiberglass, or a mix of both will work best for your home. Once installed, these updates can bring lasting peace of mind to you and your household.

Ready to lower your energy bills? Schedule your free estimate with FOR Energy today.