Attic Insulation Materials: Which Type Is Right for Your Home?

R-value tells you how much your attic insulation material resists heat flow, but it can’t tell you whether that material fits neatly around your attic framing or addresses unwanted air movement. When you’re comparing attic insulation materials, the R-value on the label is only part of the bigger picture. 

At Koala Insulation of Greater Knoxville, we’ll inspect your attic before recommending a material. In this guide, we draw on years of professional experience to compare the types of attic insulation materials and outline the factors that influence your decision.

Types of Attic Insulation Materials

The table below provides a side-by-side comparison before we dig into each attic insulation material.

Material	Approx. R-Value Per Inch	Best For	Key Advantage	Main Limitation	Air Sealing
Fiberglass	~ R-2.2 to R-2.7	Open joist bays and accessible attics	Widely available	Poor fitting can hurt performance	No
Cellulose	~ R-3.2 to R-3.8	Adding insulation to existing attics	Fills irregular spaces	Sensitive to moisture	No
Spray Foam	~ R-3.7 to R-6.5, depending on type	Attics needing insulation and air sealing	Creates an air barrier when properly applied	Higher cost	Yes
Mineral Wool	~ R-3.0 to R-3.3	Areas where fire resistance is a priority	Naturally fire-resistant	Typically costs more	No

 

Fiberglass Batts and Rolls

Fiberglass batts come in rolls or pre-cut sections for common 16- and 24-inch framing. At roughly R-2.2 to R-2.7 per inch, they’re well suited to open joist bays when properly fitted. 

Gaps or compression around joists, wiring, and penetrations can reduce performance, so batt insulation installation requires a close fit. A crowded or irregular attic can make that fit harder to achieve.

Blown-In Fiberglass and Cellulose

Blown-in insulation settles around joists and other hard-to-reach areas, making it useful when you’re adding to existing insulation or working with an irregular attic layout.

Fiberglass provides about R-2.2 to R-2.7 per inch, while cellulose offers around R-3.2 to R-3.8.

Cellulose is made from recycled paper and treated for fire resistance, making it one of the recycled materials you may come across when comparing insulation. The trade-off with cellulose is moisture: it absorbs water, so you need to address leaks or other moisture problems before adding it.

Spray Foam (Open-Cell and Closed-Cell)

Spray foam expands as it’s applied, insulating and sealing gaps at the same time. Open-cell foam provides around R-3.7 per inch, while closed-cell can reach around R-6.5 and offers greater resistance to vapor movement.

That dual purpose is one reason spray foam attic insulation may make sense when air leakage is part of the problem. 

Mineral Wool (Rock Wool / Slag Wool)

Mineral wool, including rock wool and slag wool, is designed to provide insulation with strong fire-resistance properties. At roughly R-3.0 to R-3.3 per inch, mineral wool also offers natural fire resistance. You may have a harder time finding it than fiberglass or cellulose, though, and it typically costs more.

How to Choose the Right Attic Insulation Material for Your Knoxville Home

Once you know how the materials compare, consider:

• Existing Insulation Depth and What to Add: If your insulation is still in good condition, you can often add blown-in fiberglass or cellulose on top to raise the total R-value. In Climate Zone 4A, ENERGY STAR recommends R-49 for attics with about 3 to 4 inches already in place and R-60 for uninsulated attics.
   
• Attic Structure: Cathedral vs. Vented vs. Unvented: Vented attics with accessible floor joists work well with batts or blown-in insulation. Cathedral ceilings and unvented attics may need insulation along the roofline instead, which changes the materials and installation methods you can consider.
• Budget and Performance Trade-Offs: Fiberglass batts generally cost less, while blown-in insulation falls in the middle and spray foam costs more. But price doesn’t always indicate performance, and the material you choose won’t perform well if air leaks remain underneath the insulation. ENERGY STAR estimates you can save an average of 15% on heating and cooling costs by air sealing and adding insulation in recommended areas.

What to Check Before Adding New Attic Insulation

Before you start installing new insulation on top of the old, spend a little time looking around the attic. A few clues can tell you whether you need to do some homework first.

Check the Condition of Existing Insulation

Old insulation doesn’t need to come out simply because it’s old. If it’s dry and in good condition, you can often add to it. Wet, moldy, contaminated, or damaged material may require insulation removal services once you correct the underlying problem.

Look for Air Leaks Before Insulating

The attic hatch, recessed lights, plumbing and electrical penetrations, duct openings, and gaps around framing can all let conditioned air escape. Insulation won’t seal those openings, so schedule air sealing service to address them before they’re buried beneath new material.

Check for Moisture Problems

Fresh insulation is a bad place to hide a leak. If you spot damp material or water stains, track down the source before adding anything new. A roof leak, plumbing problem, or condensation issue won’t disappear because you switch insulation materials.

Check Attic Ventilation and Access

If you have a vented attic, keep soffit and roof vents clear. Baffles can keep loose-fill insulation from blocking airflow at the eaves. Layout matters here, too. An open attic floor may work well with batts or blown-in insulation, while limited access can change how you approach the job.

Get an Attic Insulation Estimate

Contact Koala Insulation when you need a second set of eyes in your attic. We’ll inspect your space and find the right attic insulation materials and approach that fits your home improvement goals. 

FAQ — Attic Insulation Materials

What is the best attic insulation material for Knoxville, TN?

Blown-in insulation is an effective option when you’re adding to existing insulation, while spray foam has advantages in certain attic setups. Koala Insulation will inspect your attic and recommend the best approach based on its layout and insulation conditions.

What is the R-value of common attic insulation materials?

Fiberglass is usually around R-2.2 to R-2.7 per inch, cellulose around R-3.2 to R-3.8, and mineral wool around R-3.0 to R-3.3. Spray foam ranges from roughly R-3.7 for open-cell to R-6.5 for closed-cell.

Can you add spray foam insulation to an existing attic?

You can. But where the foam goes and how your attic is built determine whether you can keep the existing insulation. Check the attic first to avoid making the wrong move.

What are the different types of attic insulation in older homes?

You might find fiberglass batts, rock wool, loose-fill insulation, or vermiculite. Sometimes there’s more than one material depending on the work done over the years. If you can’t identify the material, especially vermiculite, leave it alone until a professional attic insulation company has inspected your space.