Blown in Insulation R Value: What Homeowners Need to Know

Understanding blown in insulation R value is the key to knowing whether your attic will actually keep your house comfortable and your energy bills down. R-value measures resistance to heat flow, and the higher the number, the better the insulation slows heat moving into your home in summer and out in winter. But R-value depends heavily on material, installed depth, and how much the insulation settles over time. This guide breaks down the per-inch ratings of common blown-in materials, how deep to install them, and the Department of Energy targets for your climate zone.

What R-Value Actually Measures

R-value is a rating of thermal resistance. A higher R-value means the material transfers heat more slowly. Insulation R-values are additive, so if you add R-19 of new material over an existing R-19, you get roughly R-38 total. For attics, R-value is the single most important number because the attic is where most homes lose and gain the most heat. The goal is to reach the total R-value recommended for your climate, then keep it there by accounting for settling and even coverage.

R-Value Per Inch by Material

Blown-in (also called loose-fill) insulation comes in three main materials, each with a different R-value per inch:

• Blown fiberglass: Roughly R-2.2 to R-2.7 per inch. Lightweight, does not absorb much moisture, and resists settling relatively well.
• Blown cellulose: Roughly R-3.2 to R-3.8 per inch. Made from recycled paper treated with fire retardant, it is denser, fills gaps well, and offers slightly better air-blocking, but it settles more.
• Blown mineral wool (rock wool): Roughly R-3.0 to R-3.3 per inch. Fire-resistant and moisture-tolerant, less common as loose fill but excellent where fire performance matters.

Because cellulose delivers more R-value per inch, it reaches a target R-value in less depth than fiberglass, though fiberglass is lighter on ceilings and resists moisture better.

How Deep for a Given R-Value

Depth is what determines your total R-value, so installers mark it and often staple depth rulers to the trusses. Approximate depths to reach common attic targets:

• R-38: About 14 to 17 inches of fiberglass, or about 10 to 12 inches of cellulose.
• R-49: About 18 to 22 inches of fiberglass, or about 13 to 16 inches of cellulose.
• R-60: About 22 to 27 inches of fiberglass, or about 16 to 19 inches of cellulose.

Every insulation bag lists a coverage chart on the label giving the exact depth, bags per 1,000 square feet, and settled R-value for that specific product. Always follow the bag chart rather than a generic rule of thumb, and blow slightly extra to account for settling.

DOE Recommended R-Values by Climate Zone

The US Department of Energy recommends attic R-values based on climate zone. General attic targets for most homes:

• Warm southern zones (1-2), such as Florida and south Texas: R-30 to R-49.
• Mixed zones (3-4), such as the Carolinas and the mid-Atlantic: R-38 to R-60.
• Cold zones (5-7), such as the Midwest, Northeast, and Mountain West: R-49 to R-60.

Most newer energy codes push attics toward R-49 to R-60 nationwide, and adding insulation up to R-60 rarely hurts. If your attic currently has only a few inches (common in older homes at R-11 to R-19), topping up to the recommended level is one of the highest-return energy upgrades available.

Settling: Why Installed Depth Matters

Loose-fill insulation compresses over time, and that settling reduces depth and therefore R-value. Cellulose can settle 15 to 20 percent, so installers blow it deeper than the final target, and manufacturers rate the settled R-value on the label. Blown fiberglass settles much less. When you buy by the bag, the coverage chart accounts for settled performance, but if an installer under-fills, your attic can end up well below the R-value you paid for. This is why measuring depth after installation, and comparing it to the number of bags used, matters.

Blown Fiberglass vs. Cellulose

Both are good, and the choice depends on priorities:

• Cellulose pros: Higher R per inch, better at filling irregular cavities, made from recycled content, and slightly better at slowing air movement.
• Cellulose cons: Heavier, settles more, and can absorb and hold moisture if the roof leaks.
• Fiberglass pros: Lighter, moisture-resistant, settles little, and often less dusty in the living space over time.
• Fiberglass cons: Lower R per inch (needs more depth) and can lose some performance in very cold attics due to convection unless installed deep enough.

Coverage Math and Cost

To estimate a job, multiply your attic square footage by the target, then read the bag’s coverage chart for bags needed. For example, a 1,000-square-foot attic reaching R-49 might need roughly 25 to 35 bags of cellulose or a comparable coverage of fiberglass, depending on the product. Typical installed cost for blown-in attic insulation runs about $1.50 to $3.50 per square foot, or $1,500 to $3,500 for a mid-size attic. DIY material cost is far lower, often $0.50 to $1.50 per square foot, and many home centers lend or rent a blower with a bag purchase.

DIY vs. Hiring a Pro

Blowing attic insulation is a feasible DIY project for a fit homeowner willing to work in a hot, cramped attic. Rent the blower, wear a respirator and long sleeves, and have a helper feed bags while you direct the hose. That said, professionals reach consistent depth quickly, handle tricky areas, and can spot air-sealing needs. Whichever route you choose, air-seal the attic (top plates, penetrations, and the attic hatch) before adding insulation, because insulation slows heat but does not stop air leaks.

Common Mistakes to Avoid

• Skipping air sealing: Insulation over leaky gaps still lets conditioned air escape. Seal first.
• Blocking soffit vents: Install baffles so blown insulation does not smother attic ventilation.
• Burying recessed lights improperly: Only cover IC-rated fixtures; others need clearance to prevent fire risk.
• Under-filling for R-value: Measure final depth against the bag chart to confirm you hit the target.
• Ignoring the attic hatch: An uninsulated hatch is a major heat leak; add a rigid insulated cover.
• Adding on top of wet or moldy insulation: Fix roof leaks first, then insulate.

Maximizing Your R-Value Over Time

Check attic insulation depth every few years, especially in older cellulose that has settled. Top up if it has dropped below your target. Keep the attic dry and ventilated so insulation stays at its rated performance, since wet insulation loses R-value dramatically. Combine adequate blown-in R-value with good air sealing and ventilation, and your attic will hold comfortable temperatures with far less strain on your heating and cooling system.

Blown-In vs. Batt Insulation R-Value

Homeowners often weigh blown-in loose fill against fiberglass or mineral wool batts. For attics, blown-in usually wins on total installed R-value because it flows into every gap, around wiring, and into irregular spaces, leaving fewer voids and gaps that let heat escape. Batts, by contrast, must be cut to fit and often leave gaps around obstructions, which reduces their real-world performance below their labeled R-value. Batts do have advantages in walls and floors where you need a defined layer, and they are easy to install between accessible framing. For most attic top-ups, though, blowing loose fill over the existing insulation gives more uniform coverage and a higher effective R-value for the money, which is why it is the standard for attic retrofits.

Payback and Energy Savings

Adding attic insulation to reach the recommended R-value is one of the highest-return home energy projects available. Homes that start with only R-11 to R-19 (a few inches) and top up to R-49 or R-60 often see meaningful heating and cooling savings, and the project frequently pays for itself within a handful of years through lower utility bills. The exact savings depend on your climate, energy prices, and how leaky the house was to begin with. To maximize the return, combine the insulation upgrade with air sealing, which addresses the drafts that insulation alone cannot stop. Many utilities offer rebates for attic insulation, and federal energy-efficiency tax credits may apply, further shortening the payback period. Check for local incentives before you buy, since they can offset a significant share of the cost.

The bottom line on blown in insulation R value: choose your material, follow the bag’s coverage chart to reach your climate zone’s DOE target (commonly R-49 to R-60), account for settling by installing extra depth, and air-seal before you blow. Get those steps right and you will lock in year-round comfort and meaningful energy savings for decades.