Cellulose insulation is made mostly from recycled paper that's been treated with fire retardants. In attics it's blown in as loose fill over the attic floor, where it fills around joists, wiring, and other obstructions. It's one of the two most common materials for adding insulation to an existing attic, along with blown fiberglass.
The main things to know are:
- R-value per inch is relatively high. Settled loose-fill cellulose is rated at roughly R-3.2 to R-3.8 per inch.
- It settles. The depth you see on installation day isn't the depth you'll keep, so the job has to be blown deeper to start.
- It's heavier than fiberglass.
- It doesn't handle getting soaked well. A roof leak over cellulose needs attention quickly.
For a typical San Antonio attic top-off, cellulose is a solid option when it's blown to the right depth over dry, sound insulation.
What Cellulose Insulation Is
Cellulose is made mostly from recycled newsprint, according to the U.S. Department of Energy. The paper is shredded into fibers and treated with fire-retardant chemicals. GreenFiber, one of the larger cellulose manufacturers, uses borate-stabilized formulas.
Cellulose is installed two main ways:
- Loose fill is blown over an open attic floor. This is the most common attic use.
- Dense pack is blown tightly into closed cavities, such as existing walls, through small holes. The high density keeps it from settling and slows air moving through the cavity.
This article focuses on attic cellulose insulation. Dense-pack walls are a separate job, covered on our wall insulation page.
Cellulose Insulation R-Value
The cellulose R-value per inch is usually quoted as R-3.2 to R-3.8 after settling, based on DOE data from the Building America Solution Center. ENERGY STAR puts most common insulation types, including cellulose, at roughly R-3 to R-3.5 per inch for quick estimates.
A product's own coverage chart gives the real number. For GreenFiber's loose-fill cellulose, reaching R-38 takes at least 11.7 inches when installed, settling to 10.5 inches. That works out to about R-3.6 per settled inch.
For comparison, blown fiberglass needs more depth for the same R-value. That doesn't make cellulose better in every attic. R-38 is R-38 whatever it's made of. It does mean cellulose reaches a target in less height, which helps near the eaves where the roof comes down low.
Settling: Why the Installed Depth Matters
Loose-fill cellulose compacts after it's installed. DOE data puts cellulose settling at 10 to 20 percent over time, compared with 2 to 4 percent for loose-fill fiberglass.
Manufacturers account for this. Under the Federal Trade Commission's R-Value Rule, loose-fill products have to publish coverage charts showing the minimum settled thickness, weight per square foot, and coverage per bag for each R-value. The chart also lists an initial installed thickness, which is higher than the settled number.
When you check a new cellulose job, compare the depth to the initial installed thickness on the bag chart. An installer who stops at the settled number leaves you below the R-value you paid for once it settles.
Blown-In Cellulose in a San Antonio Attic
A few things are specific to local attics:
- Target levels. San Antonio is in Climate Zone 2. ENERGY STAR's retrofit guidance for Zone 2 recommends adding R-49 to an uninsulated attic, or adding R-38 when there are already 3 to 4 inches. These are reference points. What your attic actually needs depends on what's there now and how it's built. Our attic insulation service starts with measuring that.
- Adding over existing insulation. ENERGY STAR notes you don't have to match the existing type, so blown cellulose over older fiberglass is common. The existing layer should be dry, free of mold and pest damage, and not something that needs to come out first.
- Roof leaks. The Building America Solution Center notes that cellulose breaks down when saturated. After a storm or roof leak, wet cellulose should be checked and the leak repaired. Covering damp insulation with more insulation hides the problem.
- Heat. Cellulose doesn't change how hot the attic itself gets. In a vented attic it slows heat moving down through the ceiling into the rooms below.
Weight
Cellulose is heavier than fiberglass. DOE data puts settled loose-fill cellulose at 1.5 to 2.0 pounds per cubic foot, versus 0.5 to 1.0 for loose-fill fiberglass. The Building America Solution Center recommends against cellulose in mobile home ceilings and floors because of its weight. In a standard site-built home, the installer should still note the ceiling's condition, especially if the drywall is sagging or has water damage.
Is Cellulose Safe?
Cellulose is treated with fire retardants during manufacturing, and it's widely used in residential attics and walls when installed according to the manufacturer's instructions.
The main safety issues come during installation:
Blowing cellulose is dusty. Anyone in the attic should wear eye protection, gloves, long sleeves, and a properly fitted particulate respirator or dust mask, as ENERGY STAR recommends for attic insulation work.
San Antonio attics get extremely hot. Plan attic work for early morning.
Walk only on joists or truss chords, never on bare drywall.
Keep loose fill away from recessed lights that aren't rated IC (insulation contact), and away from furnace flues and chimneys. Use metal dams to hold it back.
If the existing insulation looks like pebbly, gray-brown or gold granules, it may be vermiculite. The EPA advises treating vermiculite as possibly containing asbestos and leaving it undisturbed. Don't blow new insulation over it without professional advice.
Common Mistakes With Attic Cellulose
- Measuring to the settled depth instead of the initial installed depth on the bag chart.
- Burying air leaks. Gaps around wiring, plumbing, and the tops of walls should be sealed before cellulose goes on top.
- Blowing insulation into the soffits. Rafter vents (baffles) keep the eaves open so the attic can still breathe.
- Covering wet or damaged insulation instead of fixing the cause first.
- Uneven depth. Low spots near the hatch and along the eaves are where attics most often come up short.
If you're not sure how much insulation you have or what condition it's in, an inspection will tell you whether a cellulose top-off makes sense. Learn more on our blown-in cellulose insulation page, or Get an Estimate.
Frequently Asked Questions
Can you blow cellulose over existing fiberglass?
Yes, as long as the existing fiberglass is dry and in good condition. ENERGY STAR notes that new insulation doesn't have to match the type already in the attic. The new layer is sized to reach the total R-value you want.
What should the receipt show after a cellulose job?
The FTC's R-Value Rule requires the receipt for loose-fill insulation to list the coverage area, initial installed thickness, minimum settled thickness, R-value, and number of bags used. It's the easiest way to confirm you got the depth you paid for.
Can I install blown-in cellulose myself?
It's possible. ENERGY STAR notes that some home improvement stores rent blowing machines, but also says hiring a professional may be wise for loose fill because it takes a blowing machine to do well. Getting an even depth across the whole attic, including the eaves, is the hard part.
Does cellulose work in hot, humid climates like San Antonio?
Yes. It's one of the most common attic insulation materials. The points that matter most locally are keeping it dry (fix roof leaks promptly) and keeping the attic ventilation clear.
The Takeaway
Cellulose is a practical choice for attics:
- R-value: it delivers roughly R-3.2 to R-3.8 per settled inch, so it reaches common attic targets in fewer inches than blown fiberglass.
- Settling: it settles noticeably, so check the depth against the initial installed thickness on the bag chart.
- Weight and moisture: it's heavier than fiberglass and shouldn't stay wet.
- Before installing: seal air leaks and keep the soffit vents clear.
Blown to the right depth over a dry attic, it does the job well.
References
- U.S. Department of Energy, Energy Saver: Types of Insulation. energy.gov
- U.S. Department of Energy, Building America Solution Center (PNNL): Loose-Fill Insulation and Dense-Pack Insulation. basc.pnnl.gov
- ENERGY STAR: Recommended Home Insulation R-Values and Well-Insulated and Sealed Attic. energystar.gov
- ENERGY STAR: A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR. energystar.gov
- Federal Trade Commission: Labeling and Advertising of Home Insulation (R-Value Rule), 16 CFR Part 460. ftc.gov
- GreenFiber: Loose Fill Cellulose Insulation Fact Sheet (INS551LD) and All Borate Stabilized Cellulose Insulation Fact Sheet. greenfiber.com
- U.S. Environmental Protection Agency: Asbestos, vermiculite insulation guidance. epa.gov/asbestos