Blown-In Attic Insulation: Cellulose, Fiberglass, and Why Air Sealing Comes First

Blown-in insulation, also sold as loose-fill, is fiber that arrives compressed in bags and gets fluffed through a blowing machine and a long hose onto the attic floor, where it settles into a continuous blanket over the ceiling joists, the wiring, and every odd-shaped bay a batt would leave gaps in. Three fibers are sold this way. They differ in what they are made of, how much R-value each inch carries, and how much they settle after the installer leaves.

Cellulose Insulation: Recycled Paper With a Fire Retardant

Cellulose is shredded paper fiber. The Cellulose Insulation Manufacturers Association describes it as up to 85 percent recycled paper and cardboard, and all of it is treated with fire retardants; boric acid is the common one, used both as a flame retardant and to repel insects, and some products, such as GreenFiber's INS765LD, are sold as all-borate formulas. The Department of Energy's Building America program lists loose-fill cellulose at a settled density of 1.5 to 2.0 pounds per cubic foot and a settled R-value of about R-3.2 to R-3.8 per inch. That density is the source of most of its strengths: it is heavier and tighter than blown fiberglass, so more of it is needed by weight, and it does more to slow air moving through the layer.

Blown Fiberglass Insulation

Loose-fill fiberglass is glass fiber made for blowing rather than rolling into batts. The same DOE source lists it at 0.5 to 1.0 pounds per cubic foot and about R-2.2 to R-2.7 per inch, so an attic needs more inches of fiberglass than of cellulose to reach the same R-value. It does not absorb moisture under the ASTM test cited on Owens Corning's AttiCat data sheet, and it settles very little, which is why fiberglass coverage charts list a minimum thickness that equals the settled thickness.

Loose-Fill Mineral Wool

Rock or slag wool blown as loose-fill runs denser than fiberglass, about 1.7 pounds per cubic foot settled and R-3.0 to R-3.3 per inch by the DOE figures, and it settles about as little as fiberglass. If a bid offers it, hold it to the same paperwork as the others: a product name and a coverage chart.

Installed Depth, Settled Depth, and the Coverage Chart

Loose-fill is sold by the bag, and its R-value depends on how much material lands per square foot, not on how tall the fluff looks on install day. That is why the Federal Trade Commission's R-value rule, 16 CFR 460.12, makes every loose-fill bag carry a coverage chart giving, for each R-value, the minimum settled thickness, the initial installed thickness, the maximum net coverage area, the number of bags per 1,000 square feet, and the minimum weight per square foot. The bag count is the honest check. A blowing machine can fluff a thin, light layer to the right height; it cannot fake the number of empty bags.

Cellulose is blown deeper than its target because it settles. DOE puts loose-fill cellulose settling at 10 to 20 percent over time; CIMA's installation bulletin puts applied thickness 5 to 12 percent above settled thickness, and the gap is built into every chart. On GreenFiber's INS765LD chart, R-49 calls for at least 56.6 bags per 1,000 square feet, blown to 14.78 inches so it settles to 13.30. Blown fiberglass settles only 2 to 4 percent, so Owens Corning's AttiCat chart lists R-49 at 25 bags per 1,000 square feet and 17 inches, with the same 17 inches as the settled figure. Neither number means much on its own. Together, bags and depth tell you whether the attic actually got the R-value on the invoice.

ENERGY STAR's attic guidance says it in one sentence: EPA recommends air sealing the attic before adding attic insulation. DOE's insulation guide for homeowners is blunter: most insulation does not stop airflow, money spent on insulation can be wasted if the holes and gaps are not sealed first, and adding insulation without air sealing risks serious moisture problems. Loose-fill is the clearest case. Blow 14 inches of it over a leaky ceiling and the warm air keeps rising through it, carrying heat and household moisture into the attic just as it did before, only now through a layer that hides every leak from the next person who looks.

Attic Air Leaks Loose-Fill Can't Stop

ENERGY STAR's do-it-yourself guide tells homeowners to plug the big holes first: where inner and outer walls meet the attic floor, dropped soffits, and the spaces behind and under kneewalls. Then the smaller ones: plumbing vent pipes, wiring holes drilled through top plates, furnace flue and duct chases, recessed lights, and the attic hatch itself, which the guide says to treat like a door to the outside. Caulk handles gaps up to about a quarter inch and expanding foam handles larger ones, up to about three inches. The one exception is the gap around a chimney or metal flue, which ENERGY STAR says to close with aluminum flashing and high-temperature caulk, not spray foam.

Older Eau Claire houses carry more of these leaks than the guide's diagram shows. The balloon-framed houses around the Third Ward and Randall Park have wall cavities that run open from the basement to the attic like flues, and every one of those bays is an air path that loose-fill sitting on top will not close.

Attic Frost and Ice Dams in a Climate Zone 6 Winter

Eau Claire County sits in IECC climate zone 6A, and the city logs 7,923 heating degree days in a normal year, measured at Chippewa Valley Regional Airport. In that climate, air leaking up through the ceiling is not just lost heat. DOE notes that some of the moisture in a house, from cooking, bathing, and breathing, ends up in the attic by that route, where it meets roof sheathing that sits far below freezing for months and can condense or frost on roof sheathing that sits below freezing for months. The same leaking warmth melts snow from below and feeds ice dams at the eaves; ENERGY STAR counts a history of ice dams as a sign of serious air leaks. Our attic insulation page covers why ice dams end at the ceiling plane. A thicker loose-fill layer helps; sealing the leaks is what removes the heat source.

Who Does the Attic Air Sealing

Some loose-fill installers air-seal as part of the job and price it in, some sell it as a separate line, and some leave it out. Ask which, in writing. Air-sealing the attic floor plane with foam is work this crew does, top plates, chases, and penetrations, on the floor approach described on the attic insulation page. When the honest plan is foam to seal and loose-fill to build the depth, we can do the sealing and you hire a loose-fill installer for the depth, and we will tell you that on the walkthrough rather than quote foam for the whole attic.

The material is the easy part. What separates a good loose-fill job from a pile of fluff over problems is the prep, and most of it is spelled out in code or federal guidance.

Eave Baffles and Open Soffit Vents

ENERGY STAR's guide puts it in capitals: never cover attic soffit vents with insulation; use rafter vents, also called insulation baffles, to keep the air moving. The model energy code, IECC R402.2.3, requires a baffle next to soffit and eave vents for air-permeable insulation in vented attics, holding an opening at least as large as the vent and extending above the top of the insulation, and the IRC wants at least a 1-inch space between insulation and roof sheathing at the vent. Wisconsin's dwelling code says simply that insulation shall not block the free flow of air. A baffle at every vented rafter bay belongs in the bid, and so does a block at the outer edge of the top plate so the blowing hose does not fill the soffit itself.

Attic Hatch Dams and Weatherstripping

An attic hatch is a hole in the ceiling with a lid on it. Wisconsin's UDC, SPS 322.32, requires the access cover in an unconditioned attic to be weatherstripped and insulated to match the surrounding ceiling, and where loose-fill is installed, a wood-framed baffle or retainer around the opening so insulation does not spill into the house when the hatch opens and the R-value around the opening holds. That rule governs new homes; a retrofit bid should meet it anyway.

Chimney, Flue, and Recessed Light Clearances

DOE's guide notes that cellulose must be kept away from chimneys and metal flues. ENERGY STAR describes the usual building code clearance as 1 inch from metal flues and 2 inches from masonry chimneys to any combustible, including insulation, with a sheet-metal dam to hold loose-fill back. Recessed can lights are the other hazard: ENERGY STAR says to keep all insulation at least 3 inches away unless the fixture is rated IC for insulation contact, and the electrical code says the same for non-IC fixtures. ENERGY STAR recommends IC-rated, airtight (ICAT) fixtures, which insulation can touch; an older non-IC can needs a dam that holds the insulation back, or replacement by an electrician.

Knob-and-Tube Wiring in Older Attics

The Chippewa Valley's lumber-era houses, in Eau Claire and Chippewa Falls alike, date from the knob-and-tube years, so an attic in one of them is worth checking before anything is blown in: porcelain knobs and separated single wires are pre-1930 electrical work. DOE's guide explains that insulation can make the wiring overheat, and ENERGY STAR calls it a fire hazard when in contact with insulation, and DOE's guide says not to proceed with air sealing or insulating where active knob-and-tube is found in the attic; the house must be rewired first. An electrician decides whether it is live. A loose-fill bid should not just blow over it.

Bath Fans That Dump Into the Attic

Bath and kitchen fans that end at the attic floor, or at a duct lying loose near the soffit, blow moist air straight into the coldest part of the house. The IRC, at M1501.1, says exhaust air goes to the outdoors and not into an attic, soffit, ridge vent, or crawl space, and ENERGY STAR lists fans exhausting into the attic as a problem for a contractor to fix. Get every fan ducted outdoors before it disappears under a foot of insulation.

Existing Insulation, Wet Spots, and Vermiculite

Dry, clean old insulation, batts or loose-fill, can usually stay, and new loose-fill goes on top; the existing layer counts toward the target, and ENERGY STAR's recommendation for zones 6 and 7 is R-49 for an attic that already has 3 to 4 inches and R-60 for an uninsulated one. Air sealing still comes first, which may mean pulling back old insulation to reach the plates. Wet, matted, moldy, or rodent-fouled material comes out, and the leak or entry point gets fixed before replacement. Vermiculite is different: a pebbly, gray-brown or silver-gold pour-in fill, much of it mined near Libby, Montana, and sold as Zonolite. EPA's advice is to assume it contains asbestos and not disturb it, and ENERGY STAR says not to disturb it unless an approved lab has tested it. No one should blow new insulation over vermiculite, vacuum it out, or crawl through it to air-seal until that question is answered by a professional.

If you are searching for blown-in insulation near you in the Chippewa Valley, you want an insulation contractor who blows loose-fill as a regular part of the business, not a sideline. We are not that contractor, and we will not recommend one by name here. What we can do is tell you what a bid that deserves your signature looks like, because the bid is where most of a loose-fill job's quality gets decided. Get at least two written bids and compare them line by line.

What a Good Blown-In Bid States

Questions to Ask a Loose-Fill Installer

• Is air sealing in this price, and can you show me on the attic floor what gets sealed?
• How many bags will go in, and will you leave the empty bags or a bag count I can check against the chart?
• What will the depth be on install day, and what will it settle to?
• How do you handle the eaves where the roof comes down low over the top plates?
• What do you do about the can lights, the flue, and the hatch?
• If you find knob-and-tube, vermiculite, a wet spot, or a fan venting into the attic, what happens to the schedule and the price?

The Attic Card and Depth Markers at the Hatch

Federal law already requires part of this. Under the FTC's R-value rule, 16 CFR 460.17, an installer must give the customer a contract or receipt, and for loose-fill it must show the coverage area, initial installed thickness, minimum settled thickness, R-value, and number of bags used. Wisconsin's dwelling code goes further for new homes: SPS 322.20 requires depth markers in blown-in attics, at least one per 300 square feet, fixed to the trusses or joists, showing both the minimum initial installed thickness and the minimum settled thickness in numbers at least an inch tall, each facing the attic access, plus a permanent insulation certificate posted at or next to the electrical panel. The IECC adds a signed, dated installer certificate listing installed density and bag count. On a retrofit, ask for all of it. A ruler you can read from the hatch, and a card that matches the bag count, is what lets you check the job without hiring anyone to climb up there.