Blown-in attic insulation: how it works, and how to get it right

By Insulation Report Editorial. Last reviewed August 2026. How we research this.

What blowing actually is

The materials hub compares every attic insulation material against every other one, including where blown-in loses to spray foam or to batts. This page is about the method itself: what the machine does, what a correct install looks like from inside the attic, and how to tell afterwards whether you got what you paid for.

The equipment is one machine and one hose. Bags of compressed fibre are torn open into a hopper, an agitator breaks the compressed slabs back into loose fibre, and a blower pushes that fibre down a long flexible hose to whoever is holding the far end. Rental machines and contractor machines differ in throughput and in how much they fluff the material, not in principle. The hose is the reason the whole method exists: it reaches across an attic with no walking surface, over the top of ducts and framing, into the shallow triangle at the eaves where nobody can kneel.

It is a two-person job by design, not by preference. One person stays at the machine at ground level, cutting bags and feeding the hopper at a steady rate, because a starved hopper blows thin and an overloaded one jams the hose. The other works the hose in the attic. Blowing an attic floor runs about 9 to 12 crew-hours per 1,000 square feet, which on a 1,200 square foot floor is 10.8 to 14.4 crew-hours, or 5.4 to 7.2 hours on site for two people. That bracket is an estimate from secondary trade guidance rather than a measurement, and setup, air sealing and cleanup sit outside it, which is why a job quoted as an afternoon usually is not one.

Why loose fibre beats a cut piece on an attic floor

An attic floor is not the tidy rectangle a batt is cut for: irregular joist spacing, truss webs crossing the bays, cables and boxes on the ceiling, and a space at the eaves that narrows to nothing. Loose fibre takes the shape of whatever it lands in, so none of that needs cutting around. That advantage is specific to the open attic floor. Blown material is not a substitute for air sealing, it does not stop air moving through gaps in the ceiling plane below it, and in a closed cavity such as a wall or a sloped ceiling it is a different job with a different density target. Everything below is about open-blow over an attic floor. For regular, accessible framing bays such as knee walls and the hatch itself, cut material still wins, which is the case the fibreglass batts page makes.

What changes at the hose, once the material is chosen

R per inch, water behaviour and settling are how the two loose-fills are chosen between, and the materials hub sets them side by side against foam, mineral wool and batts. Three differences only show up after that choice is made, at the machine and on the ceiling, and none of them appear in a material table.

Why the same bag reads differently on day one

The agitator in the machine tears the compressed slab apart and the blower drives air through it, so material lands with far more air in it than it had in the bag. Fibreglass fluffs the hardest and lands lightest, which is why a fresh fibreglass blow can measure at target on the afternoon it goes in and measure short a season later. Cellulose leaves the hose closer to its settled state, so day-one depth is a better predictor of where it ends up, and it still packs down further than fibreglass does. Neither behaviour is a defect. It is the reason the coverage chart carries two thicknesses, and the reason a bag count beats a ruler as evidence.

Weight, which is the one nobody checks

One product carries a measured anchor here, Owens Corning AttiCat blown-in fibreglass, 27.5 lb bag, and its published coverage is 39.5 square feet at R-49. That is about 0.70 lb per square foot, or roughly 835 lb spread over a 1,200 square foot ceiling, and about 0.86 lb per square foot at the manufacturer's published R-60 coverage. Spread over every joist in the house that is a light load, which is why fibreglass rarely raises a question. Cellulose is denser and therefore heavier over the same area, and there is no published density figure here to turn that into pounds per square foot. If the ceiling below is lath and plaster, or drywall that already sags between joists, look at it from below before adding depth in any material, and price the ceiling repair into the decision rather than after it.

Pests, and what the additives do not do

Neither material is a pest control product. The borate in cellulose is a fire retardant, carried to specifications such as ASTM C739, and it is not sold or tested as a rodent treatment. Rodents nest in both materials, tunnel through both, and contaminate both, and no additive on the market changes that enough to count on. This matters at the hose because blowing is the one operation that makes contamination invisible: new fibre over an old nest looks finished from the hatch and smells the same in July. Where there is a rodent history, the cleanup comes first and the insulation second, which is the whole argument of the rodent contamination page.

How a correct install is done

The order matters more than the material. Every step below has to happen before the hose is switched on, because each one becomes either impossible or expensive once the floor is under a deep layer of loose fibre.

1. Air seal the ceiling plane first. Top plates, wiring and plumbing penetrations, dropped soffits over cabinets, the chase around a chimney or flue with materials rated for the clearance, bath fan housings, and the perimeter of the hatch. Insulation slows conduction; it does not stop air moving through a hole. Blowing over unsealed penetrations buries them, and the air keeps moving, carrying moisture with it. This is the slow, unglamorous half of the job and it is the half that decides the result.
2. Baffle every eave before material goes anywhere near the soffits. Rigid baffles, sometimes called vent chutes, staple into each rafter bay at the eaves and hold a clear air path from the soffit vent up over the insulation. Without them, loose fibre drifts into the soffit and blocks the intake, which is how a working ventilated attic becomes a damp one. This is not optional in a vented attic and it cannot be fixed afterwards without crawling back to the eaves.
3. Build dams where material must not go. Around the attic hatch, around a masonry chimney, and around any recessed light fixture not marked as suitable for insulation contact, which is what an IC rating on the fixture means, at the clearance the fixture's own label states. Clearances for fixtures without that rating come from the National Electrical Code, a model code each state or city adopts with its own amendments, so the label in front of you is the authority, and a fixture with no legible label gets treated as if it must be kept clear.
4. Staple depth markers to the framing. Printed rulers marked in inches, stapled to joists or trusses, spread across the attic including the far corners, each one showing the target depth. They are cheap, they are what an inspector looks for, and they turn "it looks deep enough" into a number anyone can read later from the hatch. Where material is going over an existing layer, set them to the finished total measured from the drywall, not to what is being added on top, or the job reads correct and is short.
5. Blow from the far corner back toward the hatch. The hose gets shorter as the work proceeds and nobody has to wade back across finished material, which compresses it. Even depth is checked against the markers as the work goes, not judged at the end.
6. Treat the hatch separately. A hatch or pull-down stair gets rigid board or a batt fixed to its back and weatherstripping around the frame. It never gets buried: an unsealed hatch under a pile of loose fibre is both an air leak and a mess waiting for the next person who opens it.
7. Count the bags. The number of bags emptied, checked against the coverage chart for the product, is the only measurement that survives settling. See the next section for why.

What a bad install looks like

Bad blown-in jobs fail in a small number of repeatable ways, and every one of them is visible from the hatch or from a short crawl. Depth drifts: thick where the installer stood near the hatch, thin at the eaves and in the corners the hose barely reached. Soffit vents are packed solid because no baffles went in first. There are no depth markers anywhere, so nothing can be checked. Material sits banked against a recessed light fixture that carries no insulation contact rating. The hatch is buried. Nothing was air sealed, so the ceiling penetrations are still open, just invisible. And the whole floor was blown to the settled target on the day, which reads correct with a ruler that afternoon and reads short a season later.

Settling, and why installed depth has to exceed label depth

Loose fibre leaves the hose full of entrained air. Over the following weeks and months it compacts under its own weight, and the layer that measured correctly on install day measures less afterwards. That is why the coverage chart on a loose-fill package, required to be there under the FTC R-value Rule (16 CFR Part 460), states both an installed thickness and a settled thickness for each labelled R-value. The settled figure is the one that has to meet your target.

The clearest evidence is in the manufacturer's own coverage numbers. Take the published coverage for the anchor product at three labelled R-values, and multiply R by the square feet one bag covers:

If R-value scaled cleanly with depth, that third column would be flat: one bag would deliver the same R times area however thick you spread it. It does not. It falls by about 5.6 per unit of R, so the same bag delivers roughly 8 percent less at R-49 than at R-19. The material is compressing itself. Deeper piles are denser piles, and denser loose-fill gives less R per inch than the shallow layer the R per inch figure was measured on.

The practical consequence is an arithmetic trap. Scaling the R-19 coverage straight to R-49 predicts 42.9 square feet per bag, where the manufacturer publishes 39.5. An order built on that prediction comes up short by the same 8 percent the row above lost, because it is the same yield loss seen from the other end, and it shows up at the end of the day as either a second trip to the counter or a floor spread thinner than the target. The same trap sits inside any calculator that divides a target R-value by a fixed R per inch, which is why the attic insulation calculator here interpolates the R times coverage product rather than the coverage.

So: work to bags per area and to settled thickness, not to inches alone. On a 1,200 square foot attic, the anchor product's published R-49 coverage gives about 30 bags, and its published R-60 coverage about 38. Round up rather than down: a bag over is a shelf in the garage, a bag short is a thin stripe across the far end of the attic and a second trip. Every product differs, and the bag label for whatever is actually blown into your attic is the authority over any table, including this one.

Inspect the finished job in ten minutes

Do this before the crew leaves if you can, and before the invoice is paid either way. A head and shoulders through the hatch with a torch and a rigid ruler covers most of it.

1. Depth at five points. Push the ruler down to the drywall, not to the top of a joist, at five places spread across the attic including one near the eaves and one in a far corner. All five should read the installed thickness on the invoice. Any point noticeably shallower than the others is a drift problem, not a measuring error.
2. Joist tops. If the tops of the ceiling joists are visible anywhere, the depth there is no greater than the joist. Dressed lumber under the American Softwood Lumber Standard PS 20 measures 5.5 inches for a 2x6, 7.25 for a 2x8 and 9.25 for a 2x10, so even the deepest of those filled to the top with blown fibreglass is about R-20 to R-25. That is short of every ENERGY STAR attic recommendation except the R-25 retrofit figure for zone 1, which the top of the range just reaches. On a finished blow, visible joist tops mean the hose did not get there.
3. Eaves. Look along the perimeter for baffles standing in the rafter bays and a clear gap above the soffit vents. Loose fibre packed into a soffit is a defect, not a thorough job.
4. Bag count. Ask for the empty bags to be left in the driveway, or for the count and the product name written on the invoice. Compare it with the coverage chart. It is the one check settling cannot erase.
5. Fixtures and the hatch. Dams standing around any recessed fixture without an insulation contact rating, and around the chimney. The hatch insulated on its back and weatherstripped, not buried.
6. Paperwork. Product name, labelled R-value, number of bags installed, and installed and settled thickness. A quote that says only "blow attic to R-49" leaves nothing to check later.

If the depth already in place is what you are checking rather than a new install, the depth checker converts inches into an R-value range and compares it with the recommendation for your climate zone.

Rent the machine, or hire it out

Work down the table and stop at the first row that describes your attic. The rows are ordered so that the ones that overrule everything else come first.

The recommendation. The first two rows are stop signs rather than hiring decisions: fix the attic before anyone blows anything into it. Rows three to five are hire. Cellulose from a rental machine is a judgement call rather than a bar. If only the last row applies, rent, and budget a full day rather than an afternoon, because the air sealing underneath is the part that takes it. Material for the anchor product at retail shelf price on a 1,200 square foot attic to R-49 is roughly $2,050 before the machine, the baffles or the respirators, which is the figure to compare a quote against. The blown-in cost page builds the installed number from the BLS trade wage and that same bag price, and states which of its inputs are estimates.

Either way, wear a fitted respirator, sealed eye protection and gloves, and cover exposed skin. Both materials are respiratory irritants in a confined, unventilated, hot space. Worth knowing which way the rules run: OSHA's respiratory protection standard binds an employer with respect to the crew it sends, so a contractor's people work under it, while a homeowner in their own attic is outside it entirely. On a rented machine the protection is nobody's job but yours, and the fit of the respirator is the part people skip.

You may not need this

A machine day is the easiest thing to book and often not the thing that is wrong. Three checks before any bags are ordered.

Measure the settled depth yourself, rather than trusting the last invoice. A layer blown to a settled target years ago has been compacting ever since, and a layer blown to the wrong number was never at target at all, so the paperwork is not evidence. Push a ruler to the drywall at five points and take the shallowest. ENERGY STAR recommends R-49 for an attic in climate zones 4 through 8 that already has 3 to 4 inches of material, and R-60 for a bare one, with lower targets in the warmer zones. Within about R-10 of your zone's figure, more depth is a slow payback. That threshold is an editorial judgement labelled as an estimate rather than a measurement, and it follows from heat flow falling as 1 over R, so the last increment saves a small fraction of what the first ones did.

If only the eaves are short, only the eaves need blowing. A blown attic is rarely uniformly thin. It is thin in the last few feet at the perimeter, where the hose reached worst and where a missing baffle let material drift into the soffit. If the middle of the floor measures at target and the perimeter measures half of it, the job is a few bags and an afternoon at the edges, with baffles going in first, not a whole-floor quote. Measure the perimeter separately before accepting a price for the whole area.

Sometimes the depth is fine and the holes are not. An attic already at its zone's recommendation gains little measurable from another layer while it is still losing air through an uninsulated hatch, unsealed top plates and open plumbing chases, because loose fibre resists conduction and does nothing to air moving through a hole. Rigid board on the back of the hatch, weatherstripping around its frame, and sealant at the penetrations is a weekend and a small bill. The remove-or-add-over guide runs the related test on whether the old material should come out at all. We do not sell insulation and we do not install it, which is why this page can tell you that the cheapest correct answer is often a caulk gun and a weekend rather than a machine and 30 bags.

Recommended levels above are ENERGY STAR retrofit guidance, which is based on the 2021 IECC model code. Where the 2021 IECC calls for R-60 in the ceiling, installing R-49 over the whole ceiling satisfies it if the insulation carries over the wall top plate at full uncompressed height, which normally means a raised-heel truss. Energy codes apply to new construction and permitted work, not usually to voluntarily adding insulation to an attic you already have. Which edition applies where you live depends on your state and often your city, and your local building department is the only authority on it.

Related

• Every attic insulation material compared
• Fibreglass batts, and where cut material still wins
• What blown-in insulation costs installed
• How many bags and how much depth
• What R-value to aim for, by climate zone