The essentials in 30 seconds
Blown-in insulation, also called loose-fill, is made of small pieces of cellulose, fiberglass or mineral (rock) wool that a machine blows through a hose. It is used two ways: spread loose across an attic floor, and dense-packed into closed wall cavities through small holes. Seal air leaks first, fit baffles at the soffit vents, then blow to an even depth right out to the edges. Cellulose settles more than the others, about 10 to 20 % over time against 2 to 4 % for fiberglass and rock wool, so installers must reach the settled thickness printed on the bag chart.
- For the R-value you need in your climate zone, see attic insulation.
- You can rent a blowing machine for an attic; walls are a job for a professional.
- In the UK the attic is the loft, and blowing insulation into closed walls is usually called cavity wall insulation.
What blown-in insulation is and where it is used
Loose-fill insulation comes compressed in bags. A blowing machine breaks it up and pushes it through a long hose, so it settles into every gap around wiring, pipes and framing. That makes it well suited to:
- Attic floors in vented attics, either on bare drywall or on top of old insulation that is dry and clean.
- Closed walls in older houses, filled through holes drilled from inside or outside, a method known as drill-and-fill or dense-pack.
- Hard-to-reach spaces where cutting and fitting batts would leave gaps.
If the roof leaks or a bath fan vents into the attic, fix that before adding any insulation.
Cellulose vs fiberglass vs mineral wool
Data from a Department of Energy (DOE) loose-fill guide, reproduced by the Pacific Northwest National Laboratory:
| Material | Made from | Settled R-value per inch | Settling over time | Moisture | Points to know |
|---|---|---|---|---|---|
| Cellulose | Recycled newspaper and cardboard, treated as a fire retardant | 3.2 to 3.8 | 10 to 20 % | Holds small amounts of water but breaks down when saturated | Heavy; slows air movement well; not recommended for mobile home ceilings |
| Fiberglass | Spun glass | 2.2 to 2.7 | 2 to 4 % | Not permanently damaged if soaked | Light; less effective at slowing air; easy to "fluff" below its rated density |
| Mineral (rock) wool | Spun blast-furnace slag | 3.0 to 3.3 | 2 to 4 % | — | Least common of the three |
Because R-value per inch differs, the depth for the same R-value differs too. For R-38, the same guide gives about 10 to 12 inches of cellulose, 14 to 17 inches of fiberglass and 11.5 to 13 inches of rock wool. Always go by the chart on the bag you are using.
Fluffing happens when loose-fill goes in below the manufacturer's minimum density: the layer looks deep but holds too little material and falls short of its R-value. Counting bags against the chart is the simplest guard against it.
How to blow insulation into an attic
DOE's Building America guidance for existing vented attics, in order:
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01
Inspect
Look for vermiculite, knob-and-tube wiring, bath or kitchen fans venting into the attic, and roof leaks. Fix or remove these hazards before anything else.
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02
Air seal
Seal top plates, wiring and plumbing holes, dropped soffits and the hatch. Our attic insulation guide lists where to look. Leave combustion air openings and soffit, ridge and gable vents open.
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03
Protect hot and electrical items
Fit metal blocking and collars around hot flues, and cover or dam recessed lights that are not IC-rated. Raise junction boxes above the future insulation level or mark them with a flag.
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04
Fit baffles
Install one at every soffit vent, running from the top plate to above the planned insulation height, with at least a 2-inch air gap. They keep the vents open and stop wind from blowing the insulation back from the eaves.
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05
Measure and count
Work out the attic area and read the bag chart for the number of bags per 1,000 square feet at your target R-value. Staple a few depth rulers to joists so you can see the level while you work.
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06
Blow from the far end
Start at the edges farthest from the hatch and work back. Fill every joist bay, cover the tops of the joists and reach all the way to the top plates at an even depth.
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07
Check and record
Compare depth and bags used against the chart, insulate the hatch, and note the date, product, depth and bag count for future reference.
Dense-pack insulation in existing walls
Dense-packing fills empty or poorly insulated wall cavities without opening the whole wall. DOE's Building America Solution Center describes the method:
- Check what is there through a small inspection hole, an outlet box with the power switched off, or an infrared camera.
- Choose inside or outside: from inside, holes go through the drywall; from outside, through siding and sheathing, ideally when the house is being re-sided so the drainage plane can be checked and restored.
- Drill 1- or 2-inch holes about 6 inches below the ceiling in each cavity, with extra holes above and below windows, above door headers and below any blocking.
- Blow to the manufacturer's recommended density for dense installation, given in pounds per cubic foot, so every cavity is packed full.
- Verify with an infrared camera before patching, ideally on a day with a big indoor-outdoor temperature difference.
- Close balloon-framed cavities at the top and bottom first, so insulation stays out of the attic and basement.
This is a job for an experienced contractor. Ask how they will find hidden blocking, restore the exterior water barrier and handle lead paint in older homes. In cold climates, ask how the wall will manage moisture; damp walls lead to the problems in our black mould guide.
Settling, depth and what the bag label must show
All loose-fill settles a little; what matters is reaching the right settled depth. In the US, the FTC's R-value Rule requires each bag of loose-fill to carry a chart showing, for a range of R-values, the initial installed thickness, the minimum settled thickness, the maximum coverage per bag, the number of bags per 1,000 square feet and the minimum weight per square foot. Installers must give you a receipt showing the coverage area, initial installed thickness, minimum settled thickness, R-value and number of bags used.
| What you see | Likely cause | What to do |
|---|---|---|
| Depth well below the rulers after a year or two | Normal settling, more with cellulose | Compare with the minimum settled thickness on the receipt; top up if it falls short |
| Fewer bags used than the chart calls for | Fluffing: blown too light | Ask the installer to add material to the charted bag count |
| Bare or thin strip along the eaves | Wind-washing or missing baffles | Fit baffles and fill the edges |
| Dark, dirty patches in the insulation | Air leaking up from the house | Pull it back, seal the leak, then refill |
| Wet or matted insulation | Roof leak or condensation | Fix the source first: see roof leak |
| Insulation piled in soffits or around a flue | Missing baffles or dams | Clear it and fit baffles or metal dams |
Renting a blowing machine vs hiring a pro
ENERGY STAR notes that loose-fill needs a blowing machine, that some home improvement stores rent them, and that hiring a professional may be the wiser choice.
| Situation | DIY with a rented machine | Hire a professional |
|---|---|---|
| Open, easy attic with a good hatch and headroom | Reasonable for a careful pair of people | Faster and comes with a receipt |
| Low, tight attic or long crawls | Hard and tiring | Better choice |
| Vermiculite, knob-and-tube wiring, mold or wet wood | Stop | A specialist first, then the insulator |
| Closed walls (dense-pack) | Unsuitable: needs density control and infrared checks | Yes |
| Air sealing | You can do the simple items | Ask for it as part of the job |
If you do it yourself, have one person feed the hopper and one on the hose, keep a phone within reach, start early on hot days and drink plenty of water.
When you hire, ask:
- Will you air seal first, and which leaks will you seal?
- Which material, what settled thickness and R-value, and how many bags for my area?
- Will you fit baffles, protect flues and lights, and staple depth rulers I can see from the hatch?
- Will I get the receipt the FTC rule requires?
- For walls: how will you check for voids and restore the siding or drywall?
Blown-in insulation: frequently asked questions
What is blown-in insulation made of?
Cellulose (recycled paper), fiberglass or mineral wool, broken into loose pieces and blown by machine.
Is cellulose or fiberglass better for blown-in insulation?
Cellulose has a higher R-value per inch and slows air better; fiberglass is lighter, settles less and tolerates getting wet.
Does blown-in insulation settle?
Yes. Cellulose settles about 10 to 20 % and fiberglass or rock wool about 2 to 4 %, so installers aim for the settled thickness on the bag chart.
Can I blow insulation into my attic myself?
Yes, in an open attic with a rented machine and a helper, after air sealing and fitting baffles. Leave walls and hazardous attics to professionals.
Can you blow insulation into existing walls?
Yes. Contractors drill small holes in each cavity, dense-pack the insulation and check with an infrared camera before patching.
Can blown-in insulation go over old insulation?
Yes, if the old insulation is dry, clean and free of vermiculite or mold. Seal air leaks underneath first.
How deep should blown-in insulation be?
It depends on your climate zone and the material. See our attic insulation guide for R-values and the bag chart for depth.
Is blown-in insulation a fire hazard?
Cellulose is treated as a fire retardant. Keep all insulation away from hot flues and non-IC recessed lights, using metal dams.
Sources
Official guidance on loose-fill insulation, checked on 27 October 2026.
- US DOE and PNNL · Loose-fill insulation ↗Cellulose, fiberglass and rock wool: R-value per inch, settling, moisture, density, fluffing and depth for R-38.
- US DOE Building America Solution Center · Blown insulation for existing vented attics ↗Hazard checks, air sealing, baffles with a 2-inch gap, flues, lights, junction boxes and even coverage.
- US DOE Building America Solution Center · Blown insulation for cavities of existing exterior walls ↗Drill-and-fill steps, hole size and position, infrared checks, balloon framing, lead and knob-and-tube cautions.
- FTC R-value Rule · 16 CFR 460.17, what installers must tell their customers ↗Receipt contents for loose-fill installations; the bag chart rules in section 460.12 were also checked.
- ENERGY STAR · Attic insulation project ↗Blowing machine rental, safety gear, barriers around recessed lights and unfaced batts under loose-fill.
- US EPA · Protect your family from asbestos-contaminated vermiculite insulation ↗What vermiculite looks like and why to leave it undisturbed.
Planning an attic top-up?
Measure the attic area, check for vermiculite and old wiring, and read the bag chart before you buy or book.