Blown-In or Loose-Fill Insulation

Fill the Whole Framing Cavity — Even Irregularly Shaped Ones

Bird's-Eye View

A complete fit

When insulation fibers are blown in place, the insulation fills all the nooks and crannies of the framing bay where it is installed.

Compared to wall and roof assemblies insulated with batts, assemblies insulated with blown-in insulation usually perform better because they are much less likely to have insulation gaps.

See below for:

Key Materials

Denser is better

Loose-fill insulation materials include fiberglass, mineral wool, and cellulose.

Glass fibers prepared for blown-in applications are cut to smaller lengths than glass fibers prepared for batts.

The trend with cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. is also in the direction of smaller fibers. A few decades back, cellulose insulation was manufactured by the hammer-mill process, a method that left small pieces of newsprint intact. These days, manufacturers use a process called "fiberizing." Fiberized cellulose has smaller particles than hammer-milled cellulose. While the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch of hammer-milled cellulose decreases slightly as density increases, the opposite is true for fiberized cellulose. As fiberized cellulose gets denser, its R-value per inch increases—at least until the insulation reaches a density in the range of 3.3 to 4.0 lb. per cubic foot.

Design Notes

Cold weather houses may need deeper walls

Designers of homes with blown-in insulation need to calculate whether the specified studs and rafters are deep enough to include adequate insulation. Blown-in cellulose or fiberglass insulation can achieve R-20 in a 2x6 wall cavity; higher R-values require deeper studs, double-wall construction, or foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .

Builder Tips

Blowers are available to rent or buy

Because installing blown-in insulation requires specialty equipment, most builders sub out such work to an insulation contractor. However, some builders invest in an insulation blower so that they can have more control over their schedules.

Most big-box lumberyards rent insulation blowers; some will lend the equipment to customers who buy enough insulation.

The Code

Make the R-value clear in two places

Section N1101.4.1 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. requires: "The thickness of blown-in or sprayed roof/ceiling insulation (fiberglass or cellulose) shall be written in inches (mm) on markers that are installed at least one for every 300 square feet throughout the attic space. The markers shall be affixed to the trusses or joists and marked with the minimum initial installed thickness with numbers a minimum of 1 inch high. Each marker shall face the attic access opening."

Section N1101.8 of the IRC requires: "A permanent certificate shall be posted on or in the electrical distribution panel. The certificate shall be completed by the builder or registered design professional. The certificate shall list the predominant R-values of insulation installed in or on ceiling/roof, walls, foundation (slab, basement wall, crawlspace wall and/or floor) and ducts outside conditioned spaces..."

How long does damp-spray cellulose take to dry?

Like many construction-related questions, the answer to this question is "It depends." In most cases, walls sprayed with damp-spray cellulose should be ready for the drywall crew after three days of drying. Remember, though, that drying will be slower when temperatures are cool and the relative humidity is high. If conditions are favorable, some cellulose contractors give drywallers the okay to proceed after only 24 hours of drying.

Damp-spray installers usually advise against the use of interior polyethylene vapor retarders. In climates where a vapor retarder is required, the best options are vapor-retarding paint or MemBrain.

Fluffing and settling

Cellulose manufacturers and fiberglass manufacturers have been rivals for years. Cellulose manufacturers like to point out that blown-in fiberglass is more easily "fluffed" during installation than cellulose. Insulation is fluffed by installing it with more air than recommended by the manufacturer, resulting in a lower density and a lower R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. than indicated by the insulation thickness. Fluffing is illegal; it is specifically prohibited by the federal R-value Rule.

Manufacturers of blown-in fiberglass, on the other hand, like to point out that cellulose settles over time to a greater extent than fiberglass. If an unscrupulous cellulose installer fails to account for settling, the R-value of the installed product would be less than it should be.

Insulation contractors must follow the label requirements dictating the amount of material that must be installed—the "bag count"—per square foot of attic. They must also follow label requirements for initial installed thickness. The initial installed thickness of attic cellulose is always more than the settled thickness. Labels provide the initial installed thickness required to achieve the desired settled thickness.

Wary builders may wish to perform their own bag-count calculations and collect empty bags at the end of an insulation job.


Typical wall and ceiling cavities aren't uniform in size; some are 22 1/2 in. wide, while others are narrow. They can include electrical boxes, wiring, plumbing vents, and blocking. Because of these variations, it's hard to get batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. to conform to all the voids in a stud or joist bay.

In contrast, blown-in insulation provides better performance than batts by filling odd-shaped cavities completely.


For many green builders, cellulose is the first insulation choice for above-grade walls and ceilings. It is environmentally friendly, is inexpensive, and performs well.

Although cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. is not an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., it is much more resistant to airflow than fiberglass. Cellulose insulation is blown in place, so it does a better job at filling cavities completely than fiberglass batts. For these reasons, a given thickness of cellulose almost always performs better than the same thickness of fiberglass—even though the insulating value of cellulose (R-3.1 to R-3.7 per inch) is comparable to that of fiberglass batts.

Cellulose insulation has several environmental advantages. It is made from ground-up newspaper; most brands contain 75% to 80% recycled newspaper (often post-consumer). The shredded paper is mixed with nontoxic borate or ammonium sulfate fire retardants.

When it is damp-sprayed into open cavities or blown into closed cavities at relatively high density, cellulose insulation adds to the airtightness of the house. Also, the quality of the installation is less contractor-dependent than fiberglass batts.

Cellulose can be installed in existing wall cavities through holes drilled in the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , plaster, or drywall. One or two holes are drilled in each stud bay. A good installer should understand how to achieve a so-called “dense-packed” installation.

A new home can be insulated with cellulose using one of four techniques:

  • Using the damp-spray technique, a mixture of cellulose insulation and water is sprayed into the open wall cavities. The insulation is allowed to dry for a few days before drywall is installed.
  • An air-permeable netting can be installed over the interior face of the studs, and cellulose can then be blown into the stud cavities through holes in the netting.
  • The drywall can be installed before the stud bays are insulated. In that case, a 4-in.-wide horizontal gap is left between the lower course of drywall and the upper course of drywall. The gap is 4 ft. off the floor, providing access to each stud bay for the cellulose installer. Once the walls are insulated, the 4-in. gap is patched with drywall.
  • The walls are sheathed with interior rigid foam board before the stud bays are insulated. The foam board is held in place with horizontal strapping. The cellulose contractor insulates the stud bays through holes poked in the foam; these holes are later patched with spray foam.

For more information on installing cellulose, see How to Install Cellulose Insulation.


Anyone looking for a better insulation job than can be achieved with batts might consider blown-in-place fiberglass. Known by a variety of names—well-known brands include the Blow-In Blanket System (BIBS), InsulSafe, Optima, and Johns Manville Spider insulation — the technique uses an insulation blower to install chopped fiberglass (and in some cases an adhesive) into open framing cavities. Blown fiberglass does a better job of filling framing cavities completely than fiberglass batts.

The R-value of blown fiberglass depends on the density; the higher the better. Typical R-values range from 3.2 to 4.2 per inch.

Health concerns associated with inhaling glass insulation fibers have not been substantiated, but it is generally a good idea to avoid installing loose-fill fiberglass unless the fibers can be prevented from entering occupied space or air distribution systems.


Blown Insulation for Attics: Fiberglass vs. Cellulose

Image Credits:

  1. U.S. GreenFiber
  2. CertainTeed
  3. Spray Insulation Industries
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Feb 2, 2012 6:24 AM ET

Response to Charles Eichenlaub
by Martin Holladay

In your climate, most building codes require a vapor retarder, not a vapor barrier, on the inside of your wall. You should avoid the use of polyethylene. Vapor-retarder paint (over the drywall) will work fine.

Of course, airtight construction techniques are much more important than whether or not you include a vapor retarder. Do your best to seal all air leakage points.

Here is more information on vapor retarders:

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Feb 2, 2012 12:02 AM ET

Vapor barrier
by Charles Eichenlaub

I live in Northern Michigan. What about vapor barriers and cellulose in new home construction? I am building with a double wall 8-10" of cellulose.

Nov 15, 2010 10:24 AM ET

Response to Bryce
by Martin Holladay

Thanks for your post. You bring up some important concerns. I discuss some of these issues, and more, in my recent Fine Homebuilding article, Blown Insulation for Attics: Fiberglass vs. Cellulose.

Here are the links:

Nov 15, 2010 10:13 AM ET

ORNL Study
by Bryce

The article I have in my files from Home Energy, May/June 1992, discussing the ORNL tests, indicate that the R-value of the blown fiberglass began deteriorating at approximately 30 degrees F, due to convection currents within the product. I believe the University of Illinois arrived at similar information under testing they performed on blown fiberglass. From personal history, I can attest that homes with blown fiberglass have higher ACH under a blower door test and have higher energy consumption than similar homes with cellulose.

Feb 2, 2010 3:58 AM ET

Either one
by Martin Holladay

1. Either product will work. If it were my own house, I'd choose cellulose.

2. Remember, more insulation is better than less. As long as you're installing more insulation, you might as well install as much as you can afford.

Feb 1, 2010 11:30 PM ET

I need some help on insulation
by Ryan O'Dell

I built my house 4 years ago and used fiberglass blown-in insulation in the attic. I went up and found that the fiberglass has settled down about 4 in., so i am going to get more insulation to blow in. But do I use more fiberglass or do I get some cellulose and blow it on top of the old fiberglass ?

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