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Green Basics

Blown-In or Loose-Fill Insulation

Fill the Whole Framing Cavity — Even Irregularly Shaped Ones


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 insulation 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 insulation is not an air barrier, 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 sheathing, 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:

For more information…

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  1. Ryan O'Dell | | #1

    I need some help on insulation
    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 ?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Either one
    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.

  3. Bryce | | #3

    ORNL Study
    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.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Bryce
    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:

  5. Charles Eichenlaub | | #5

    Vapor barrier
    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.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Charles Eichenlaub
    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!

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