cellulose building insulation © Daniel Friedman Properties of Cellulose Building Insulation

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Cellulose insulation: this article illustrates and describes the properties of cellulose building insulation materials. I've added these examples because of frequent questions about these materials. This document assists building buyers, owners or inspectors who need to identify building insulation materials and also people who need to recognize both asbestos materials (or probable-asbestos) in buildings as well as materials unlikely to contain asbestos - all by simple visual inspection.

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Cellulose building insulation identification guide

Cellulose insulation blown into an attic Cellulose insulation in the hand

Article Contents:

Also see BLOWN-IN INSULATION and INSULATION LOCATION - WHERE TO PUT IT. In these articles we provide photographs and descriptive text various kinds of building insulation along with description of the characteristics of each material.

Modern cellulose building insulation is basically chopped newsprint, usually treated with a fire retardant chemical. As you can see in the photos above, it looks like fluffy gray papery material. The lighter colored chips may be wood fragments that have been added to this mix.

Cellulose insulation is usually blown-in to building cavities as an insulation retrofit or into attics where it is being added or where access is physically difficult.

Cellulose building insulation has been used in buildings since or before 1937 and continues to be installed in buildings (2008) in the U.S.

Cellulose insulation produced by some manufacturers is a mixture of chopped paper and wood fibers (sawdust).

How to Inspect Cellulose Insulation for Defects

Falling down cellulose insulation at a ceiling cut

Don't cut a big hole to look for blown-in cellulose insulation - as you can see in this photo, it may simply fall out.

Cellulose building insulation blown in to walls shows up at the sills

If you inspect an older building's basement or crawl space it may be easy to see if cellulose insulation has been blown into the building's walls.

Check at the building's sills atop the foundation walls.

Often openings in building walls permit blown-in cellulose to fall onto the top of the sill as you can see in our photo at left.

Pro's and Cons of Cellulose Building Insulation

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

  • Q&A on - PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, December 1985/January 1986, adapted and updated for InspectAPedia.com December 2010.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Question: How does Cellulose Insulation Stack Up Against Fiberglass or Rockwool Insulation?

I plan to retrofit 7 inches of blown insulation over the top of 6 inches of existing fiberglass insulation, and I would like to use blown-in cellulose.

How does cellulose insulation stack up against fiberglass or rockwool with respect to

  1. R-value of cellulose insulation compared with fiberglass or mineral wool (rock wool) insulation
  2. Moisture absorption of cellulose insulation compared with fiberglass or mineral wool (rock wool) insulation
  3. Attraction to (resistance to) nesting rodents: cellulose insulation compared with fiberglass or mineral wool (rock wool) insulation
  4. Fire protection (fire resistance) - of cellulose insulation compared with fiberglass or mineral wool (rock wool) insulation.

Thanks - David Stingle, Black Creek WI

Below we add additional insulation property questions:


The R-vale per inch of loosefill insulation varies depending on its installed density and product characteristics. For that reason, the most reliable way to buy loose-fill insulation is to specify the R-value - not the thickness - and install the correct number of bags per square foot, following the loose-fill or blown-in insulation coverage chart printed on all insulation bags.

The insulation chart also shows a minimum insulation thickness necessary to guarantee the desired R-value3.

The U.S. Federal Trade Commission (FTC) requires insulation manufacturers to make available to consumers an information sheet explaining this system.

Cellulose insulation yields R-3.1 to R-3.7 per inch, compared to R-2.2 to R-2.9 per inch for blown in or loose-fill fiberglass and rockwool insulation. In general, you should compare products on the basis of cost per R-value per square foot.

Moisture absorption of cellulose insulation: Of the three insulations you named: blown-in or loose-fill cellulose, fiberglass, or rockwool, only cellulose will absorb moisture, but this is only a problem if it gets drenched, such as by roof leaks into an attic or building wall. The other two insulations will hold moisture only on their glass or mineral fiber surfaces.

Fire resistance of cellulose insulation: Of the three insulation products we are discussing, only cellulose is potentially flammable, if its fire retardant loses effectiveness over time (as some suspect of the dry-applied fire-retardants). Studies in the mid 1980's of the reliability of fire retardance of cellulose insulation over time were inconclusive. See Cellulose Insulation Fire Resistance for details.

As for rodent resistance of cellulose insulation, we (DJF) have observed that rodents are happy tunneling in just about any soft insulating material, but we have also observed that a different sort of pest, mold, is not generally found in cellulose insulation. We (DF) believe based on our own field and lab investigations that the fire retardant chemicals used to treat cellulose insulation appear to also resist mold growth.

Mold resistance of cellulose insulation: cellulose insulation appears to be resistant to mold growth. Details are at Cellulose Insulation Mold Resistance

The question-and-answer article above and also appearing at Cellulose Insulation Pros-Cons, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

What are the R Values and Effectiveness of Cellulose Building Insulation?

Cellulose insulation over a suspended ceiling © Daniel FriedmanThe thermal resistance or heat-loss resistance of cellulose insulation sold by Pal-O-Pak Insulation company, for a 10-inch depth, was reported as 0.635 per square meter. The product heat resistance ranged (by thickness) from 0.004 to 1.602 m2· K· W-1. Modern blown-in cellulose building insulation has an R-value of about 3.70 per inch.

Our photo (left) of cellulose insulation sprayed over a suspended ceiling shows an area we were investigating for mold on the upper or hidden side of the suspended ceiling tiles (none was visible). We brushed back the cellulose insulation to check the ceiling surface. But you can also see that this insulation plan was not the best. About R7.

Less than three inches of insulation had been added, and working on wiring or piping in the ceiling area means removing one of these ceiling tiles and dropping cellulose insulation into the occupied space - a bit of a mess.

What these data and most reports of insulation products' resistance to heat loss do not include is the large impact on building heat loss of the degree of care with which any insulating product has been installed.

Gaps between insulating materials and building surfaces can permit drafts which can overcome otherwise high "R" values that may be associated with the insulating material. (Just imagine a well-insulated home in the dead of winter but with a few windows open.)

Insulating materials that by their physical nature tend to fill in cracks and gaps without much human effort, such as blown-in products or foamed products, are likely to produce fewer air leaks and thus may be expected to improve the economy of heating or cooling a building when compared with construction where diligence was not a watchword.

To compare insulating material R-values see our Table of Properties of Insulating Materials

What is the Mold Resistance of Cellulose Building Insulation?

We suspect that building cavities insulated with fire-retardant treated cellulose insulation are a bit more resistant to mold-growth than cavities insulated with fiberglass, cotton, or some other materials.

Our hypothesis is that the fire-retardant chemicals happen to also discourage fungal growth.

Details about mold resistance of cellulose building insulation: field inspection & lab testing

Mold resistance of cellulose insulation: we (DJF) add to this list of properties of cellulose insulation our field and laboratory observation that cellulose insulation appears to be highly resistant to mold growth and somewhat resistant to insect activity compared with fiberglass and mineral fiber insulation.

We have inspected buildings at which cellulose insulation in walls or attics has been actually wet (from building leaks and from fighting building fires).

Testing cellulose insulation for mold contamination included both analysis of bulk samples from buildings where the insulation appeared undisturbed and others where it had been soaked. We examined the cellulose insulation microscopically in our forensic laboratory, at low magnification for evidence of visible mold contamination on the surface of the cellulose fragments, and at high power magnification up to 1200x for individual mold spores. We also collected vacuum-samples of cellulose wall insulation in the same buildings in order to more readily separate the larger cellulose insulation fragments from the generally smaller, lighter mold spores that might be present.

Testing in our laboratory did not in any case detect meaningful mold contamination nor mold growth in the cellulose.

We believe that the fire retardant chemicals used to treat cellulose insulation probably also imparts mold growth resistance.

See FIBERGLASS INSULATION MOLD for examples of mold contamination in building insulation.

Odor Properties of Cellulose Building Insulation

According to Deborah Falkow, owner of MetroNY Insulation, both National Fiber's Cel-P)ak and Nu-Wool cellulose insulations are "all-borate formulations (for fire, pest and mold resistance). Borate is an odorless mineral that doesn't outgas, which is a fancy way to say that National Fiber's cellulose products don't produce funny smells."

But some cellulose building insulations may produce a funny odor, especially right after insulation. Ms. Falkow continues: "Some cellulose manufacturers use an ammonium sulfat/borate mix. That can produce objectionable odors, under the right conditions."

Settlement or Void Problems with Various Building Insulation Products

Insulation settlement and compaction: the effectiveness and R-value of any loose fill insulation product (cellulose, chopped fiberglass, mineral wool, rock wool, vermiculite) that is poured or blown into building wall cavities, attic floors, or cathedral ceilings is at the mercy of the workmanship of the installer.

Deborah Falkow, owner of MetroNY Insulation, writes that the worry about cellulose insulation settlement in buildings is a
myth. "Dense-packed cellulose doesn't settle, because it can't." she writes. "It's installed at twice its settled density, which means that it's under slight pressure in the wall or ceiling cavity."

At BLOWN-IN INSULATION we also discuss insulation settlement in retrofit jobs and we calculate the potential impact on wall R-value.

Insulation Voids: Causes & Types

If an insulation retrofit job omits certain building areas such as the stud bay below windows, cavities above or below older framed buildings that use diagonal corner bracing, or cathedral ceilings built with fire blocking, there may be voids in the insulation blanket. In field inspection we have not observed insulation voids that appeared to be due to the material rather than workmanship of the installer, with the exceptions of:

  • UFFI, Urea Formaldehyde Foam Insulation: foam shrinkage can leave significant gaps at the top and sides of UFFI that was injected into building wall cavities. See UREA FORMALDEHYDE FOAM INSULATION, UFFI.
  • Fiberglass insulation: custom-cut fiberglass batts that are retrofitted to older frame buildings whose framing members were not placed on uniformly-spaced centers may be poorly fit if workmanship is not careful. Also see : INSULATION AIR & HEAT LEAKS
  • Foam board insulation: custom-cut foam insulating board sections that are retrofitted to older frame buildings whose framing members were not placed on uniformly-spaced centers may be poorly fit if workmanship is not careful.
  • Poured insulation or blown-in attic floor insulation: is sometimes distributed unevenly in insulation retrofit jobs leaving areas of thin or even no insulation, especially in hard-to-reach areas, if workmanship is not careful.
  • Also see : /insulation/Fiberglass_Insulation_Leaks.php and AIR LEAK DETECTION TOOLS

Other Voids in Blown-in Building Wall or Ceiling Insulation

As discussed at BLOWN-IN INSULATION:

Watch out: in some blown-in building insulation retrofit projects we have occasionally found significant insulation voids where the installer was careless, or where the installer did not anticipate blockages in the wall cavity formed by diagonal bracing or fire blocking. An infra-red or thermal scan of a heated building during cold weather will make such insulation voids obvious - DF.

[We did indeed observe significant shrinkage, not settlement, in UFFI blown-in insulation in some homes insulated with that product in the 1970's, particularly if the product was not properly mixed in the first place. See UREA FORMALDEHYDE FOAM INSULATION, UFFI. - DF]

Watch out: other insulation properties such as air flow resistance and moisture resistance may be very important in some cases, such as choosing an insulation to use in or over a crawl space that may be damp, or against basement foundation walls - Ed. See INSULATION INSPECTION & IMPROVEMENT and FIBERGLASS INSULATION MOLD for examples- Ed.

Details about the R-values and other properties of various insulation products can be found in our Insulation Table at INSULATION R-VALUES & PROPERTIES.

Manufacturers of Cellulose Insulation

Some of the cellulose manufacturers registered by NIST include

  • National Cellulose Corp.,
  • National Fiber Corp. (Cel-Pak & Nu-Wool)
  • U.S. Insulation Sales Corp.,
  • Pal-O-Pak Insulation Company (aka National Cooperatives, Inc.),
  • United Materials Corporation, and
  • Insul-Wool Insulation Corporation.

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CELLULOSE LOOSE FILL INSULATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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