cellulose building insulation © Daniel Friedman Properties of Cellulose Building Insulation

  • CELLULOSE LOOSE FILL INSULATION - CONTENTS: Cellulose building insulation identification guide - photo guide to identifying & inspecting cellulose building insulation. How to Inspect Cellulose Insulation for Defects. Pro's and Cons of Cellulose Building Insulation. What are the R Values and Effectiveness of Cellulose Building Insulation? What is the Mold Resistance of Cellulose Building Insulation? Odor Properties of Cellulose Building Insulation. Settlement or Void Problems with Cellulose or Other Building Insulation Products. List of Manufacturers of Cellulose Insulation
  • POST a QUESTION or READ FAQs about the identification, contents, & properties of cellulose insulation.
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Cellulose building insulation:

This article illustrates and describes the properties of cellulose building insulation materials.

We 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:

Visual Details Help Identify Loose-Fill Cellulose Insulation

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).

Close-up view of paper fragments in cellulose insulation (C) Daniel Friedman

Above, a close-up look at cellulose insulation will often show small fragments paper on which you may make out printed characters or partial characters (photo above, red arrow). Or you may see small fragments of paper of different colors (photo below). These details can help identify cellulose insulation and will distinguish it from just about any other building insulation product. [Click to enlarge any image]

Tiny fragments of colored paper help identify cellulose insulation (C) Daniel Friedman

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.

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

Question: changing estimates of R-values for cellulose insulation

2016/08/15 Z Wilfre said:

I have had about 75 bags of cellulose insulation, 30 pounds each, stored since 1989 in my summer residence. Now I am retiring and want to blow them into the attic. The information on the old bags show that a R32 is 8.8 inches settled, with a density of 1.88 per square foot, installed. 1000 sq. ft. would require 62.6 bags (30 lbs. each) net.

Since the place is large, I went to Menards website which shows that R30 to be 8.6 inches settled and require 46 bags (18.1 lbs each) with a density of 8.78 lbs per sq. ft. installed. This is about half the density of the 1989 cellulose that I already have.

My questions: What has changed? Is the cellulose produced today approximately twice as efficient? Has the measurement of "R" values changed? Is the product in the old 30 lbs bags somehow not as "fluffy" or does a newer blower chop it up better and make it a better insulator.


I suspect that different people are making the same calculation with perhaps very slight changes in product rating, test results, or other factors. I'd ignore it. After all, real on-site variations such as how densely or loosely you blow the insulation will overwhelm the slight R difference in the two theoretical claims.

Z Wilfre said:

Perhaps all my information was confusing, I think I got the Menards density wrong by a factor of 10. (should be 0.878 lbs/ sq ft. for R30)

But a simpler calculation shows that the old cellulose gives 0.059 lbs/sq ft per R. Menards gives 0.029 lbs/sq ft per R. This is not "slight". This is double. I take it that in your experience the "lbs/sq ft per R" has not changed with time. I guess I will experiment a bit and see if I can get the old cellulose to be as effective as the Menards cellulose. Which would save me about $300. I'll let you know. Thanks for your response.

(mod) said:

To be clear and candid, my search didn't find a list of historical R-values for loose-fill cellulose. I don't think we can reliably compare R-vales from different eras without first knowing exactly how the R-value was determined. Historically those were "measured" in the field by obtaining U values and converting them to R. (See DEFINITION of HEATING & COOLING TERMS at ) Briefly R is heat transfer resistance while U is the more easily-measured heat-conductance.

A problem with that approach is that it doesn't account for variation in how the insulation was installed: how loose, how tightly packed, or perhaps other factors affecting the measured wall-area heat transmission. A simple change in test conditions, such as starting with different indoor and outdoor temperatures will change the measurement (heat transfer rate is greater when there is greater difference between the two areas). 15K is the ideal difference when making measurements, and other factors such as solar radiation, wind, moisture and humidity, infiltration all need to be held constant.

There might also differences in the cellulose application method: loose-fill dry-spray vs wet-spray methods. And in walls, settlement vs when measurements are made would be a factor.

R-value measurement, claims, and advertising remain controversial in the insulation industry. Typical R-values claimed for cellulose insulation by either method are R 3 to 3.8 per inch.

For other readers,

There, a 3.5" thickness (presumably in a wall and without accounting for settlement or voids) of loose fill celluose = R13. That's an R of 3.71 / inch and right in line with industry claims.

All of the R-value or even density per "square foot" claims are to me confusing since when a homeowner is installing cellulose insulation she is not going to have any idea the density at which it is being applied, nor the thickness of "a square foot" since by definition squares omit a thickness dimension.

2016/08/19 Z Wilfre said:

I did find a history of cellulose insulation. So it appears that my old cellulose won't perform as well as the new cellulose for the same density. The web link is:

"Material density of loose-fill cellulose is one example of the progress that has occurred. As recently as 1980 the settled density of a typical cellulose product was over 42kg/m3. By the mid-80s average density had decreased to about 40kg/m3. A couple years later 38kg/m3 was common. This was done by improving and refining production technology that had been used for 40 years or more.

At the end of the 1980s new manufacturing technologies became available, and the density of cellulose insulation dropped suddenly by about a 2-5kg/m3. Today the average settled density of cellulose is about 30-35kg/m3"

" Cellulose insulation is now produced on very good technology based on fiberization systems, which maximize fiber separation. These attenuated, long and slender fibers are consistent, uniform and offer much better comfort during application."

Moderator said:

ZW thanks; we'll research and add that information.

The source you cite is a recently-formed insulation industry association, the European Cellulose Insulation Association, Website:, Administrator Lucia Gross

In North America, for the U.S. see CIMA, the Cellulose Insulation Manufacturers Association, 136 South Keowee St., Dayton OH 45402, Tel: 937-222-2462. CIMA provides a helpful document listing cellulose insulation buildnig codes, regulations, specifications and voluntary standards, CIMA Technical Bulletin #1, [PDF] at

Certainly casual observation of cellulose in older vs newer homes shows that newer cellulose *looks* more finely fluffed as individual particles.

Still I'd consider these average density numbers and variations in rated R-value and weight as underlying theory rather than as accurate predictors of the insulation performance in a given installation.

Not included in the ECIA's otherwise helpful description of the properties of cellulose insulation, and pertaining to all insulation products, I emphasize that having inspected many cellulose-insulated homes, including opening or disassembling older homes containing various qualities and types of insulating materials, I almost always find installation variations whose effect on the building's overall heat loss or heat gain will swamp the underlying theory. A few missed cavities or over-pumped overly-dense insulating material along a wall can add up to a heat loss effect similar to leaving a window open. What this means to us who care about a home's thermal performance is that attention to detail during an insulation job is important.

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."

Question: cellulose insulation odor complaints, diagnosis, solutions

(May 13, 2015) Cellulose Odors said:

I built a new home in New England and moved in last August. We noticed over the winter, a slight odor in our attic which is a walk in attic, located off of the master bedroom closet. That odor has drastically increased as the weather becomes warmer, its overwhelming.

What could the odor be? We are worried it could be toxic and want to understand what to do to resolve. We have contacted our building and insulation provider but I'd like to have my facts ready for that discussion and would appreciate any insight folks can offer.

(Jan 20, 2016) joseph fish said:

I have this house that has blown insulation in the floors and drywall holding it up. It's 10" thick and notice smell and mold issues in the house.I been living here 3 yrs. now and think I need to get it out.

Prior to living here the basement was cealed up with foam sheets on center block foundation and 10" out with paneling, had no air flow at all and electric heating. I took out all the paneling and insulation sheets but the insulation in the flooring is still there. Issue is smell and mold.



Thanks for the question: but from just this text no sensible building consultant would volunteer what the odor is or even is likely to be - we have little cellulose insulation odor data except for Falkow's comments above and the research on odors in cellulose insulation that I cite below.

I don't know if it's a coating, a dead animal, something spilled, a water leak that got something wet, or what. Your comment is on a cellulose insulation page. If your insulation is cellulose, that material is rather mold resistant. If a leak got drywall from a ceiling below wet or wet something else there could be a mold problem.

That's already more arm-waving than fits the facts. All I can say from the facts of your note is that often an existing problem smells worse when things warm up.

I'd look for candidates: spills, leaks, animals, materials that maybe got wet.

Mr. Fish

Cellulose insulation is not particularly conducive to mold growth, but there could be mold on the cavity side of drywall if it was wet. I'd make some test cuts to inspect the most-suspect areas first.

ASTM C-739 provides tests for R-value, odor, moisture vapor accordance with ASTM Standard C-1015, Standard Practice for Installation of Cellulosic and Mineral Insulation.

Research on odors in cellulose building insulation

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:

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 & Cellulose Insulation Information, codes, standards

Cellulose insulation manufacturers registered by NIST

In these building insulation articles we provide photographs and descriptive text various kinds of building insulation along with description of the characteristics of each material.

Cellulose Insulation History, Properties, Codes, Standards

Cellulose Insulation Articles


Continue reading at CELLULOSE INSULATION FIRE RESISTANCE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see INSULATION R-VALUES & PROPERTIES for an extensive table describing the properties of different insulating materials used in buildings.


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


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