photo of balsam wool building insulationProperties of Balsam Wool & Wood Product Building Insulation

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Balsam wool insulation guide:

This article illustrates and describes balsam wool and wood product insulation materials. We discuss the properties of balsam wool, what it is made from, its fire resistance, insect resistance, insulation properties (R-value). We address the frequent question: does balsam wool insulation contain asbestos and we give the history of use and manufacture of balsam wool. We added these examples because of frequent questions about these materials.

This document assists building buyers, owners or inspectors who need to identify various insulation materials in buildings by simple visual inspection. We provide photographs and descriptive text of balsam wool insulation and other wood insulating products to permit identification of these materials in buildings.

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Balsam Wool Insulation Properties, Composition, History, Photos, Tests

photo of balsam wool building insulation

Article Contents

History of the Development & Production of Balsam Wool Insulation

"Balsam Wool" is a wood fiber product or "cellulose" insulation that was widely used in homes and in a variety of other applications in the U.S. from at least the 1930's.

Our Balsam-Wool insulation photo (above left) includes a Weyerhaeuser Forest Products logo in the bottom of the red diamond.

[Click to enlarge any image]

We learn from the Minnesota Historical Society [5] as well as from reviewing patents describing the history of production of balsam wool fiber insulation [4] that the Wood Conversion Company of Cloquet, Minnesota, incorporated in 1922 in Delaware.

The Wood Conversion Company of Cloquet was a Weyerhaeuser By-Products Division, and was a key participant in the development & production of Balsam Wool Insulation. Wood Conversion was the assignee of a variety of patents describing the production of balsam wool and other wood fiber (and even animal hair) insulation products.

The wood product insulating batts shown in these photos was produced by Weyerhaeuser corporation.

In the right hand (and page top) photo of this insulation (basically chopped wood fibers, probably treated with a fire retardant), you can see the brown/tan fibers of insulating material.

Properties of Balsam Wool Building Insulation

photo of balsam wool building insulationBalsam wool insulating blankets, also referred to as "wood cotton" in some patent literature, are shown in our photographs here. These insulating batts of "balsam wool" were produced using wood fibers from tree bark, wood pulp, or other lumber byproducts.

A review of some of the key patents, (described below in the references section of this article) a variety of tree species were described including tree bark and wood byproducts from redwood, [balsam] fir, and others. [4]

Various sprays were used to give desired properties including for adhesion, fiber control, sizing, and possibly insect and rodent resistance.

When inspecting balsam wool insulation we advise against cutting the kraft paper covering these insulation batts as if the paper is cut a large amount of the insulating material is likely to simply fall out.

If you must make a cut to test or confirm the material or its condition, the cut should be just an inch or so, and you should tape the cut closed after your inspection.

Reader question: Is this Balsam Wool? what is this dark brown fibrous insulation?

Possible wood fiber or Balsam wool insulation Steve S. (C) InspectApediaI Wonder if you recognize the loose fill insulation in attached photo?

This insulation was found in an attic in 1944 house. Reddish brown, strands.

Would ignite and smolder but not really burn. Same house has some Kimsul insulation but this is quite different. Friends upstate say its been a bad winter, its been poor here in Raleigh but nowhere near what you guys suffer through. - S.S., Raleigh N.C. 3/10/2014


What you have in hand looks like wet Balsam Wool - a wood fiber insulating product I describe in the article above, though your insulation sample is more coarse and has larger chunks than I've seen.

- Was the insulation wet?

- Was it wrapped in black or tan Kraft paper?

I'll post the photo here to invite other reader comment. - DF

Does Balsam Wool Insulation Contain Asbestos?

Balsam wool under the microscope (C) D FriedmanBalsam wool insulating blanket used wood fibers from tree bark, wood pulp, or other lumber byproducts. As such it is a wood or cellulose product and not an asbestos-containing material.

Watch out: however, from 1958 through 1974 asbestos was used in the production of other products at the Conwed Corporation (Wood Conversion Company) plant in Cloquet, Minnesota, including the production of ceiling tiles and other products.

Our photo at left illustrates a stereo-microscopic view of balsam wool fibers. You can see that the manufacturing process separated individual long wood fibers and that there was little extraneous material in this insulation product.

12/5/2014 Ron said:

I have Balsam Wool Batt insulation in my house. It is labeled with the same Weyerhaeuser label that you have pictured. I want to replace the insulation but need to know if the black paper backing contains asbestos. thanks, Ron.


Ron, while there are some curious manufacturing site overlaps between producers of asbestos-containing products and manufacturers of wood-fiber based products that *might* have produced some asbestos cross-contamination among products (Robins 1988), we have not yet been able to find credible research asserting that problem is a real one.

Balsam wool is a chemically treated wood fiber used as insulation material.

Other than that, wood fiber insulation products are not asbestos-products nor was their paper covering. If you want an absolutely reliable answer however, you'd want to spend about $50. to send a sample of your material to a certified asbestos testing lab. Keep us posted if you take that step. I'd be surprised if asbestos were detected in any balsam wool product. Here are some interesting research citations Take note of Baird (1946) in re your question as he cites "boards containing wool and asbestos" suggesing that mineral wool or balsam wool might have appeared with asbestos in some products.

  • Allcut, E. A. "Heat Insulation as Applied to Buildings and Structures." Proceedings of the Institution of Mechanical Engineers 128, no. 1 (1934): 195-251.
  • Baird, Parker K., Robert J. Seidl, and Donald J. Fahey. "PAPER-BASE PLASTICS 423 Wool Goldsmid, P. Analysis of fabrics, papers, and boards containing wool and asbestos. Papeterie 68, no. 2: 34-35, 37-39 (Feb., 1946); BIPC 17: 223. Zinc." Pulp and Paper Manufacture; Bibliography (1946): 423.
  • Blondeel, Arlette, Jacques Oleffe, and Georges Achten. "Contact allergy in 330 dermatological patients." Contact dermatitis 4, no. 5 (1978): 270-276. - Includes citations of balsam wool contact.
  • Cayuga lumber company. Building material catalog. Webber Lumber & Supply Co., 1941. - includes Balsam Wool insulation.
  • Cummings, J. M. "Possibilities for the Manufacture of Mineral Wool in British Columbia" [PDF], British Columbia Department of Mines, (1937)- includes tables comparing properties of various insulation and other materials to mineral wool and balsam wool, retrieved 12/6/2014, original source:
  • Hahn, David W., and M. Necati Özişik. "Appendix I: Physical Properties." Heat Conduction, Third Edition: 679-684. includes physical properties of balsam wool
  • Robins, Thomas G., and Margaret A. Green. "Respiratory morbidity in workers exposed to asbestos in the primary manufacture of building materials." American journal of industrial medicine 14, no. 4 (1988): 433-448.
  • Hooper, William James. "The Deposition of Dust on Walls." Journal of Applied Physics 1, no. 1 (1931): 61-68.

Reader Question: can I get my insulation tested to see if this (apparently Balsam Wool) insulation contains asbestos?

(Jan 7, 2013) Matt said:

Hello. I think I may have found something like Balsam wool in my attic. Would love to get it tested (to make sure it's not asbestos.) Any recommendations? I'm in Nashville, TN.


Matt most forensic labs can identify different types of insulation; but there is little or no physical resemblance between balsam wool and asbestos, nor was asbestos used as a brown fiber enclosed in batts, nor does asbestos resemble long woody fibers. Take a look at the insulation; Balsm wool is usually pretty easy to identify by eye.

We perform limited forensic lab testing on a pro-bono basis for people of limited means. Please do NOT send us any samples or materials for testing without prior arrangement.

Fire Resistance of Balsam-Wool Insulation - lab test results

In 2012 we performed a simple combustion test of a sample of Balsam-Wool insulation from an older home. Our photos below illustrate the result. Using an alcohol lamp as a flame source in our lab, and holding a sample of balsam-wool insulation in the flame until its edges glowed, we found on removal from the flame that the insulation would not support combustion.

Balsam wool insulation fire resistance test (C) D Friedman Balsam wool insulation fire resistance test (C) D Friedman

Insect Resistance of Balsam Wool Insulation

Balsam wool, depending on the wood species used to produce it, may be naturally insect resistant (Balsam) as are cedar, cypress, and some other wood products. It might have been treated with a fire retardant, I don't yet know (am researching the question). Some fire retardants (borate salts used in cellulose insulation) also seem to be mold resistant and may also be insect resistant.

Reader Question: is balsam fiber insulation treated to prevent attracting insects?

(Mar 25, 2012) Anonymous said:

Is the balsam fiber treated to prevent attracting wood destroying insects. Have a crawlspace that has allot of balsam batt insulation that's deterioated and falling on ground.


Balsam wool, depending on the wood species used to produce it, may be naturally insect resistant (Balsam) as are cedar, cypress, and some other wood products. It might have been treated with a fire retardant, I don't yet know (am researching the question). Some fire retardants (borate salts used in cellulose insulation) also seem to be mold resistant and may also be insect resistant.

Reader question: is balsam wool insulation mold resistant?

(Sept 16, 2012) Geoff In Oregon said:

Question: Any anti-fungal/mold properties to balsam ?


Good question. I haven't come across moldy balsam wool insulation. Although there are plenty of molds that like to grow on cellulose - wood fibers - the number that grow happily on dry or even damp balsam fir may be much smaller.

If you have some that is suspect, mail me a few cubic inches in a labelled new clean Ziploc bag and I'll test it pro bono next January when our lab returns to the US.

R-Values of Balsam Wool Insulation

We estimate that balsam wool has an R value of about 2 to 3.5 in loose fill and about 2 to 3.25 in batts.

We have seen some estimates of an R value of 3.4 per inch for versions of insulation that contain a mixture of wood fibers and chopped paper.

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

Balsam Wool Historical Documents & Installation Instructions

Thought you'd appreciate these instructions for Balsam Wool. We've been removing it because we have limited space for fiberglass, and the packaging is displacing too much space. The balsam inside has disintegrated and packed, thus not too useful. - D.G. 3/10/2014

Balsam wool insulation installation instructions (C) - D.G. Balsam wool insulation installation instructions (C) - D.G.

[Click to enlarge any image]

Blackweld Wood Fiber / Cellulose Insulation?

Blackweld wood fiber cellulose insulation (C) D Friedman S Shockley

Professional home inspector Stephen P. Shockley sent along this excellent photo of 1940's vintage insulation found in a home below a more recent layer of fiberglass insulation.

We think this is "Blackweld Insulation", another wood-fiber insulation product similar to the Balsam Wool product described above, and we are looking for a positive ID, product label photos, or a sample to examine in our forensic lab. CONTACT us with feedback.

Had Mr. Shockley been able to send a small sample (a cubic inch or less would be sufficient) to our forensic lab we would have examined the material to identify its constituents.




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BALSAM WOOL BATT INSULATION at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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