photo of balsam wool building insulationBalsam Wool & Wood Product Building Insulation

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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. Synonyms for balsam wool insulation include wood fiber insulation, wood insulation, wood-wool insulation, and blackweld or blackwell insulation.

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 insulation

Balsam 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) InspectApedia

I 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. Most likely this is SILVAWOOL INSULATION or a similar product. - DF

Reader Question: Is this balsam wool insulation or vermiculite?

30 June 2015 Joshie said:

Hey. Is this balsam wool insulation or vermiculite?

Vacuum removal of attic insulation (C) excerpt from YouTube Video

link to YouTube youtube [dot] com/watch?v=nVq6NWfGcCk



The insulation in the video you cite (and these three photographs) is not balsam wool and it is not vermiculite. The insulation in the video shown by the link you provided (and excerpted here in stills) is most likely fiberglass batts or perhaps a mix of fiberglass batts and chopped (or damaged) fiberglass, or possibly fiberglass batts along with some older mineral wool or rock wool.

Vacuum removal of attic insulation (C) excerpt from YouTube Video

See MINERAL WOOL - ROCK WOOL INSULATION to learn what mineral wool insulation looks-like.

Vacuum removal of attic insulation (C) excerpt from YouTube Video

See VERMICULITE INSULATION to learn what vermiculite looks like.

Or see INSULATION IDENTIFICATION GUIDE - for a guide to identifying all types of building insulation.

It's not apparent why the vacuum removal of this attic insulation was undertaken. Just why was not given. But it would perhaps be reasonable to remove old fiberglass or mineral wool insulation if it were badly damaged, ineffective, or if the insulation were contaminated by rodents, insects, or if the insulation had become wet and was a mold hazard.

Does Balsam Wool Insulation Contain Asbestos?

Balsam wool under the microscope © D Friedman at

Balsam 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" suggesting that mineral wool or balsam wool might have appeared with asbestos in some products.

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 © D Friedman at


Balsam wool insulation fire resistance test © D Friedman at

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 deteriorated 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 labeled 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 & Silvawool 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.

[Click to enlarge any image]

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

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.

Some readers refer to this insulating material as "Blackwell Insulation" but that's probably an error. Google Scholar searches for BlackWeld Insulation and for blackwell insulation made in 2009 and again in 2017 have not found patents nor specific uses of the terms blackweld insulation nor blackwell insulation.

Silvawool & Kimsul Wood Fiber Insulation

Silvawool like wood fiber insulation (C) InspectApedia JR

Question: is this wood fiber insulation Silvawool?

I ran across you site while doing some research on insulation inside a vintage refrigerator. I'm refurbing (Admiral Refrigerator 1949 - 1950) when I removed the interior I came across what looked similar to Silva Wool or some type of shredded wood insulation but much more fine that what was pictured on your site.

It's reddish brown, mixed sized pieces some look like wood, while others almost hair. It burns for a short second than goes out much like your tests. It wasn't held together with any paper backing but looks like it may have been glued in sheets but now is very loose and falls apart very easily.

Have you come across anything like this in your research? From what I found and reaching out to some antique appliance folks also, it sounds like refrigerators in that era typically used compressed sawdust which hopefully means something similar or Silva Wool (I've read horse hair too).

I have attached an image (with penny for scale) if you'd be willing to take a look - and possibly help identify, if anything for peace of mind - I would greatly appreciate it! - J.R. 2015/12/28 by private email

Reply: Silvawool® & Weyerhaeuser wood fiber insulation history & dates

Silvawool like wood fiber insulation (C) InspectApedia JR

Thanks for the excellent photos. Your insulating material may be Silvawool or Kimsul or a similar chemically treated wood fiber insulating material: at least to me this looks very much like a treated wood fiber insulating product. Indeed the fibers in your photos are longer than in some balsam wool insulation products, but if we consider that a wide variety of cellulose fibers were used for insulating materials that is not a surprise.

Even sugar cane was used.

Silvawool® was indeed a shredded wood fiber insulation product, one of many popular in North America beginning in the 1950's. There is no doubt that wood fiber insulation was used not just in buildings but within refrigerators, freezers and in other special applications.

SilvaWool was a Weyerhaeuser product - the trademarked brand name was first used in 1949 and was filed in a trademark application by Weyerhaeuser (Tacoma Washington) on 5 August 1950 as # 0562777. The Silvawool trademark was fully registered in 1952. The company's literature (found by trademark search) described Silvawool as "... Silvawool is a product related to chemically impregnated wood fiber for use as insulation material. " Silvawool insulation is no longer in production.

The material is likely to have been treated with a fire retardant, perhaps the same borate salt as was later used in cellulose (chopped newsprint) insulation.

As with other wood and cellulose insulations, this is not an asbestos-suspect material though we have read reports of possible cross-contamination when some companies processed non-asbestos insulation products at the same plant where asbestos products were handled. Generally testing for asbestos is not performed unless we find some particular reason to do so such as facing a high-cost dusty demolition project.

Kimsul Crepe-paper-like Insulation

Kimsul insulation from Kimberly Clark  - catalog page (C) InspectApedia

Kimsul™, produced by Kimberly Clark, in Neenah Wisconsin in the U.S., is an older insulating material used from about 1930 into at least the 1950's and that has been described by some building history sources as a creped paper insulating material that was impregnated by asphalt.

Although Kimsul was described by its manufacturer as a wood-fiber product, It won't look like the wood fiber insulation described at BALSAM WOOL INSULATION PROPERTIES though I've seen Silvawool and "Kimsul" described together in some sources. Kimsul, according to Bill Kibbel at, was used in refrigerators, car dashboards, in trains, and as an insulation in Quonset huts during WWII. - retrieved 2015/12/29, original source

Kimsul, a Kimberly-Clark insulation brand copyrighted in 1949, was sold in round rolls or in stitched batts, was still in distribution in 1956 when Kimberly-Clark advertised Kimsul for use as building insulation in the April 1956 issue of Popular Science - ad excerpt shown here.

Kimsul was produced in reflective-faced batts which, if spread apart, showed what look like soft, brown, accordion-pleated crepe paper. The company described Kimsul insulation with its reflective "vaporseal" as resisting radiant heat loss as well as providing insulation value in ceilings and walls.

Kimsul insulation ad Popular Science April 1956 (C)

The Kimsul manufacturer provided charts of percent of loss in a Kimsul-insulated wall as ranging from 0 to 90% but did not provide the R-values used today. Typical heat loss savings estimated by the company for homes using Kimsul in walls and roofs ranged from about 25% to 38%. A K-factor of 0.27 Btu/sq.ft. was quoted in the company's Kimsul catalog.

Kimsul's ASTM tested fire resistance was quoted in the same catalog as resisting flame temperatures as high as 1700 degF. You may see Kimsul Insulation printed on the Kimsul insulating batts in an attic or wall of a home built in the 1940's and 1950's. Each roll of Kimsul contained 200 sq. ft. of insulating material.

More Balsam Wool Insulation History, Patents & References


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

Or see BALSAM WOOL BATT INSULATION FAQs - questions and answers about balsam wool insulation posted originally on this page.



Suggested citation for this web page

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


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman