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How do you know that?

    Are your recommendations based on accurate information?

                               By Roger Hankey, ASHI Certified Inspector


In the fall and winter of 2011 I inspected a 10-12 year old townhouse complex to assess chronic ice damming and moisture intrusion concerns.  



The work required inspecting the attics which were insulated with blown fiberglass.  Many of the attics had evidence consistent with widespread signs of rodent infestation including trails and tunnel openings atop the insulation, as well as small feces.



While developing recommendations for the Home Owner’s Association and their property management vendor, I contacted a trusted pest control operator to learn if he would be interested in doing the rodent control work on this project. To my surprise, he said his services might not be needed if the attics were re-insulated with blown cellulose. He said the fire retardant in blown cellulose (boric acid) would provide enough rodent resistance that other pest control measures might not be needed. 


I’d also found the attics had numerous attic bypasses (air leaks from the homes into the attic) including many leaky recessed light fixtures.  Given the widespread bypasses and the compromised insulation, my recommendations included removal of the existing rodent infested fiberglass.  This would facilitate finding and sealing the bypasses.  An insulation firm suggested installing of a layer of spray foam insulation would be a cost effective method, eliminating the need to locate and seal the leaks individually.  They further stated that mice typically do not chew through petroleum based closed cell foam insulation.


Based on the comments of the pest control operator and the insulation firm, I recommended the replacement insulation be a base layer of spray foam topped with blown cellulose since the insulation firm said this would be more cost effective than using spray foam to establish the necessary R-44 insulation. 


Since 2011, I’ve found hundreds of attics and many unfinished basement walkout walls with fiberglass insulation damaged by rodents.  I’ve repeated the recommendation of removal of the insulation (professional cleanup, usually by a mold mitigation firm) and re-insulation with spray foam and/or blown cellulose since (as I state) they are both “rodent resistant” materials.   I was comfortable with these recommendations since I rarely find signs of rodent activity in attics with blown cellulose insulation. 


Recently, I began to rethink these recommendations, and did some internet searches on the rodent resistive properties of blown cellulose.  One manufacturer claimed that their cellulose had pest control properties from the borate fire retardant, but only on insects, not rodents.   Therefore, as a caution, I sent an email to the pest control operator to check on “how he knew” cellulose was rodent resistant.

He said it came from a guest speaker at a pest control conference he attended. He directed me to and said he thought I could get some “good information” from it.   This California insulating contractor’s website states,

“An additional benefit of the cellulose being treated with borate is that it is resistant to insects, rodents, molds, mildew and fungus.”  

I also checked with the Cellulose Insulation Manufacturers Association (CIMA), and received this response from its Executive Director:

“The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) makes it illegal for anyone to make pest control claims about any product that is not specifically registered with and approved by USEPA as a safe and effective pest control product.  Accordingly, CIMA makes no statements about pest control properties of cellulose insulation as a generic material.”  Notice he did NOT say cellulose has no pest control properties, he said it is illegal to make such claims.  

Based on the CIMA statement, a 1992 University of Nebraska study on rodent damage to insulating materials,  and many internet technical forum posts about mice being able to chew through almost anything, I’ve stopped making specific recommendations for spray foam topped with cellulose. 


I checked further with Jeff May, of May Indoor Air, a trusted expert on indoor air quality and a former home inspector.  His view is that “there are simple physical and chemical reasons why mice might prefer fiberglass to cellulose. When burrowed through, fiberglass retains its shape, creating a nice shaped space for nesting. I do not think that this holds true for cellulose. A rodent is less likely to burrow through cellulose for the same reason that I am hesitant to walk through the stuff: it's a pain in the butt to clean off!” I agree fiberglass maintains the tunnel shapes and I believe it is far less likely that cellulose would.


The Material Safety Data Sheets (MSDS) for boric acid and cellulose insulation treated with boric acid both state that boric acid is a skin and eye irritant for humans.  Mice are used to test cosmetics since their eyes and skin are similar to humans.   


It is unwise to accept information from a trusted source without asking “How do you know that?” Trust but verify is a prudent maxim.  It is easy to accept information we would LIKE to be true without pursuing the information from multiple sources and from the source with the most at stake in the quality and accuracy of the information. Thanks to the internet and modern communication, information sources are abundant, but unless we carefully check our sources, we may be relying on rumors, guesswork, hype, and/or misinformation.  


Home inspectors need to do a careful job researching the basis of their recommendations and not recommend the use of any specific material or product unless there is a consensus from multiple sources and authoritative information about its properties.


Finally, avoid designing or specifying corrective measures.  A material or product may not be suitable or cost effective in all cases, so it is wise to have the qualified contractor choose the materials and methods best suited to project.  Recommend the type of contractor to do the job so they are responsible for the materials and methods used.   Wise inspectors avoid the trap of designing the “fix”. 


Thanks to Kevin O’Hornett for reviewing this article.


© 2016