We own an old two storey heritage house in Passmore. I was in the attic recently and lifted up the fiberglass batt insulation. Under the fiberglass I found sawdust and wood chips. What is this all about?

Back in the day before mineral wool, rock wool, fiberglass, vermiculite and cellulose insulation products were available, sawdust and cork were the two commonly used insulators. Cork, of course, was the expensive commercial product used in buildings where refrigeration (with ice) was a requirement for production of a product. Sawdust and wood shavings were the only affordable substitute for home owners. In fact, most builders and home owners didn’t insulate attics or walls at all. So a home that had sawdust insulation was better than average in that respect.

However, if you compare the R-value of sawdust and wood chips to our modern insulation products, the insulation value is poor. As well, sawdust can spontaneously combust through a process known as pyrolysis in which heat builds up to a kindling point in the sawdust. Of course this hasn't happened in your home in its lifetime so you may take some comfort in this fact. The only other consideration is insect infestations. If the sawdust becomes wet due to roof leaks, insects may be attracted to the wet wood. Sometimes, folks who went to the trouble of installing insulation mixed lime into the sawdust to help protect their attics from this problem. I have been told that the lime also helps control pyrolysis, but I cannot confirm this rural legend. 

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This article was written by Steven Cannon, a Registered Building Official and partner in Lynch Building Inspection Services Ltd. of Nelson, British Columbia. It originally appeared in The Nelson Express.

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