Bob Anders wrote:Could it be rock wool? It was made with asbestosis for a lot of years.
This was installed in 2010 after a PG&E audit found our dwelling inadequately insulated... pretty sure it isn't asbestos-based.
After spending an hour hunting online, it does seem prudent to not use this insulation in a California hugelkultur bed. Boric acid is indeed the major component of fire retardant treatment of it and as John noted, California (or at least southeastern California) has a bountiful accumulation of boron and not enough rain to make it disperse very quickly.
If I were in the rainy Pacific northwest, I'd have no qualms about building beds and reusing old cellulose insulation.
I'm going to see if our waste and recycling people accept it as compostable or not. As it is, I've decided, "Not in My backyard".
Thank you for your input!