Can mold grow in cellulose insulation?

Yes, mold can grow in cellulose insulation, but it is more resistant to mold than many people assume when installed correctly. Cellulose insulation is made from recycled newspaper treated with boric acid and borate compounds that serve as fire retardants and also act as mold inhibitors. Borate raises the pH and makes the material less hospitable to mold and pests. Laboratory tests show that properly treated cellulose resists mold growth better than untreated wood and standard fiberglass. However, cellulose is still made from paper fiber, which is organic and can support mold if it gets and stays wet. If a roof leak, plumbing leak, or high humidity causes cellulose to become saturated and remain damp for more than a few days, mold can grow on it, especially if there is dust or organic debris present. Wet cellulose also loses R-value and can become heavy and compress. Fiberglass and mineral wool are inorganic and do not support mold themselves, but they can still have mold growing on the dust and dirt that collects on them if they get wet. The key to preventing mold in any insulation, including cellulose, is moisture control. Fix roof leaks, ensure proper attic ventilation with soffit and ridge vents, use bathroom exhaust fans vented outside, not into the attic, and keep indoor humidity below 60 percent. If cellulose does get wet from a one-time leak and is dried quickly within 48 hours, it can often be saved. If it stays wet or shows visible mold, it should be removed and replaced.