Is Blown Cellulose Insulation Mold Resistant?

Blown cellulose insulation is a natural and eco-friendly alternative to fiberglass insulation, consisting largely of recycled materials and naturally occurring compounds like boric acid. Blown cellulose insulation is durable and weather resistant, and it also has impressive mold resistance. However, it is not completely mold proof due to the nature of its design.

Mold Resistance

Blown cellulose insulation is resistant to mildew and mold because it contains acids that can kill mold on contact. Specifically, it contains boric acid, a compound commonly sold as “borax” in stores and used as a commercial pesticide. Though boric acid is used in cellulose insulation because of its flame retardant properties, the mold resistance is an important added bonus if you live in a humid region or if you ever suffer any sort of water damage.

The Problem with Cellulose

Though boric acid can offer considerable mold resistance, cellulose itself is a mold nutrient. Cellulose is an organism found in all plant material, like leaves, wood, cotton and the shredded paper used in blown cellulose insulation. This paper, in addition to providing food for mold, is highly absorbent. If enough moisture presents itself, mold can still grow because it has the all the ingredients that it needs within the insulation. In order for mold to grow, however, the moisture must be excessive — not the kind that results from day-to-day humidity or a small roof leak.

Circumstances for Growth

Cellulose insulation is about as mold-resistant a material as you can install. Still, in heavy cases of water penetration, like the aftermath of a flood or the result of a burst pipe, mold spores can still emerge from within the insulation. This does not necessarily mean that cellulose insulation is a waste of money because under the right circumstances, mold can grow on almost any resistant surface, including mildew-resistant paint, moisture-resistant drywall and even concrete.


As long as you keep your home from falling into disrepair, you should never experience mold in your cellulose insulation. The borates within the insulation material will protect against mold growth even when the insulation retains moisture for long periods of time as long as you do not allow the moisture in your home to get out of control. While few things are actually mold-proof, cellulose insulation offers enough natural mold resistance to protect against most mold spores.

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