Insulation That Won’t Mold

Most insulation products do not support mold growth. However, some types of insulations are accessories to mold growth. Once the mold is entrenched on the insulation, it will spread to adjacent materials.

Mold-Resistant Insulation

Causes of Mold

Mold is one of the most well-known home infestations. It ranks with rodents and termites. Spores are everywhere and move with air currents. Some can remain dormant for hundreds of years–just needing moisture to start growing, Mold needs three things to grow.

  • Moisture. Mold spores are dormant without moisture.
  • Warmth. Mold will not grow in freezing temperatures.
  • Food. Any type of biodegradable material like wood or dust.

Moisture is the catalyst that sets mold in action. Wall leaks. Plumbing leaks. Roof leaks. Inadequate vapor barriers and/or poor exterior waterproofing. Some insulation products absorb moisture and are incapable of expelling it. Mold will grow on the dust in the insulation or on framing members touching the wet insulation.

8 Insulations and Mold Growth

Insulation gets the blame for mold growth, but it is usually only an enabler. Insulation is almost always in contact with wood framing members. Mold growing on insulation will migrate to the wood and eventually cause rot.


Fiberglass insulation is light and fluffy. It holds sawdust and any other dirt it comes in contact with. Add moisture and mold starts growing on the dust. The Kraft paper used on faced fiberglass provides a food source for mold Some manufacturers use mold-resistant chemicals on the paper. Not all. The fiberglass itself is not consumed by mold.

Mineral Wool

Mineral wool insulation is made from melted rock and iron slag. It is inorganic and mold will not feed on it. Like fiberglass, it holds organic material the mold feeds on. Mineral wool absorbs moisture. The dirt in the product or the surrounding framing provides the food source for mold to grow.

Spray Foam

Closed cell spray foam insulation is inorganic. It does not have any openings that attract dust. One of its big advantages is that it completely seals all gaps and cracks–preventing water penetration. This eliminates the main ingredient needed for mold spore growth. Open cell spray foam holds dust and absorbs moisture. Mold can grow on it.

Rigid Foam Boards

Rigid insulation boards like extruded polystyrene and polyisocyanurate are closed cell products that act much the same as closed cell spray foam without the crack-sealing properties. Two-inch thick foam acts as a vapor barrier–provided that all gaps are sealed. Expanded polystyrene boards can hold dust and moisture–allowing mold growth.


Cellulose insulation is treated with borates to resist mold, pests, and fire. If cellulose gets wet, it will trap the moisture against adjacent wood framing members. The borates may resist mold on the cellulose but wet wood is a perfect host for mold growth.

Sheep Wool

Sheep wool insulation is naturally mold-repellent. It can absorb up to 35% of its weight in moisture without losing its insulation value or having mold growth. No additives. All natural insulation.

Foam Glass Insulation

Foam glass insulation is made of tiny closed cell glass bubbles. It will not absorb any moisture during its lifetime. Making it almost mold-proof. Glass does not support mold growth. Dirt and dust rarely adhere to it. Any mold that may start growing if the circumstances are right will quickly die out.

Cementitious Foam

Cementitious foam completely seals stud cavities–filling gaps and cracks to keep out moisture. It does not absorb water and resists mold growth even in high-humidity locations. Dirt and dust can adhere to it but without moisture, mold will not grow.