Styrofoam, by its very nature, contains no properties that naturally support mold growth.
Moisture Retention
Mold needs moisture to grow, because mold spores cannot germinate on a dry surface. For ideal mold growth, moisture should penetrate a porous surface, where it becomes trapped for long enough periods of time to enable mold to grow properly. Styrofoam does not hold water, and therefore presents extreme difficulty for mold spores. Except in instances of extreme saturation, like flooding, Styrofoam contains enough natural water-resistance to stop mold in its tracks.
Lack of Nutrients
Though water is essential, mold spores require more than water in order to grow. Mold, like all living organisms, can only grow when it has a source of “food.” Mold can feed on any plant organism like cotton, wood and paper, because plants contain a nutrient called cellulose. Mold feeds on other organic materials such as hair and soil, but Styrofoam contains no organic materials and does not support mold growth.
Other Factors
Styrofoam is one of the most naturally mold-resistant forms of insulation, but in very extreme cases, it can still grow mold on its surface. This happens due to a combination of excessive moisture, along with an accumulation of organic material such as dirt. In most cases, this happens when a home becomes subject to a natural disaster that compromises the insulation, like a collapsed roof or a severe flood.
Other Insulation Types
Other common forms of insulation include fiberglass insulation that consists of inorganic, or non-mold-friendly materials but contains no fungicidal coatings. Fiberglass insulation can collect organic materials like dust, however, and it absorbs moisture more readily than Styrofoam. Cellulose insulation is also available, and although it contains a coating of fungicidal chemicals (boric acid), it consists entirely or absorbent, organic material (recycled paper) and therefore can support mold growth. As a result, Styrofoam is one of your most mold-resistant options when it comes to insulation.