Substrate Requirements

Mold growth requires a physical substrate that provides structural support and nutrients. Material composition, surface characteristics, and contamination determine susceptibility to fungal colonization.

Nutrient Requirements

Mold fungi are heterotrophic organisms requiring organic carbon sources for energy and biomass synthesis.

Essential Nutrients

Carbon Sources

Simple sugars (glucose, fructose)
Complex carbohydrates (cellulose, hemicellulose, starch)
Lignin (partially degraded by some species)
Proteins and amino acids
Lipids and fatty acids

Nitrogen Sources

Organic nitrogen from proteins and amino acids
Inorganic nitrogen (NH₄⁺, NO₃⁻) if available
Typical C:N ratio requirement: 10:1 to 30:1
Lower nitrogen availability slows growth but does not prevent it

Mineral Requirements

Phosphorus (P) for ATP synthesis and nucleic acids
Potassium (K) for enzyme activation
Magnesium (Mg) for chlorophyll analogs and enzymes
Trace elements: iron, zinc, copper, manganese

Nutrient Acquisition

Molds secrete extracellular enzymes to break down complex organic polymers:

Cellulases: Degrade cellulose to glucose units
Hemicellulases: Break down hemicellulose
Proteases: Hydrolyze proteins to amino acids
Lipases: Break down fats and oils
Pectinases: Degrade pectin in plant materials

Most building materials contain sufficient trace nutrients. Carbon availability is the primary limiting factor.

Substrate Categories

Class I: Highly Susceptible Materials

Materials with high organic content and easily accessible nutrients.

Cellulosic Materials

Paper facing on gypsum board
Cardboard and paper products
Natural fiber insulation (cellulose, cotton)
Wood chips and fiberboard
Kraft paper vapor retarders

Wood Products

Softwoods (pine, fir, spruce): Higher susceptibility
Hardwoods (oak, maple): Moderate to high susceptibility
Oriented strand board (OSB)
Plywood with softwood veneers
Particleboard and medium-density fiberboard (MDF)

Natural Organic Materials

Cotton and wool insulation
Natural fiber textiles
Jute backing on carpets
Cork materials
Straw and plant-based materials

Class II: Moderately Susceptible Materials

Materials with organic content requiring more aggressive conditions for colonization.

Composite Materials

Gypsum board core (calcium sulfate with organic additives)
Fiberglass insulation with organic binders
Mineral wool with phenolic binders
Acoustical ceiling tiles with organic components
Vinyl wallcoverings on paper substrate

Treated Wood Products

Pressure-treated lumber (preservative-dependent)
Hardwoods with natural resistance (cedar, redwood)
Engineered lumber with chemical additives

Organic-Modified Inorganic Materials

Painted surfaces (organic paint films)
Caulks and sealants with organic components
Adhesives and mastics

Class III: Low Susceptibility Materials

Materials requiring surface contamination or extended exposure for fungal growth.

Inorganic Building Materials

Concrete and masonry (growth on surface deposits)
Ceramic tile and porcelain
Glass and glazing
Metals (growth on surface contamination only)
Pure gypsum without paper facing

Synthetic Polymers

Polyethylene and polypropylene films
PVC piping and membranes
Polyurethane foam (closed-cell, no additives)
Synthetic rubber

Class IV: Non-Susceptible Materials

Materials that do not support mold growth without external contamination.

Stainless steel
Aluminum
Copper
Glass
Glazed ceramic
Solid plastic without fillers

Material Susceptibility Ratings

Material	Susceptibility Class	Time to Colonization (RH > 80%)	Required RH for Growth
Gypsum board (paper-faced)	I - Very High	7-14 days	75-80%
Softwood lumber	I - Very High	14-21 days	80%
OSB and plywood	I - Very High	10-18 days	80%
Cellulose insulation	I - Very High	5-10 days	75%
Kraft paper	I - Very High	5-7 days	75%
Hardwood lumber	II - High	21-28 days	80-85%
Gypsum core	II - Moderate	28-42 days	85-90%
Fiberglass (organic binder)	II - Moderate	30-45 days	85%
Mineral wool (phenolic)	II - Moderate	35-50 days	85%
Concrete (clean)	III - Low	60+ days	90%+
Painted surfaces	III - Low	45-60 days	85-90%
PVC materials	III - Low	Not typical	95%+
Ceramic tile	IV - Very Low	Requires contamination	N/A
Metals (clean)	IV - Very Low	Requires contamination	N/A

Notes:

Times assume continuous moisture exposure at stated RH
Lower temperatures extend colonization times
Surface contamination accelerates growth on all materials

Surface Contamination

Surface deposits provide nutrients on otherwise non-susceptible substrates.

Contamination Sources

Dust and Dirt

Organic matter: 5-30% by mass in building dust
Skin cells and hair
Food particles
Pollen and plant debris
Insect fragments
Textile fibers

Biofilms and Deposits

Bacterial biofilms on wet surfaces
Algae growth (exterior surfaces)
Previous mold growth residue
Water stains with dissolved organics

Chemical Contamination

Soap residues
Cooking grease and oils
Personal care product residues
Cleaning product residues containing organic surfactants

Contamination Effects

Nutrient Layer Formation

Dust accumulation creates a nutrient-rich layer on inert surfaces:

1 mm dust layer contains sufficient nutrients for visible growth
Organic content: 50-200 mg/m² typical in commercial buildings
Growth occurs on glass, metal, ceramic when dust is present

Critical Dust Thickness

Surface Type	Dust Layer for Mold Growth	Time to Visible Growth (Wet Conditions)
Glass	> 0.5 mm	21-30 days
Ceramic tile	> 0.5 mm	21-35 days
Painted metal	> 0.3 mm	14-21 days
Concrete	> 0.2 mm	14-21 days
Vinyl flooring	> 0.3 mm	18-25 days

Hygroscopic Salt Deposition

Water-soluble salts increase surface moisture availability:

Chlorides from de-icing salt spray
Sulfates from combustion products
Nitrates from atmospheric deposition
Organic salts from cleaning products

These salts absorb moisture from air, maintaining high surface RH even when ambient RH is moderate.

Cleaning Effects on Substrate Susceptibility

Cleaning Impact

Nutrient Removal

Physical cleaning removes surface contamination
Reduces available nutrients on Class III and IV materials
May not penetrate porous materials (Class I and II)

Surface Modification

Abrasive cleaning creates surface texture
Increased surface area for colonization
Micro-scratches retain moisture

Residue Effects

Organic cleaning products leave residues
Detergent films provide additional nutrients
Incomplete rinsing increases susceptibility

Cleaning Effectiveness

Cleaning Method	Nutrient Removal	Surface Impact	Residual Risk
Water rinse	20-40%	None	Low
Detergent wash	60-80%	Minimal	Moderate (residue)
Alkaline cleaner	70-90%	Slight roughening	Low (if rinsed)
Acid cleaner	60-80%	Etching possible	Low (if rinsed)
Abrasive scrubbing	80-95%	Roughening	Low
Steam cleaning	85-95%	None	Very low
HEPA vacuum	50-70% (surface only)	None	Low

Post-Cleaning Colonization

Cleaned surfaces show delayed colonization compared to contaminated surfaces:

Glass Surface Example

Uncleaned with dust: Growth in 21 days at 85% RH
Water-rinsed: Growth in 35 days at 85% RH
Detergent-cleaned: Growth in 45 days at 85% RH
Steam-cleaned: Growth in 60+ days at 85% RH

Porous Material Example (Gypsum Board)

Uncleaned: Growth in 7 days at 80% RH
Surface-cleaned: Growth in 9 days at 80% RH (minimal improvement)
Cleaning does not significantly affect Class I materials

Material Selection Criteria

Design Phase Considerations

Moisture-Prone Areas: Specify Class III or IV materials
Concealed Spaces: Avoid Class I materials where inspection is difficult
High-Humidity Zones: Use inorganic or treated materials
Maintenance Access: Select cleanable surfaces in critical areas

Susceptibility Reduction Strategies

Use paper-free gypsum board in wet areas
Specify treated lumber for below-grade applications
Apply antimicrobial coatings to Class I materials in high-risk zones
Design for surface cleaning access
Minimize horizontal surfaces that collect dust
Select smooth, non-porous finishes where possible