Mold-Resistant Construction Materials: Godsend Or Gimmick?

Quality moisture management relies heavily on guidance intended to reduce surface relative humidity below thresholds for mold.   The companion to this approach is centered on using materials that are more resistant to mold either by using naturally resistant materials or by employing materials and products manufactured with chemical inhibitors.  Mold-resistance strategies assume that materials can sustain greater moisture levels without the risk of degradation.   When both strategies are combined, objectives for minimizing degradation can be distilled to this simple equation:

Moisture Management + Mold-Resistance = Improved Durability

Godsend or Gimmick?
Moisture management and resiliency both have their place in achieving improved durability.  But what we currently see is an alarming dependence on mold-resistant materials while moisture management practices have shown only modest improvement.   Costs figure prominently in the above equation as it is significantly less expensive to integrate mold-resistant materials than it is to integrate safeguards for moisture control.  Moreover, manufacturers see mold-resistance as a competitive advantage and a potential boon to any product line having perceived resiliency.  Even ASHRAE Standard 160 recognizes the relevance of mold-resistant materials:

“Materials that are naturally resistant to mold or have been chemically treated to resist mold growth may be able to resist higher surface relative humidities and/or to resist for longer periods as specified by the manufacturer.  The criteria used in evaluation shall be stated in the report.”

Fungal Degradation of Tyvek Housewrap   Fungal Degradation of Tyvek Housewrap

Fungal Degradation of Tyvek Housewrap by the mold Cladosporium

The exemption by ASHRAE 160 does not imply that “mold-resistant” materials will invariably perform in the face of ever-increasing moisture levels and ever-lengthening durations.  The above photos attest to the fact that any material is subject to Nature’s wrath.  The standard merely stipulates that assumptions regarding mold resistance should be explicitly stated.   Such assumptions rely on manufacturers’ claims derived from standardized mold-resistance tests.  Therein lies the problem.  While mold-resistance tests provide insight into short-term performance under controlled conditions, they offer little information regarding the durability of a product’s long-term performance in real-world applications.  Below we examine four widely utilized tests and discuss some of the limitations of mold-resistance testing.

ASTM G-21 ASTM C-1338 ASTM D-3273 ASTM D-5590
Materials* Synthetic Polymerics: PVCs and Plastics Insulation &
Paints & Coatings Paints & Coatings
Test Species Penicillium,
A. niger,
A. versicolor,
P. funiculosum,
A. flavus
A. niger,
A. niger,
Test Chamber Petri Dishes Petri Dishes Suspended over a soil test bed Petri Dishes
Inoculum Spores
directly to
directly to
Specimen is suspended above soil bed Spores
directly to
Nutrient Agar No, only non-nutritive salts No No Yes; but failed tests may be reassessed without nutrient agar
Temperature 28-30°C
30   ± 2°C
R.H. 85-90% 95 ±4% 95-98% 85-90%
Duration 28 days 28 days 28 days 28 days
Microscopy Only to confirm growth
40x mag. No No
Interpretation 6-point   rating scale based on percent coverage; Test lacks pass / fail criteria 1) Fail if growth
is larger on test
specimen; or
2) No growth
criterion: fail if
any growth
Compared to photographic reference; rated based on a scale from 0(heavy growth) to 10 (no growth) Growth
assessed on
degree of coverage; 0 = no growth; 4 = heavy growth

*Each test may involve materials other than those listed.

Mother Nature Does not Comply with ASTM Standards  

Care should be taken when extrapolating test results to real-world applications.  Scored results, based on percent coverage or comparisons to reference samples, convey the degree of fungal resistance for a given set of tests.  Such scores are subject to low reproducibility, error, and subjectivity.  Despite manufacturers’ claims, the results do not represent a metric of absolute fungal resistance for comparison between tests or between products.  Here are some additional reasons why the extrapolation can go only so far.

Time – Tests are generally limited to 28 days.  Although this is a long time in the lab, it is merely a brief moment in a building’s service life.   Mold-resistance testing evaluates short-term resistance under controlled conditions that may or may not favor fungal growth.   The results are not applicable to the products’ long-term performance.

Soiled Materials – Fungal resistance testing employs pristine materials, not materials removed from the jobsite.   Resistance shown in the lab is rendered moot when the material is subject to soiling or depositions.  Just read the fine print of any product datasheet or installation instructions.

Chemical Leaching – The industry relies on a myriad of chemical compounds to prevent defacement by mold and other microorganisms.  Unfortunately, many of these chemicals are also water-soluble or are otherwise modified by photo-chemical processes.  Presumed resistance, as demonstrated in the laboratory, is not necessarily achieved in the field due to chemical leaching or modification.  Exposures to ultraviolet light, oxidation, acids, and even biological degradation can also significantly reduce product resistance.

Number of Test Organisms – There are over 65,000 known species of fungi.  Estimates of undiscovered species range in the hundreds of thousands.  It would be wrong to assume that three to five test organisms represent the entirety of risk.

Spores vs Hyphae (cells) – All of the reviewed tests utilize spore suspensions as the means for inoculating test specimens.  In fact, standardized test methods go to great lengths to rid the spore suspensions of stray hyphae, which often proliferate quite well under conditions that are restrictive to spore germination.  It is important to note that fungal hyphae, like fungal spores, are ubiquitous in nature, jobsites, and the built environment.

Visible Mold – Tests generally emphasize non-microscopic means for assessing percent coverage and comparisons with reference controls.  Materials may actually contain a thriving lawn of fungi undetected by the resistance test.  Such oversights are often due to fungal hyphae being less pigmented than spores and spore-bearing structures.  Without this pigmentation, hyphae appear more translucent or only faintly colored.

Relative Humidity vs Bulk Water – The standard test protocols rely on high humidity as the moisture source.  Although specimens may come in partial contact with wetted agar, they are generally not subject to direct wetting.  In the real world, it is reasonable to assume that building enclosure components will be subject to occasional wetting as well as high humidity.  Punctuated wetting events coupled with high humidity can significantly improve the likelihood of spore germination and proliferation.  Standard mold-resistance tests do not account for this condition.

Implications for Hygrothermal Modeling
As building mycologists and long-time users of hygrothermal modeling, we are often asked to model mold-resistant materials.   Like other WUFI practitioners, we use ASHRAE 160’s criteria for evaluating assembly performance.  Of ASHRAE 160’s criteria triad, the one most often breached is the 30-Day running average at 80% relative humidity and 41°F.  This is a reasonable standard for untreated wood and other lingocellulosic materials.  But it may or may not represent a realistic threshold for mold-resistant materials.  Moisture-temperature isopleths for various fungal species and resistant building materials can be found in the scientific literature.  When in doubt, we encourage modelers to understand and reference these studies.

In our simulations, we generally use two thresholds when evaluating mold-resistant materials.  ASHRAE’s 30-day running average is viewed only as a conditional threshold – in other words, conditions neither pass nor fail.  We also use a 30-day running average at 95% RH and 41°F as an absolute threshold.  Meeting or surpassing these conditions during a 30-day running average results in a ‘fail’ performance rating for all potentially nutritive materials, including mold-resistant materials.  The modeler’s best judgment should be used when conditions occur between these two thresholds.  It is important to note that our 95% / 41°F criterion is not part of ASHRAE 160 but is derived from published isopleths for numerous mold-resistant materials.  No endpoint criterion is perfect for all materials.   After all, the role of hygrothermal modeling is to seek and resolve tendencies, not absolutes.  A good portrayal of these tendencies emerges when WUFI results are plotted as 30-day running averages with the ASHRAE 30 day criteria and our 30-day resistant criteria.  Further comparison with published fungal isopleths can vastly improve confidence in model outcomes.

Hygrothermal Snapshot 5


ASTM C1338. Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings.

ASTM D3273. Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber

ASTM D5590.  Standard Test Method for Determining the Resistance of Paint Films and Related Coatings to Fungal Defacement by Accelerated Four-Week Agar Plate Assay

ASTM D4300.  Standard Test Methods for Ability of Adhesive Films to Support or Resist the Growth of Fungi

D4783 Test Methods for Resistance of Adhesive Preparations in Container to Attack by Bacteria, Yeast, and Fungi

ASTM E2471.  Standard Test Method for Using Seeded-Agar for the Screening Assessment of Antimicrobial Activity in Carpets

ASTM G21.  Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi

ASTM D5324.  Standard Guide for Testing Water-Borne Architectural Coatings

ASTM D3730.  Standard Guide for Testing High-Performance Interior Architectural Wall Coatings