Energy efficiency
Moisture control
Health & safety

Designing with mold and mildew resistant insulation

Ryan Martin
Ryan Martin
July 11, 2019

Material properties are the key to achieving mold and moisture-resistant assemblies

Designing with moisture resistant insulation when paired with moisture resistant drywall helps avoid mildew on walls, the negative health effects of mold

Energy efficiency has become a key objective in today’s built environment. It’s an important goal, given that buildings currently consume 30 percent of the world’s energy use, and it’s estimated that up to 30 percent of that is wasted.

To address the issue, architects and designers are looking to improve energy performance by increasing building insulation levels and creating tighter building envelopes.  Both are important strategies that can lead to improved comfort and durability, reduced environmental impact, and long-term energy savings over the life of the building.

That said, high-performance envelopes can bring their own set of challenges, especially if moisture issues arise.  Why?  Higher levels of insulation can lead to less energy transfer through the building envelope which slows or prevents drying of any moisture present in the wall assembly. In addition, tighter envelopes can result in higher concentrated air leakage areas.

Without adequate air ventilation, this can lead to mold, bacterial growth and rot, which ultimately can impact the durability and safety of the structure and the health and wellbeing of its occupants. However, it is possible to effectively prevent mold if you understand the fundamentals.

That’s where mold- and mildew-resistant insulation and other water-resistant building materials should be considered with every project to create a healthy and productive living environment.

This article is structured in five sections beginning with what is mold and what causes mildew, working into preventing mold in your indoor environment, a case study on resilient building practices, the codes and standards you need to know, and finally, mold and mildew proof insulation solutions.


What is mold?

Mold is a fungus that is a normal part of the outdoor environment, aiding in the decomposition process of organic materials such as fallen leaves and trees.  While mold can play a beneficial role in nature, it can be very problematic in an indoor space, feeding off wood, gypsum, wallpaper, and other organic materials. There are many different kinds of mold including black molds, brown molds, white molds, gray molds, green molds and many other colors, with some species being more concerning than others.  Below is a brief description of the most common type of mold.

Black molds are usually dark green, gray or black in color, and typically grow in warm, moist environments such as kitchens, basements, showers, toilets, and baths.  Most of these molds release toxins that can cause allergic reactions in people such as itchy eyes, itchy skin, stuffiness, wheezing and coughing or more severe symptoms such as headaches, exhaustion, fever and difficulty breathing.

White molds typically grow in water-damaged areas of a home, they get their white color because their spores are not pigmented.  White mold is sometimes less obvious because it has a powdery appearance that often blends with the material on which it grows.  Sometimes, people confuse white mold with mildew.  However, mildew rarely grows on surfaces other than plants and doesn’t destroy materials.  Conversely, white mold penetrates the surface of porous materials like wood or drywall and can cause rot.  White mold poses similar negative health effects, or problems, to those of black molds.

Green molds are abundant, with thousands of green mold species documented.  Aspergillus, Cladosporium, and Penicillium, are the most common molds that produce a green hue.  The green colour is often caused by the material they grow and feed on, the climate or region they inhabit.  Most green mold species produce mycotoxins (a toxic substance produced by a fungus) and are associated with allergic symptoms such as watery eyes, itchy skin, sneezing, coughing or respiratory issues.

Brown molds can appear brown, tan or dark yellow and appear as dark patches on wood or tiles.  Common types include Pithomyces, Chartarum, Aureobasidium and Stemonitis.  Brown molds, like most molds, can damage structures including homes and buildings, and the spores from brown molds can cause asthmatic or allergic reactions.


How does mold spread?

Mold can reproduce quickly and easily through airborne spores that are invisible to the eye. Because spores travel easily, they can enter indoor environments through the air or water, through open doors and windows and on the clothing and shoes of occupants and guests.

Spores can also be introduced into an indoor space through building materials or other contents that have had previous exposure to spores or existing mold.  It is impossible to prevent the migration of mold spores, which can be detected in virtually all outdoor and indoor spaces. However, spores alone do not lead to mold as the right conditions must exist to support growth.

What causes mildew?

The cause of mildew is best explained by focusing on the spores that can grow into mold colonies under the right circumstances.  Those circumstances can be best explained through four variables that are needed for mold spores to grow and thrive indoors:

  • Moisture/water/humidity
  • A food source (organic matter) 
  • Oxygen 
  • A temperature between 40 degrees and 100 degrees F (provide Celsius conversion)

Why is mold concerning?

The health effects of mold can be drastic and in some cases, hazardous. Beginning with the negative impact toward indoor air quality (IAQ), contributing to the release of microbiological volatile organic compound (MVOCs) and toxins. As a result, mold can adversely harm your health and wellbeing.

The presence of mold and/or mildew in homes and buildings has been linked to a variety of health conditions for occupants including allergic reactions, respiratory illness, skin, eye, nose and throat irritation or infection. Some studies including the ASHRAE Position Document on Limiting Indoor Mold and Dampness in Buildings, have found strong evidence that indoor mold exposure can lead to asthma development in children and other sensitive persons such as people receiving treatment for cancer, those who have had an organ or stem cell transplant, and individuals taking medicines that suppress their immune system.  Some highly-allergic individuals may even have life-threatening reactions to mold, depending on the time and nature of exposure.


The best and most reliable way to prevent mold or mildew growth is to keep a building dry.  Mold cannot grow without the presence of moisture.  The most effective time to employ moisture prevention measures is during the design and construction phases of a home or building – applicable to both new build and renovation or retrofit projects.

Creating a tight building envelope will help to discourage moisture penetration and air leakage that can be a significant source of moisture issues that can lead to mold problems. It is for that reason that in this section we consider both the interior and exterior applications for mold and mildew prevention. This is followed by guidance on material selection and a checklist for evaluating insulation solutions.

Interior walls, attics, and ceilings

While exterior walls remain a primary focus, it’s equally critical to prevent and address moisture issues in other key areas of the building envelope and to consider all potential moisture sources.  Moisture can exist in interior areas of a home or building and causes of excessive moisture can include:

  • Rainwater leaks through roofs and walls,
  • Leakage of moist air,
  • Diffusion of water vapor through walls, roofs, and floors,
  • Groundwater intrusion into basements and crawl spaces through walls, floors and foundations,
  • Leaking or burst water pipes,
  • Indoor moisture sources, and
  • Construction moisture.

Interior walls and ceilings

The same is true of interior walls.  What causes mold or mildew on walls or ceilings? The most common cause of excess moisture in interior walls or ceilings is a leaky pipe.  Secondary causes can be due to high humidity levels, particularly in moist rooms such as bathrooms and kitchens.  A non-organic, vapour permeable insulation with excellent drying potential such as stone wool is ideal for these applications.  

When it comes to ceilings, a wide variety of options exist, including mold-resistant stone wool ceiling tiles. These are a top choice in all applications, but particularly where indoor air quality (IAQ) is paramount, such as in hospitals, where the health effects of mold present a more serious threat to immune-compromised patients. 

Attics and roofs

Common areas of concern include the roof and attic.  Moisture, wetting or leaks can be caused by damaged or deteriorated shingles, failed flashing details at penetrations or walls or ice dams caused by warm air leakage into the attic.  Sure-fire signs of moisture in the attic or roof of your home or building is a noticeable leak on visual inspection, wet attic insulation and drywall (when moisture resistant drywall was not used), wet insulation in your crawl space, and mold or mildew in the attic.

Exterior walls

When it comes to moisture prevention, energy and building codes are a good place to start. In fact, several variables must first be considered during the design and specification phase of a project, including the following:

  • Climate zone: Wall assemblies need to be constructed to suit their unique climate zone, since each zone will experience different moisture/rain and humidity levels, and temperatures. Depending on climate zone, the building codes and standards may also have specific vapor control requirements.
  • Vapour diffusion: Vapor diffusion is the movement of moisture in the vapor state because of a vapor pressure difference (concentration gradient) or a temperature difference (thermal gradient). Overall, the direction of vapor drive has important ramifications to the placement of materials within a wall assembly.  Improper placement of materials in a wall can lead to condensation on colder surfaces, water-damaged insulation and building materials, as well as fungal growth.  Often referred to as breathability, vapor permeability describes a material’s ability to allow water vapor to pass through it (measured in units called perms).  Materials with lower perm ratings are better at stopping the movement of water vapor. It should be noted that all materials have a vapor resistance, also referred to as permeance, and can be categorized as permeable (greater than 10 perms), semi-permeable (10 perms or less and greater than 1.0 perm), semi-impermeable (1.0 perms or less and greater than 0.1 perm) or impermeable (0.1 perms of less).
  • Air pressure: The difference in air pressure between the interior and exterior of a building drive air through a wall assembly from the high-pressure side to the low-pressure side.  To control air leakage in building enclosure assemblies, an air barrier membrane or system is installed.  Attention to connections and the use of tapes and sealants is important to ensure a continuous air barrier.  Air leakage is not dependent on the material properties of the air barrier, but most often occurs due to holes in the air barrier.  When air leakage does occur, it does carry moisture.  If the air then meets a surface below the dew point temperature of the air, condensation can occur that can lead to fungal growth or degradation of the assembly and its components.
  • Vapour retarders, membranes and more: Vapour-retarding materials are used to control vapour diffusion in the wall assembly.  Their purpose is to manage moisture and maximize drying potential should moisture be present in assembly materials during construction or somehow become wet in service. Special consideration must be given to placement of the vapor control layer, which will be highly dependent on climate and seasonal vapor drive where a given building is being constructed.  Improper placement can create condensation issues and inhibit drying potential of the wall, leading to failure of the wall system.  Further, taping and detailing must be undertaken with care and precision to prevent air leakage and potential moisture issues that can result.

Mold-resistant insulation options

The makeup or composition of the material is key when looking for mold-resistant insulation.  Stone wool, for example, is made primarily of basalt rock – a natural volcanic rock that is found abundantly in the earth.  The rock is melted, along with recycled slag and spun into stone wool insulation.  Because it is made of stone, an inorganic material, it does not act as a food source for spores or mold.  As such, stone wool insulation will not contribute to the growth of mold, mildew, bacteria or rot.  It will instead resist mold, helping to preserve indoor air quality (IAQ) for occupants of the home or building.

Several of our ROCKWOOL insulation products are tested to ASTM C1338 – A Standard Test for Determining Fungi Resistance to determine the relative ability of insulation and its facing to resist fungal growth under conditions favorable to their development. Stone wool consistently passes with zero fungal growth. Some of our products have also been GREENGUARD® Certified to the highest level for their ability to promote excellent indoor air quality.

We receive quite a few questions and inquiries about the different types of insulation that exist in the marketplace and which type is most suitable for mold-resistant construction.

Many homeowners and building owners wonder about the specific type of insulation present in their existing home or building.  They scour resources for information such as: Is fiberglass insulation mold resistant? Will fiberglass insulation mold? Can mold grow on spray foam insulation? Does foam insulation mold?  If insulation gets wet is it ruined? Will insulation mold if it gets wet? Do I have to replace insulation that gets wet?  These are very relevant questions for architects, contractors, and homeowners alike.

Properly installing the right materials, in the right places, can be helpful in reducing the potential for mold and mildew growth in the first place.  The answer to which insulation is best when considering the potential for mold lies in the material properties. 

Checklist: the material properties of mold-resistant insulation

The below five properties provide an overview of what to look for when you’re in the market for mold and mildew resistant insulation materials for the walls of your built environment.

  • Non-organic composition – Building materials composed of non-organic raw materials do not provide a food source for mold growth, and therefore, will not promote the growth of mold or other fungi.  
  • Moisture resistance – Because the presence of moisture is the biggest factor in creating suitable conditions for mold growth, it is best to select an insulation material that demonstrates moisture resistance -- or better yet, hydrophobic properties.  Using materials that limit or reduce the potential for moisture absorption can help lower the potential for conditions conducive to mold growth.
  • Vapour permeanceVapor diffusion is the movement of water vapor molecules through porous materials because of vapor pressure differences. In a cold climate, vapor typically diffuses from warm indoors to cold outdoors.  Conversely, in a warm climate, vapor typically diffuses from warm, humid exteriors to cool, air-conditioned interiors.  This can vary, based on weather conditions and air pressure.  In climates with extreme fluctuations from harsh, bitterly cold winters to humid, hot summers, vapor drive changes seasonally. This movement directly influences material placement in your wall assembly. When it comes to the building envelope, the goal should be to prevent moisture from getting trapped and condensing, providing a key condition for mold growth.
  • Drying potential – Moisture can enter a wall or roof assembly in a variety of ways such as rainwater intrusion, vapor diffusion, during construction, leaky pipes, or because of high indoor humidity.  Some building materials hold moisture and have poor drying potential, creating ideal conditions for mold growth.  Consider choosing insulation with high drying potential, so that should it encounter water, the insulation will dry out and retain its original R-value.  
  • Dimensional stability –  Some insulations by nature of their composition or structure can shift, settle or slump in the wall cavity, while others—in the case of roofing insulation—can shrink over time.  Any gaps or voids in an assembly can be a source of air leakage, which can carry with it significant moisture loads.   Therefore, insulation that remains dimensionally stable over time, like stone wool, can help prevent voids and gapping which can lead to air leakage and moisture in the wall cavity or roof.


It’s time that we started rethinking the status quo. How wall systems are designed has changed a great deal over the years.

Building science has taught us that changing one component can affect a whole system, so as we change our way of building or the materials we build with, it can impact moisture management and mold potential.  In fact, the advancement in building materials has made it easier to design buildings that are more resistant to mold.

Some examples include moisture resistant paneling for bathrooms, moisture resistant paneling for walls, moisture-resistant drywall, drywall with a fiberglass facing instead of paper; mold-resistant stucco, paint and caulking; smart membranes that allow vapor to flow in either direction depending on conditions, as well as the increased use of non-organic insulations with high drying potential, such as stone wool insulation.

Mold-resistant building materials often complement each other well.  A combination of new materials and a better understanding of moisture management and factors that fuel mold growth is allowing designers, architects, and builders to create wall systems and buildings that are more resilient and resistant to mold growth.

A case study: The Better Basement Wall – Doug Tarry Homes (St. Thomas, Ontario, Canada)

Premium Custom Home Builder, Doug Tarry of Doug Tarry Homes noticed moisture condensing on the polyethylene vapor barrier in the basements of some new builds, causing water to pool on the basement floor and creating conditions favorable to mold growth. Homeowners called him in to inspect, mistakenly concerned that the basement foundation was leaking.

The condition was particularly notable during the summer months, in homes with poured concrete walls, and where full-height batt/bag wrap insulation was used with full-height polyethylene on the interior side of the stand-off wall.  Below grade, the wall could not dry to the outside due to damp-proofing, drainage membrane, and soil.  It also could not dry to the inside due to the poly layer. The issue was not uncommon in many new homes across the province due to similar wall assemblies despite meeting and at times, exceeding code requirements.

The goal was to bring the air barrier to the warm side of the wall and use poly as a combination air/vapor barrier.  The other step was to change the top portion of the poly to a membrane that can change permeability, sometimes referred to as a smart membrane, to permit the summer vapor migration to dry through the wall into the conditioned basement. Using materials that wouldn’t trap moisture was integral to the new wall system.  

The Solution

A new wall design was developed to create a more forgiving wall system based on these principles:

  1. Use materials that won’t trap moisture: The smart membrane allows vapor to flow in the correct direction, even with seasonal changes in weather conditions.  One solution is to use stone wool insulation, which offers excellent drying potential and does not promote mold growth.  It allows vapor to flow through without collecting while remaining dimensionally stable meaning it holds its shape and performance decade after decade.
  2. Change the location of the air barrier: bringing the air barrier to the warm side of the insulation reduces air and vapor flow into the wall cavity during the winter.
  3. Allow drying paths for moisture migration: that way any vapor that does enter the wall assembly and condenses can migrate down and under the floor slab.

See the full case study



Over time, learnings from the building science community and professionals within the building industry has advanced our knowledge and approach to building design.

While codes and standards provide adequate guidance pertaining to the construction of homes and buildings—from energy efficiency to safety and fire resilience—it can be difficult to determine best practices and solutions for mold and moisture prevention, given the multitude of variables, building material innovations and construction practices.


ANSI/ASHRAE 160 – Criteria for Moisture-Control Design Analysis in Buildings serves as the industry standard on how to build the hygrothermal model and how to assess the performance and associated risks based on validated research and references.  According to ASHRAE themselves, the standard specifies performance-based design criteria for predicting, mitigating, or reducing moisture damage to the building envelope, materials, components, system and furnishings, while taking into account the variables of climate, construction type and HVAC system operation.

While new tools and modelling software such as WUFI, HAM and hygRIC are available to designers to assist with their envelope design and materials selection, it’s important to keep in mind that hygrothermal modeling can be challenging and ensuring appropriate inputs is critical for accuracy is the assessment.

Below you can find a map of ASHRAE building code adoption by U.S. state and a second image featuring adoption of the National Energy Code for Buildings (NECB) by province / territory in Canada. Note that some provinces have developed their own building standards to reflect the specific requirements and needs for their regions. 

Understanding the impact of building codes and standards

“Codes and standards continue to adopt considerations for energy efficiency, resiliency and sustainability. In turn, as a design industry we need to adapt our way of designing building enclosures to account for both the required changes but importantly, ensure we are not creating unintended consequences. We need to understand the characteristics of the materials we choose for our designs, and their specific control function (be it either thermal, air, water, vapor, and/or sound and fire).”

- Alejandra Nieto, Research and Development (R&D) Project Manager, ROCKWOOL 


Working with ROCKWOOL Building Science

Those looking for tailored solutions, building science support or modelling can connect with ROCKWOOL Building Science (RBS).  Our team provides advisory services to unlock the potential in buildings and is led by building science and energy-efficiency experts from top-tier industry and educational institutions. The RBS team offers resources and services in five core competencies:

  • Building Science Expertise: Building enclosure analysis, detailing and material specifications
  • WUFI Modeling: 1-d transient hygrothermal analysis, heat, air and moisture analysis, roofing heat transfer models (climate-driven R-value)
  • Thermal Bridging: THERM models (2D) / HEAT 3 Models (3D), overall U-value analysis, insulation detailing analysis
  • R-value Calculations: Code and standards compliance, overall effective R-value calculations, dew point analysis
  • Full Building Modeling: DesignBuilder and IES-VE energy modeling, included HVAC and electricity (default) and building envelope sensitivity analysis.


Designing and building with the best mold and mildew resistant insulation and building materials will help you be proactive in creating more resilient residential and commercial structures.

Choose to design and build with the insulation products that contribute toward healthier indoor living and working environments to support the improved wellbeing of building occupants. ROCKWOOL stone wool moisture resistant insulation will not absorb or hold water and offers excellent drying potential in your wall assembly.

Whether the use is for the exterior walls, roof, and/or interior applications, we have the products that meet your needs in residential and commercial construction.

  • Residential: SAFE’n’SOUND®
    • ROCKWOOL SAFE‘n’SOUND® is a stone wool insulation for use in interior partitions of residential wood and steel stud construction where superior fire resistance and acoustical performance are required.
    • Applications: SAFE’n’SOUND is ideal for interior wall partitions of residential wood and steel stud construction, as well as ceiling and floor applications where acoustic dampening performance is fire resistance is required. SAFE’n’SOUND is ideal for bathrooms, bedrooms, nurseries, home offices and theaters, laundry rooms, garages and in-between floors, ROCKWOOL Safe’n’Sound stone wool insulation promotes a quieter, more tranquil home.
    • Benefits: Non-combustible, resisting fire up to 2,150˚F or 1,177˚C, will not promote flame spread or smoke development and provides passive fire protection; offers excellent sound dampening due to its density and non-directional fibre structure; easy handling, friction fit and dimensional stability; GREENGUARD Gold certified.
  • Residential/Commercial: AFB®
    • ROCKWOOL AFB® is light-weight and semi-rigid batt insulation designed specifically for commercial green-building applications, where fire resistance and acoustic performance is required. For steel stud and interior wall and floor applications, ROCKWOOL AFB® is available in thicknesses of 1 to 6 inches, making it suitable for both retrofit and new construction applications.
    • Applications: It is ideal for friction fit into wall partitions and system applications from party walls to plant/manufacturing walls.  
    • Benefits:  Resistant to mold, mildew and bacterial growth, water repellent, non-combustible, chemically inert, and dimensionally stable
  • Residential: COMFORTBATT®
    • ROCKWOOL COMFORTBATT® is a semi-rigid, water repellant, dimensionally stable and fire resistant thermal batt insulation designed for wood and steel frame construction. Its unique flexible edge compresses and springs back to prevent gaps, maximizing thermal performance.  
    • Applications: Ideal for exterior walls, attics, basement headers, ceiling and floor frames. 
    • Benefits: Resistant to mold, mildew and bacterial growth, COMFORTBATT® is easy to handle and installs with a standard bread knife.  It is available in a variety of R-values and thicknesses.
  • Residential: COMFORTBOARD™ 80
    • ROCKWOOL COMFORTBOARD™ 80 is a rigid, high-density mineral wool insulated sheathing board that provides superior protection against moisture, fire and sound. 
    • Applications: This non-combustible and UV-resistant insulation is intended for exterior and interior non-structural insulation sheathing applications including continuous exterior insulation, basement wall, under slab and below-grade exterior wall applications.
    • Benefits: COMFORTBOARD™ 80 is thermally efficient, vapor permeable and allows for superior drying potential. Offering a stable, long-term R-value, COMFORTBOARD™ 80 is available in thicknesses of 1.25 to 3 inches.
  • Commercial: COMFORTBOARD™ 110
    • ROCKWOOL COMFORTBOARD™ 110 is a rigid, high density mineral wool insulated sheathing board that provides superior protection against moisture, fire and sound. 
    • Applications: This non-combustible and UV-resistant insulation is intended for exterior non-structural commercial and industrial high performance insulation sheathing applications.
    • Benefits: COMFORTBOARD™ 110 is thermally efficient, vapor permeable and allows for superior drying potential. Offering a stable, long-term R-value, COMFORTBOARD™ 110 is available in thicknesses of 1 to 3 inches.
  • Commercial: CAVITYROCK®
    • ROCKWOOL CAVITYROCK® is a non-combustible, lightweight and water repellent high-density, semi-rigid insulation board offering durability and enhanced strength. 
    • Applications: Used as an insulating layer in exterior cavity and rainscreen applications to provide valuable thermal, fire and moisture protection. It is compatible with most air/vapor barrier systems, adhesives and wall ties.
    • Benefits: CAVITYROCK® delivers an R-value of 4.3/inch. Available in mono-density (up to 2 inches in thicknesses) or dual-density (from 2.5 to 6 inches.

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