Insulating Against Mold Growth

Michael Andrew

Malou Hughes

October 1, 2003

Mold is probably the best known and certainly the most frequently publicized problem in indoor environments. Mold contamination cases continue to rise at alarming rates, leading to everything from finger pointing to lawsuits that demand millions of dollars in damages. These disputes can and often do envelop impact everyone involved in the design, construction, and commissioning of a building. The truth is, mold contamination cases are complicated because the causes of mold are complicated. There is no single activity, material, or precaution that you can take to prevent mold from growing in an indoor environment. Mold prevention and management begins before the first brick is laid and extend throughout the life of the building; they involve complex building systems, multiple players, and ongoing activities within the building.

Insulation’s ability to help manage temperature and moisture makes it one of the most powerful preventative tools against indoor mold growth. Correctly installed, it supports dry, temperature-controlled conditions that inhibit mold growth. Improperly installed, it can contribute to the development of mold deep within the building walls. This article examines the role of insulation systems in maintaining healthy indoor environments, common pitfalls, and best practices for selection and installation. Steps that manufacturers and installers can take to protect themselves if a mold problem does occur are also considered. While examples given don’t include an exhaustive list of insulation types or configurations, they do represent some of the most common cases, and hopefully provide a framework for selecting and installing effective insulation systems.

Understanding Insulation’s Role

To fully understand insulation’s role in maintaining mold-free buildings, it’s important to have a basic understanding of mold. In addition to a source of spores, mold requires three conditions to grow: humidity (generally relative humidity of 60 percent or more will enable growth), warm temperatures (warmer air holds more moisture than cool air), and a source of food (any biodegradable material).

What may happen in wall systems and around heating, ventilating, and air-conditioning (HVAC) systems is that warm, moist air comes in contact with cooler surfaces and condenses. The condensation and the warm indoor temperatures provide an excellent place for mold spores to germinate. As long as there’s enough moisture and the right temperature conditions, mold can grow on any surface in which there’s a food source. Even aluminum duct work, brick or stone, plastics, glass and plaster can grow mold if enough dust and other organic particles have collected on the surface.

Insulation is important because of its ability to control temperature and, by extension, moisture. Insulation is the first defense against moisture creation in a building, by preventing air from reaching dew point temperatures of surrounding air. At dew point temperatures, condensation forms, producing the moisture necessary for the development of mold. This is true both within building wall and ceiling systems and in and around HVAC systems. If insulation can effectively eliminate the source of moisture, the building is much less likely to ever have a mold problem. However, in order to serve these purposes, insulation must be properly specified, designed, and installed.

Select Proper Insulation

The first step in creating a dry, properly-functioning indoor environment is to select the best kind of insulation for a given application and location. There are numerous types of insulation available, from fiberglass to cellulose-based, from rigid to flexible. Each type is best suited for a specific use. Architects and specifiers select insulation based on their objectives, the building structure, budget, and availability. However, specifiers should add mold prevention to the long list of considerations that weigh in during the selection process. Each of the following attributes plays a role in how insulation contributes to the prevention of mold growth.


Different materials are susceptible to mold growth in varying degrees. On one hand, fiberglass, a non-biodegradable material, is resistant to mold growth. On the other hand, cellulose-based insulation, made of paper materials, is the ideal food source for mold if it becomes wet. Selecting the correct insulation material for the environment and location is essential to preventing mold growth. For example, either cellulose-based or fiberglass insulation can be used for the building envelope, but the specifier should select fiberglass insulation or some other non-biodegradable material if there is any doubt that moisture may be present in the space. Cellulose-based material should be used only when the architect can be reasonably sure that a space will be kept completely dry.


The backing on insulation can exacerbate or inhibit mold growth. Some insulation is porous with no water vapor retarder ability, and others are lined with foil, paper, or other material with possible water vapor retarder ability. Non-backed insulation allows water vapor to move freely through the material. Insulation backed with vapor retarders inhibits such movement. Selecting the right type is dependent on the moisture migration dynamics of the assembly under consideration and can inhibit or be a significant contributor to a mold problem.

Insulation that lines the interior of HVAC ducts is particularly susceptible to dust accumulation and subsequent mold growth under the right moisture conditions. Large volumes of air running through ducts deposit particles on the porous surface of the insulation, providing a food source and ideal home for mold if and when moisture is present. In areas where high levels of particulate matter, dust, or dirt are likely present, specifiers should carefully select insulation. Consideration should be made to eliminate the potential for dust build up by either externally insulating ductwork or using foil faced rigid material internally. This keeps dust deposits from collecting in the material and enabling mold growth.

In some cases, backing can support rather than inhibit mold growth. Specifiers that select fiberglass insulation for wall or ceiling systems because they worry about the possibility of water intrusion may not have considered the impact of the paper backing. If fiberglass insulation with the paper backing becomes wet, mold will grow on the paper, eliminating the whole advantage of selecting a non-biodegradable based insulation. In this case, fiberglass insulation without backing may be the best choice.


Insulation comes in as many different configurations as it does materials. Insulation should be selected in light of the area or environment in which it will be used. For example, if an area is subject to high traffic or maintenance, it’s a good idea to select rigid insulation with a durable facing to prevent damage from wear-and-tear. Insulation around ductwork can present a particular challenge. Many HVAC system components are internally lined near the fan discharge, where the insulation is subject to a lot of turbulence. In many cases, exterior insulation of the plenum isn’t possible because it would hinder easy accessibility required for maintenance. Sometimes, the boxes are lined with porous insulation, which degrades significantly over time because of frequent movement and service. Clumps of insulation eventually wear away, exposing cold surfaces and causing condensation. Although it’s more expensive, an investment in insulation packed between two layers of galvanized steel or rigid, foil-faced insulation in these areas in the immediate area of the fan discharge can prevent problems in the long-run.

Specifiers may also select uniquely-configured insulation to facilitate proper installation and ensure more complete and consistent coverage. Pre-insulated, flexible ductwork, for example, ensures that duct runs are completely and consistently insulated.

In many commercial buildings, chilled water piping is used to distribute cool water to HVAC units throughout the building. Closed-cell insulation is ideal for this application, as its solid surface is impermeable to moisture. In addition, the non-porous nature of this insulation helps prevent against the collection of dust and particles on the insulated surface.

In short, when selecting insulation, specifiers, architects, or engineers should carefully consider the environment and circumstances in which the insulation will be used in order to reduce the possibility of mold growth.

Install Correctly

If poorly installed, even the best insulation can fail miserably to fulfill its intended purpose. Once the appropriate type of insulation has been selected, installing it correctly is the next obvious, but somewhat challenging step. There are many factors to consider when installing insulation.

Insulate Completely

Insulating completely and consistently is the first rule of correct installation. Even small spaces that are left uninsulated can result in major mold problems. This is a common pitfall when it comes to insulating HVAC ductwork and chilled water lines. In many cases where correct installation becomes more difficult or takes additional time, insulation is cut short at joints and other transitions to save time. It’s critical that every point along the line be insulated so as to prevent condensation. Even a little bit of condensation on a single point along the run can formulate a constant source stream of water that can drip and migrate to areas where biodegradable materials are located, such as ceiling or wall surfaces, and initiate mold growth.

Consult the Architect

The proper installation of insulation is not as simple as it may seem. This is particularly true in wall systems, where vapor retarders are often used in areas that experience extreme temperatures. The location of the vapor retarders, if needed, in wall construction belongs should be toward the warmer side of the building: it would be toward the outside of the wall in warm climates, and toward the inside of the wall in cooler climates. Some insulation is prefabricated with instructions to install it with the vapor barrier toward the inside of the wall. However, this would not be the proper configuration in warm weather climates. These incorrect instructions may be attributed to a lack of education or simply to the climate in the location where the insulation was manufactured. In any case, installers should consult an architect on the proper configuration of a wall system to ensure that the insulation is properly installed for that particular climatic region.

Consider Interrelated "Systems"

No matter how well selected and installed, insulation alone can’t prevent a mold problem. Part of what makes the construction industry so interesting and exciting is the complexity of modern buildings and the dynamics surrounding interrelated building systems. Insulation is just one of many materials and functions that works with and among other systems in a building to create optimal, comfortable indoor environments. It’s well worth the time for specifiers, installers, and maintenance professionals to consider other building systems that are immediately related to insulation and adjust them to support its functionality. This is a particularly effective strategy in existing buildings where insulation may have already been installed and replacement isn’t a feasible option. The objective here it to identify and adjust factors directly related to, although not precisely part of, the insulation systems.

Prevent Water Intrusion

In spaces where cellulose material may have been used though fiberglass may have been a better choice, it is important to take every possible precaution to ensure that the insulation isn’t exposed to water. Avoiding water intrusion is always the most basic rule of mold prevention. Water can come from many sources, some more easily controllable than others. An inspection of the exterior walls and roof can help identify potential problems before they present themselves.

When working at a new building site, it’s important to store materials properly. If insulation and other building materials are stored outdoors or in uncovered areas, they can get wet or grow mold before they are even installed.

Increase Air Filtration Efficiency

In many older buildings, ductwork was internally insulated with porous, open-faced insulation. The cost of reinsulating is prohibitive in many cases. However, building managers and maintenance professionals can help reduce the risk of mold growth in ducts by increasing the air filtration efficiency of the HVAC unit in order to reduce the amount of particulate matter and dust that flows through the ducts. This is a cost effective and relatively easy way to protect already-installed insulation.

Incorrectly Compiled Wall Systems

If you suspect a vapor retarder is installed incorrectly, consider retaining an experienced building forensics expert to identify such problems and recommend solutions to protect against material degradation and IAQ problems. In many cases, pressure relations can offset improper vapor retarders.

Protect Yourself

Even buildings with the optimal, properly installed insulation can become contaminated with mold. In many cases, building owners-both commercial and residential-will automatically assume that the product on which the mold is growing is the cause of the problem. However, insulation within a building can become contaminated with mold, even if it was optimally selected and installed. If the building was poorly designed or constructed, or if there was a plumbing leak or an extreme weather event causing flooding, then even the best insulation system couldn’t protect against mold.

There are several precautions that insulation manufacturers and installers can take to help protect themselves against potential litigation and the inevitable finger-pointing that comes with mold contamination cases.

Have Your Products Tested For Mold Resistance

Mold resistance testing determines a product’s susceptibility to mold growth. These evaluations can test the structural integrity of a product over time as mold begins to grow on it or simply the ability of a product to resist mold under a range of "normal" to high humidity conditions. Testing can help product engineers better design insulation to withstand different conditions that may be present in different buildings. In addition, a report from mold resistance testing can be kept on file to demonstrate that the insulation product resists mold under normal conditions, implying that the abnormal conditions under which mold grew weren’t the responsibility of the insulation manufacturer.

Watch For Design Problems

Another good policy is to bring up and document obvious problems with design or construction. Take these issues to the construction team to find out if they can be corrected immediately. The contractor or designer may not have spotted the problem earlier and can make arrangements to fix it now, preventing problems later. If corrections aren’t made, a record of these problems can be used to protect the insulation manufacturer or installer from misplaced blame.

Don’t Improperly Label Insulation

Manufacturers should carefully label insulation backed with materials that are typically used as a vapor barrier. Only qualified designers or architects should provide instructions on the placement of a vapor retarder in a given climate zone. Mislabeled insulation pieces may be used in court to transfer undue liability from the architect to the manufacturer or installer. Likewise, installers should carefully follow architectural plans in conjunction with preprinted instructions when installing any material.

Insulation’s Larger Role in Creating Healthy Indoor Environments

Not all indoor air quality problems stem from mold growth. With all of the discussion and controversy surrounding mold, it’s easy to be overly focused on that single issue without considering the broader goal: to ensure acceptable, comfortable indoor environments for building occupants.

Most of the time, different insulation types involve trade-offs for indoor air quality; that is to say that one aspect of the insulation may support good indoor air quality, while another may present a threat to the indoor environment. For example, as is the case with any manufactured product, insulation materials emit volatile organic compounds (VOCs) into the air. Although fiberglass insulation is resistant to mold growth, it tends to have a high chemical content and may be a source of potentially harmful emissions. These emissions can trigger symptoms ranging from uncomfortable to debilitating. In this case, insulation could have a negative net impact on the indoor environment. On the contrary, insulation made of natural, cellulose-based fibers may have relatively low emissions, but is a sure-fire mold problem if it becomes wet. Sometimes manufacturers try to prevent mold growth on cellulose-based materials by treating them with fungicide and other chemicals, which can also emit a chemical cocktail into the air. It can be challenging to make decisions in light of these trade-offs.

Over the past few years, insulation manufacturers have come under fire regarding their emissions of formaldehyde, commonly used in the binders in fiberglass insulation. Consumers and manufacturers alike became concerned about formaldehyde emissions from their products and the potential dangers presented to building occupants. In the last year, some manufacturers have taken proactive steps to ensure that their products are acceptable for indoor environments by having them GREENGUARD Indoor Air Quality Certified®. This program provides third party guidelines for emissions levels deemed acceptable for indoor environments. Specifiers can select insulation products bearing the GREENGUARD label or can ask manufacturers for emissions information on their products. By selecting a low-emitting insulation, they can reduce the extent to which insulation negatively impacts indoor air quality, ensuring their ultimate effect is net-positive. The good specifier will consider insulation’s dynamic role in supporting good indoor air quality and will select materials that support overall health, comfort, and functionality.

Much To Be Learned

There is still much to be learned about mold contamination, the health effects, and best practices for prevention. For now, players in the construction industry must contend with litigation, negative PR, and sometimes strained relationships at the construction site.

The best way to prevent problems is to stay vigilant and to ensure complete collaboration between all players. Insulation professionals, like all others, can do their best to prevent mold growth, but will ultimately be ineffective unless other players collaborate as well. The good news is that a little prevention goes a long way and ultimately helps ensure the safety and comfort of building occupants now and for years to come.