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Why Does Insulation Turn Black?

Insulation is important in some homes, this is to regulate temperature at home. This article discusses why insulation turns black and how to dispose of damaged insulation.

A person holding a heat loss detection device.

There is nothing more shocking than opening up a wall in your home to come across a dark patch, perhaps even a full wall of black discoloured insulation. Immediately you may assume it is black mold, but first we must assess the situation before determining what really might be causing this problem.

An important element in any North American home, insulation reduces heat loss and heat gain by providing a barrier between the inside of your home and the ever-changing temperature outside. No matter the season, insulation allows the temperature indoors to stay consistent without the air conditioner or heater constantly running.

Fiberglass is the most common home insulation material in North America due to its availability and low-cost. Fiberglass insulation, also known as “glass wool” is made by weaving fine strands of recycled glass with sand, soda ash, limestone and other minerals into a material that minimizes heat transfer, reducing energy consumption in the home.

When working with this material, caution is always recommended as it will cause irritation and serious damage to eyes, lungs, and even skin if in direct contact.

A pink fiber glass insulation for ceiling.

Due to the nature of the material it is unable to truly catch on fire, but will melt at 1,000 degrees Fahrenheit (540 degrees Celsius). Though fiberglass insulation is non-flammable, it is indeed quite porous, with the ability to soak up moisture much like a sponge, holding it indefinitely until properly dried.

When was Fiberglass Insulation First Introduced into Homes?

A person with protective gloves installing fiberglass.

With the development of the famous heat-resistant “rock wool” in the late 19th century, a naturally-made, but hazardous insulation material, American researcher Dale Kleist of the Corning Glass Company believed he could create an even greater insulation.

After many failed attempts at fusing glass blocks together to create clear, waterproof walls; a stream of compressed air hit the flow of molten glass, forming a spray of tiny fibers. This sheer accident led to Corning Glass’ patent for Fiberglas* in 1936. Soon after, Corning Glass partnered with Owens-Illinois, another glass manufacturer, forming the company Owens Corning in 1938.

Why is Fiberglass Insulation Pink?

Pink fiberglass for roofing insulation.

It did not take long before rival products were created, leading Owens Corning in 1956 to add red dye to their insulation. This small visual detail set their product apart from their competition, marketing it as PINK which they later trademarked in 1985.

In August of 1980, Owens Corning partnered with the iconic Pink Panther in their advertisements. 42 years later, the panther is still found today on the outside packaging of their insulation.

So, Why Does Insulation Turn Black?

A person looking at a house through magnifying glass.

The use of low-vapour permeance membranes, as we know them today as polyethylene vapour barriers, became mandated in the 1950s as a solution to prevent condensation and moisture from building up. Homes built before this time did not use barriers, leaving the absorbent insulation material vulnerable to leaks that could go undetected for years.

Usually older homes built without polyethylene vapour barrier see damage not only from leaks, but from streaks of dark discolouration. Today, these streaks are still commonly found on insulation for one of two reasons. This dark staining could be mold, but it is most likely to be dust and dirt debris that has slowly accumulated on the surface of the batting.

It is important to assess both sides of the insulation and check where the dark streaking begins and ends. Does the wood framing around the insulation have black spots? If not, it is most likely an indicator of dirt rather than mold.

Mold tends to thrive on organic matter like wood and the paper on drywall, whereas fiberglass alone is not able to house mold unless it is constantly wet for a long duration of time. Due to its high absorbency, fiberglass insulation does not fare well with leakage, but does act as an effective filter for air-borne dust particles.

When exposed to outside elements through an exposed area on the interior or exterior wall, insulation acts as a filter, much like that of a furnace, trapping dust as the air is pulled through the material. When dust accumulates on the surface of insulation it slowly makes it less effective over time.

The best step to fix this issue would be to remove the existing damaged insulation and block the exposed area with either caulking or spray foam and insulation board. After this area is securely checked for any remaining holes, it is best to replace insulation with new batts and finish with a vapour barrier.

Black Mold on Insulation

A scientist holding a petri dish growing molds.

When dirt debris is ruled out when dealing with black streaking on your insulation and you are unable to locate an exposed area in your walls that could cause dust build up, mold is most likely the culprit.

Black mold, also known as stachybotrys chartarum, is a variety of microfungus that needs moisture to grow. It reproduces by releasing thousands of small, dangerous airborne spores. These spores are extremely hazardous if inhaled, causing severe respiratory issues.

A person with mask cleaning molds on the wall.

Technically, fiberglass itself is not a viable food source for molds. When perpetually exposed to debris, water, and organic matter like the surrounding wood beams and drywall paper, it is very possible for mold to form on the surface of insulation, especially on the insulation’s backing. Moisture and oxygen enhance mold growth, causing the mold to accelerate in reproduction

First and foremost, it is important to address the root of the problem before reinstalling the material to ensure the underlying issue does not resurface. It is always best to replace any discoloured insulation to ensure the dirt debris or mold spores, even when dried out, do not spread and travel to other areas of the house.

Innovative Insulation with an Environmentally Conscious Twist

A person holding a lab grown mold.

Although we try to keep mold out of our wall’s interior as much as possible, what if adding fungi to the walls could be a step towards a more sustainable future? As vital as the insulation industry is to North American home-owners, researchers and scientists are currently developing more sustainable materials to slowly replace foam and fiberglass insulation.

London based company Biohm has created the first ever mycelium based home insulation. Mycelium is the strong filamentous root structure of fungi that contain different characteristics depending on the strain.

In this case, researchers have studied specific strains to develop their insulation with strength and web formation in mind. Biohm’s bio-insulation is made from 100% agricultural by-products that would otherwise go to the landfill. It is used in a way that the mycelium consumes and completely envelopes the waste to form a strong and dense product.

This material is grown in forms and then heated to kill the organism, leaving a light-weight sturdy material. Not only is it fire resistant, but it does not contain synthetic resin-based materials. These resins fuel flames to spread more easily while simultaneously causing harmful smoke.

Due to its nature, it is also mold resistant. Though this research has been underway for many years, it is at the forefront of future sustainable design for the insulation industry.

How to Properly Dispose of Damaged Insulation?

With the ever-evolving improvements the insulation industry implements, fiberglass insulation is still a product that needs replacing when it no longer regulates a house’s temperature. It is suggested that fiberglass insulation in your home should be inspected every fifteen to twenty years as the insulation tends to slowly fall from fiberglass batts.

Fiberglass insulation is able to last up to one hundred years in roll form. Similarly, foam insulation or house wrap can last more than eighty years.

Replacement of an aged fiberglass insulation.


There are many factors that can compromise the life span of insulation. As noted above: mold, water damage, pest infestation, visible signs of someone in the household experiencing symptoms of allergies or illness, and an inability to regulate temperature within the house all are signs that the insulation needs to be inspected.

After assessing your insulation and deeming it damaged, it is time to safely discard and replace it. Every city has its own laws and requirements to properly dispose of certain materials.

The best way to dispose of your damaged insulation would be to call a local waste collection agency who will be able to walk you through the process of a pick up or drop off to your nearest safe disposal site. When removing the insulation, make sure to be wearing the appropriate attire and protection.

This will include: gloves, safety glasses, a respirator mask, proper closed footwear, and full length pants and sleeves. Fiberglass can irritate the skin and is dangerous to inhale, so plan to work in a well ventilated room when bagging and removing the material.

The best bags to dispose of fiberglass insulation in are contractor garbage bags that are both large and durable. After the bagging is complete, vacuum or gentle sweep the area to ensure no left over particles spread throughout the home. When disposing of the bagged material, consider asking a local waste disposal authority if there is a facility close by that recycles fiberglass insulation.

It is not common, but fiberglass insulation is able to be redeveloped into thermal or acoustic insulation. Disposing of fiberglass insulation can be a fairly simple process. It is important to remember to follow each step carefully and when unsure, always call in a professional for assistance.

A professional holding a cutter installing fiberglass insulation.

Though it might at first feel intimidating to come across a black streak in your insulated walls, it is always warranted to ask for a second opinion and assess the situation through the clues presented. Whether it be dust and debris collecting on your insulation from an exposed air pathway, or black mold itself leading you to a bigger leaking problem, both these issues are manageable and should be done with caution and safety at the forefront.