Photo: Beyza Erdem

Does mold grow behind insulation?

Technically, mold does not typically grow on the insulation itself. Fiberglass is not a viable food source for mold. Yet mold growth is often found on the top surface of attic insulation.

environix.com - Mold on Attic Insulation - Environix

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Mold on Attic Insulation - How to Deal With it Mold growth on the insulation in an attic is much less common than growth on the sheathing or framing. This area is less conducive to mold growth because the insulation is relatively warm compared to the roof sheathing. There are three main reasons we care about insulation in an attic when conducting a mold inspection. First, the insulation is often blocking the soffit venting, which limits the amount of airflow into the attic. Second, in certain circumstances, mold growth can actually occur on the insulation itself. The last one is weird. If an attic has too much insulation, condensation actually becomes more likely. Why? Because extra insulation means less radiant heat making it into the attic. Duh – this is the whole point of installing insulation. However, there is an unintended consequence. If no heat radiates into the attic, the roof sheathing will remain the same temperature as the outside air (or close). When cold, moist air enters through the attic vents and hits the underside of the roof sheathing, it now hits a very cold surface. Because the sheathing is cold, the moisture condenses on the surface and mold growth occurs.

FAQ’s related to attic insulation issues.

Why does mold growth occur on attic insulation?

Technically, mold does not typically grow on the insulation itself. Fiberglass is not a viable food source for mold. Yet mold growth is often found on the top surface of attic insulation. How does this occur?

The mold observed on the insulation is not actively growing, but is due to spores falling from the growth on the roof sheathing above. The mold is growing on the settled dust on top of the insulation.

What does the mold growth typically look like?

Attic mold often appears as a thin grey or black layer on the very top of the insulation. The dimensionality of the growth is difficult to determine, as the underlying insulation presents a rough and uneven surface. If the mold growth is due to high humidity and condensation, as is usually the case, the mold growth will only occur on top of the insulation. In the image below, notice how the mold growth uniformly covers the insulation, but is not found on the lower layers, which were disturbed during the inspection.

How is mold growth removed from the attic insulation?

First, it is important to remember that mold growth on the insulation does not negatively impact the indoor air quality. Due to the stack effect, the mold spores cannot migrate into the home. There are many unscrupulous contractors who are telling customers to replace all of their insulation, when little or no mold growth is present. This can double the cost of a remediation project and is typically not necessary. In most cases, mold growth on the insulation is found when mold on the roof sheathing is present as well. In these cases the mildicide treatment applied to the roof sheathing will also treat the mold on the insulation. This will not remove the mold spores or the discoloration. Therefore, in some cases it is best to remove the affected insulation. This can be accomplished by either removing the top several inches of insulation or removing and replacing the full depth. This can add a significant amount to the cost of remediation and therefore, we typically do not recommend it.

What steps are necessary to prevent mold growth from returning to insulation in the attic?

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Preventing mold growth on the insulation requires the same steps as outlined in the attic sheathing section. The improvements made to address the condensation on the roof sheathing will address the condensation and mold growth on the insulation as well.

Elevated moisture content found in the insulation.

This is unusual and typically only occurs when an attic suffers from extreme condensation. In this case, while the mold growth was limited to the top of the insulation, the saturation was found several inches into the fiberglass.

Close up of mold growth on white, fiberglass insulation.

This would be considered light mold growth. Visible without any magnification, but not enough to cause a blackening of the entire top surface.

Mold growth occurring beneath insulation.

In this scenario, mold growth was found on the topside of the sheetrock ceiling, but no mold was observed on the insulation. This indicates a site of a prior roof leak rather than condensation.

Visible condensation on fiberglass insulation.

Note the beads of water accumulating on the top layer of the fiberglass insulation. In a short amount of time, this moisture will lead to mold growth. It’s unusual to see the actual beads of water on the insulation. Typically the moisture accumulation is very thin (though sufficient to cause mold growth) and not visible to the naked eye.

FALSE ALARMS

EXAMPLES OF ATTIC INSULATION INSTALLED INCORRECTLY

Incorrect installation of the insulation is a common cause of attic mold growth. Below are a few examples of common issues.

Insulation installed upside down.

Note the plastic backing on the top side of the insulation batts. The kraft paper or plastic should always be facing the warm side of the ceiling or wall. In an attic, this means the paper or plastic backing should touch the ceiling. Note in this image, the plastic backing is facing upward.

Insulation incorrectly installed directly to roof sheathing

Conditioned attics are great if they are built correctly. This requires installed closed cell foam directly against the roof sheathing. In this project, the contractor had simply used standard fiberglass batts on the roof sheathing. Because these are not vapor impermeable, moisture can easily migrate through the insulation and cause mold and rot on the roof sheathing.

Insulation installed too close to sheathing, leading to mold growth.

This is the most common problem related to attic insulation. Careless insulation contractors often install the insulation too close to the sheathing near the soffit vents. This cuts off the air flow from the outside and leads to condensation and mold growth.

Insulation installed too close to electrical components

While not a mold issue, this can certainly create a fire hazard. Notice the warning on the light, indicating the insulation should remain at least 3″ from the insulation. Project Report > Inspection Regarding Mold & Moisture on Insulation Work # 101023 Weather Overcast and cold General Temperature:

Outside: 34 degrees F

Warehouse: 69 degrees F Relative Humidity (RH):

Outside: 29%

Warehouse: 34%

Dew Point: 35% @ 40 deg F.

In most cases, mold growth can occur when the relative humidity levels range 65% to 99% in a given environment. However, when cold outdoor temperatures exist, conditions can form where even low amounts of humidity can condense on poorly insulated surfaces. This “dew point” is where water vapor becomes liquid and provides the conditions needed for mold growth to begin. Carbon Monoxide (CO):

Outside: 0 ppm

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Warehouse: 0 ppm

Ideally, CO concentrations indoors are expected to be the same as CO concentrations outdoors. CO concentrations are typically around one to two parts per million parts of air (ppm) or less. Concentrations are usually lower in rural areas. Finding CO concentration higher indoors than outdoors indicates an indoor source of CO, or a source very close to your home. Carbon Dioxide (CO2):

Outside: 387 ppm

Warehouse: 779 ppm

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a concentration of CO2 no more than 700 ppm above the ambient air (outside) concentration in order to minimize human odors and maintain comfort. Observations: Water and mold damage along north wall was inspected.

Approximately 1/4 of roof insulation had been previously pulled prior to inspection.

Largest visible area of mold growth and damage was in the NE corner nearest to skylight. Area of roof decking with mold damage contained excessive moisture levels of 80% and above.

FSK facing was pulled away to inspect underlying insulation and roof decking.

Insulation behind FSK facing was wet to the touch and was stuck to the roof decking due to moisture. Several locations were noted where visible mold growth was found under the insulation. A strong, musty odor was present each time the FSK facing was pulled away to expose the underlying insulation. Each area where roof decking was inspected, visible mold growth was found in isolated patches as well as active condensation being present on OSB roof decking.

Environix conducted random inspections of insulation and roof decking throughout the warehouse.

Each inspection of the roof decking revealed moderate to heavy condensation and moisture content levels between 40% and 99%. Visible mold growth was also noted in the majority of random roof decking inspections, though it should be noted that visible mold growth was not found in every location. (See attached photos in section above) Spotty mold growth appears consistently in all locations inspected. RECOMMENDATIONS Recommend all FSK facing and insulation be removed from ceiling cavity throughout warehouse.

Recommend roof decking be below 25% moisture content before remediation is conducted.

Recommend treating roof decking with a two-stage, chemical treatment to kill and encapsulate all mold growth. Recommend consulting a commercial insulation contractor to design an insulation system that is specific to the needs of your company *Heads up – I earn a small commission on sales through Amazon links. This helps cover the expense of running the website (and answering your questions!) Got a question? Ask it here and we'll post the answer below *

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environix.com - Mold on Attic Insulation - Environix

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