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Mold on attic insulation

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Why does mold growth occur on attic insulation?

Technically, mold does not truly grow on the insulation.  Yet mold growth is often found on the top surface of attic insulation.  How does this occur?  Two possibilities exist: either the mold on the insulation is due to the falling spores from the growth on the roof sheathing above, or the mold is growing on the settled dust on top of the insulation.  Either way, it is an indication of a systemic moisture issue.

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. 

How is mold growth removed from the attic insulation?

If the layer of deposited mold is thin, the insulation may not require any treatment.  However, if the mold is significant, removal and replacement of all the insulation is typically recommended.  

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

Preventing mold growth on the insulation requires the same steps as outlined in the attic sheathing section.  A remediation contractor should both increase ventilation from the attic to the exterior and reduce infiltration from the upper floor of the home into the attic space. 

Recent Inspection Report Involving Moisture and Mold in Insulation

Insulation Mold

Work # 101023


  • Overcast and cold 


  • Temperature:
  • Outside:        34 degrees F
  • Warehouse:  69 degrees F
Saturated Insulation
  • 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.
Mold in Insulation
  • Carbon Monoxide (CO):
  • Outside:                0 ppm
  • 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 monoxide is an odorless, colorless gas often formed in the process of incomplete combustion of organic substances, including fuels. CO can cause serious health problems.
  • 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.
  • Carbon dioxide is a colorless, odorless gas. It is produced when any carbon-based material used for fuel (coal, oil, wood, etc.) is burned. When fuel burning is not a factor, the main sources are tobacco smoke, human and animal respiration. Carbon dioxide is given off whenever we exhale. Cars, trucks, industrial equipment, and burning fuel for power are some of the major contributors to CO2 in the air.


  • 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.


  • 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


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