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Mold on the attic sheathing

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Why does mold growth occur on attic sheathing?

Mold growth occurs for two reasons: roof leak or condensation.  The vast majority of cases involve condensation, which occurs when the temperature of the attic sheathing drops below the dew point.  Once this occurs, small droplets of water accumulate on the roof sheathing, creating an environment conducive to mold growth.

What does the mold growth typically look like?

Attic mold growth often has a low profile, i.e. not a 'fuzzy' texture.  Most often the mold growth results in a dark grey or black staining that looks very similar to soot.  This can occasionally lead to inspectors mistakenly describing the area as fire damaged.  

roof sheathing mold growth

Mold growth rates across different roof sheathing materials.

Ironically, despite the many gains we've made in the building codes, we've still managed to install progressively worse and worse attic sheathing materials over the decades.  Of course, the problem of mold growth is much more complicated than a simple choice of materials, involving ventilation, insulation, air sealing, etc.  But our choices of materials haven't made mold prevention any easier.  

Early 1900's to 1950's.

For many years the sheathing material of choice was skip sheeting or tongue and groove.  Both of these were natural wood products with excellent mold resistance.  

1950's to 1980's.

Natural wood is expensive.  Builders, forever looking to save a few bucks, began turning toward plywood roof sheathing.  Though certainly superior to OSB, plywood inevitably provided a food source more conducive to mold growth than its more natural cousin.  The reason is straightforward; as you compress wood into a laminate structure, the cell walls begin to break down.  In a sense, this 'pre-digests' the wood for the mold growth, allowing for much quicker decay.  

1980's to Today.

OSB reigns as the most popular sheathing material in most markets today.  Like the decision to move from natural wood to plywood, the move to OSB was motivated by price.  Unfortunately, the enormous pressure used to create OSB breaks down the cell walls of the wood at a far greater rate than plywood.  As noted above, this allows mold to attack the fibers at a much quicker rate than other materials. 

Removing mold growth from roof sheathing.

Mold remediation contractors use a variety of techniques to address attic mold growth, including dry ice blasting, hand sanding and encapsulation. The first two are effective, though often cost prohibitive.  As you can imagine, the labor required to hand scrub every square inch of roof sheathing is quite significant.  Additionally, encapsulation is still necessary, as the hand sanding and brushing will not remove the underlying stain left by the mold spores.  

Dry ice blasting is an attractive, but expensive, choice.  The technique uses solidified carbon dioxide beads to blast the surface material from the sheathing.  The concept is similar to sand blasting, minus the mess.  Unfortunately, dry ice blasting produces a large amount of carbon dioxide gas, which in a confined area, can lead to a drop in oxygen levels. This necessitates several safety measures that can quickly ruin the economic feasibility of the process.

Encapsulation addresses several of these problems in an efficient and cost-effective manner.  We have more information on the subject of mold encapsulation on our mold treatment page.

What steps are necessary to prevent mold growth from returning to the roof sheathing?

Mold growth will return if the underlying moisture issues are not addressed.  Typically, this involves increasing the roof ventilation and air sealing the ceiling of the top floor.  Combining these two techniques both limits the influx of warm moist air and also helps remove the damp air that slips through. 

3 ways to prevent mold on roof sheathing

INSPECTION REPORTS FROM RECENT PROJECTS.

PROJECT BACKGROUND INFORMATION - 103027

Mold Growth:

  • Inspection of attic revealed moderate mold growth throughout significant areas of attic sheathing.  At this time, the mold growth has not lead to significant structural damage of the underlying materials. However, the mold growth may be a health concern and is an indication of a failure of the home to adequately exhaust damp air created by the occupants. 

 Moisture:

  • No elevated moisture noted throughout significant portions of the attic sheathing.
  • Evidence of condensation was present in the form of rusty nail shafts that protrude through the sheathing as well as drip stains which are present on much of the stored contents in the attic space.

Leaks:

  • No obvious roof leaks were noted in the attic.  

Insulation:

  • Insulation shows no signs of significant water damage, or mold growth.  Spores may be present in the insulation, and it can be removed at the customer’s request, but once the recommended steps are performed the spores are unlikely to negatively impact indoor air quality.
  • Several walk boards were present throughout the attic space which can significantly decrease the effectiveness of the insulation in the attic.

Ventilation:

  • Insufficient ridge area ventilation installed throughout attic.  This limits the exhaustion of humid air through the roof assembly.
  • Insufficient soffit ventilation throughout the attic. 
  • Proper ventilation is necessary to minimize condensation and subsequent mold growth.  All areas of elevated moisture or improper ventilation must be addressed to ensure the effectiveness of our process.  Current code calls for 1sqft of ventilation for every 300 square feet of attic space, distributed evenly between the ridge and soffit area. 
  • There are several complications in venting this attic space including: shear wall blocking some ridge venting, areas above vaulted ceiling with no soffit venting, other miscellaneous complications.  It is recommended that a high-quality roofing contractor be hired to assess and create a pan for appropriate ventilation.

Air Seal:

  • The ceiling currently lacks proper air sealing, allowing excess moisture and heat to escape into the attic area.  This air leakage is a key cause of energy loss, attic condensation and mold growth. 

Trash and debris:

  • The attic is currently free from excessive amounts of trash and construction debris.
  • Stored contents located in attic, it is necessary for these contents to be completely removed prior to the start of any remediation work.

 

Remediation Report For Mold in Attic Sheathing

Work #: 202006

Mold Growth:

  • Inspection of attic revealed moderate mold growth throughout significant areas of attic sheathing.  At this time, the mold growth has not lead to significant structural damage of the underlying materials. However, the mold growth may be a health concern and is an indication of a failure of the home to adequately exhaust damp air created by the occupants. 

Moisture:

  • At the time of inspection no increased moisture content was noted in the framing or sheathing, but the mold growth suggests that moisture content is likely increased at specific times of the year.

Leaks:

  • No obvious roof leaks were observed on inspection of the attic.  Environix is not a professional roofing contractor and cannot guarantee the condition of any roofing structure.  Home owner had previously repaired a roof leak and addressed damaged sheetrock on ceiling.

Insulation:

  • Insulation shows no signs of significant water damage, or mold growth.  Spores may be present in the insulation, and it can be removed at the customer’s request, but once the recommended steps are performed the spores are unlikely to negatively impact indoor air quality.

Ventilation:

  • Current ventilation is adequate to exhaust normal amounts of humidity.
  • Proper ventilation is necessary to minimize condensation and subsequent mold growth.  All areas of elevated moisture or improper ventilation must be addressed to ensure the effectiveness of our process.  Current code calls for 1sqft of ventilation for every 300 square feet of attic space, distributed evenly between the ridge and soffit area.  Current code requires 3.21 SF ventilation.  The home currently possesses approximately 2.75SF upper ventilation, and 6.25SF soffit ventilation.
  • Baffles should be installed to protect all soffit ventilation from future blockage by insulation.
  • The attic currently possesses gable vents which are incompatible with the ventilation plan.  These vents should be sealed to prevent the short-circuiting of the convective currents.

Air Seal:

  • The ceiling currently lacks proper air sealing, allowing excess moisture and heat to escape into the attic area.  This air leakage is a key cause of energy loss, attic condensation and mold growth. 
  • The attic contains one or more large chases (areas of connectivity between inaccessible interior cavities and attic space).  It is necessary to seal these chases with the appropriate materials to provide a continuous air and thermal barrier.  The chase above the fireplace was poorly sealed with sheetrock which was broken by the inspector.  Environix will repair this area with OSB and expandable foam.

Ducting:

  • The exhaust ducting from the bathroom fan, whole house fan and/or kitchen range exhaust hood is not appropriately venting to the exterior.  This condition allows warm moist air to flow into the attic and is likely a contributing factor in condensation.  It is necessary for all exhaust ducting to vent completely to the exterior through insulated ducting and be connected to a dedicated roof jack with appropriately sized collar.

Remediation Report for Mold in Attic

Date of Project: Jan. 12th, 2012

Mold Growth:

Mold in Attic

Inspection of attic revealed moderate mold growth throughout significant areas of attic sheathing. Currently, the mold growth has not lead to significant structural damage of the underlying materials. However, the mold growth may be a health concern and is an indication of a failure of the home to adequately ventilate space.

Moisture:

  • Elevated moisture noted throughout significant portions of the attic sheathing. It is necessary to thoroughly dry the framing and sheathing prior to the application of encapsulant.

Leaks:

  • No obvious roof leaks were observed on inspection of the attic. Environix is not a professional roofing contractor and cannot guarantee the condition of any roofing structure.
Mold in Attic Sheathing

Insulation:

  • Visible water damage observed throughout the top layer of insulation in the attic. The pricing quoted in the bid includes the removal of affected insulation.

Ventilation:

  • Insufficient ridge area ventilation installed throughout attic. This limits the exhaustion of humid air through the roof assembly.
  • Insufficient soffit ventilation throughout the attic. Existing ventilation blocked by improperly installed insulation. This limits the influx of fresh air through the soffit vents, leading to excess condensation and mold growth. It is recommended that blocked soffit vents be cleared and all soffit venting be protected by the appropriate baffling.
  • Proper ventilation is necessary to minimize condensation and subsequent mold growth. All areas of elevated moisture or improper ventilation must be addressed to ensure the effectiveness of our process. Current code calls for 1sqft of ventilation for every 300 square feet of attic space, distributed evenly between the ridge and soffit area.
  • It is recommended that a qualified roof evaluate the venting and install appropriate soffit-type venting and sufficient ridge venting. This plan will likely include the addition of smart-vents to achieve a similar intake result as traditional soffit venting.

Air Seal:

  • The ceiling currently lacks proper air sealing, allowing excess moisture and heat to escape into the attic area. This air leakage is a key cause of energy loss, attic condensation and mold growth.

Ducting:

  • The exhaust ducting from the bathroom fan, whole house fan and/or kitchen range exhaust hood is not appropriately venting to the exterior. This condition allows warm moist air to flow into the attic and is likely a contributing factor in condensation. It is necessary for all exhaust ducting to vent completely to the exterior through insulated ducting and be connected to a dedicated roof jack with appropriately sized collar.

YOUR PROJECT BACKGROUND INFORMATION- Side Attics

Mold Growth:

No significant areas of mold growth noted in the side attics, apart from isolated sections of growth at areas of leakage.

Moisture:

Leaks:

  • Obvious roof leaks were noted in the attic. To prevent further damage, it is necessary to hire a professional roofing contractor to resolve the leaks immediately. Environix can provide the names of reputable contractors at the customer's request. These leaks were noted around dormers and gables.

Insulation:

  • Insulation shows no signs of significant water damage, or mold growth. Spores may be present in the insulation, and it can be removed at the customer's request, but once the recommended steps are performed the spores are unlikely to negatively impact indoor air quality.
  • Insulation is insufficient to meet current standards, it is recommended that attics be insulated to a level of R-38 where possible to prevent unnecessary heat loss.

Ventilation:

  • Insufficient ridge area ventilation installed throughout attic. This limits the exhaustion of humid air through the roof assembly.
  • Insufficient soffit ventilation throughout the attic. Existing ventilation blocked by improperly installed insulation. This limits the influx of fresh air through the soffit vents, leading to excess condensation and mold growth. It is recommended that blocked soffit vents be cleared and all soffit venting be protected by the appropriate baffling.
  • Proper ventilation is necessary to minimize condensation and subsequent mold growth. All areas of elevated moisture or improper ventilation must be addressed to ensure the effectiveness of our process. Current code calls for 1sqft of ventilation for every 300 square feet of attic space, distributed evenly between the ridge and soffit area.
  • Although the current ventilation appears to be inadequate and some soffit vents are blocked, no major condensation-based damage has occurred. It is recommended that the clients consult with the roof who will be performing repairs to determine a practical amount of ventilation to add.

Air Seal:

  • The ceiling currently lacks proper air sealing, allowing excess moisture and heat to escape into the attic area. This air leakage is a key cause of energy loss, attic condensation and mold growth.
  • Air-sealing needs to include joist-bay blocking (to limit bypass under the 2nd story floor) and construction of new sealed and insulated attic hatches.

Ducting:

  • The exhaust ducting from the bathroom fan, whole house fan and/or kitchen range exhaust hood is not appropriately venting to the exterior. This condition allows warm moist air to flow into the attic and is likely a contributing factor in condensation. It is necessary for all exhaust ducting to vent completely to the exterior through insulated ducting and be connected to a dedicated roof jack with appropriately sized collar. 

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