Skip sheathing or tongue and groove has been the material of choice for many years. Both of these are natural wood products with excellent mold resistance. While mold growth can occur on these materials, it typically requires far more moisture and time for the growth to occur. Often when we encounter attics with skip roof sheathing that have very poor ventilation and yet, astoundingly little mold growth. Part of this is due to the age of the homes and the general lack of air sealing that occurred when they were originally constructed. However, a large factor is simply natural wood’s resistance to mold growth.
Flat, Low Profile Mold Growth
Mold growth on attic roof sheathing is a relatively common issue in cool climates, like the Pacific Northwest. In most cases, mold growth is caused by condensation. This occurs when the temperature of the sheathing drops below the dew point. The dewpoint is the temperature in which the air can't hold any more water. When this happens a thin layer of moisture is created on the substrate. If this occurs regularly mold spores will spawn and begin to grow.
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Mold growth will return if the root cause of mold, moisture, is not addressed. Typically in order to solve this issue involves improving and/or increasing the roof ventilation and air sealing the ceiling of the top floor. Combining these two techniques limits the influx of warm moist air and also helps remove the damp air that slips through.
Interestingly enough, despite the many improvements we have seen made in building codes, it is still common to install bad and worse attic roof sheathing materials. Of course, the problem of mold growth is much more complicated than just a simple choice of materials. But in many cases, the choice of materials hasn't made mold prevention any easier.
Mold growth on attic roof sheathing occurs in either a flat, low profile configuration or a fuzzy, 3-dimensional pattern.
The flat, low profile growth can often look like soot or staining. This can actually in some cases fool a home inspector into misidentifying mold growth as soot from a fire. But unlike soot, this type of mold growth will not smudge or wipe off. The dark discoloration will remain long after the mold has stopped growing.
3-dimensional mold growth on the sheathing is very fuzzy and protrudes from the surface it is growing on. Beneath the fuzzy growth areas, staining can be seen as well. This type of growth is less common and typically easier to remediate. Because the staining is limited, a thinner layer of remover can be applied.
Mold remediation experts use a variety of techniques to address attic mold growth. Some of these techniques include hand sanding, dry ice blasting, and encapsulation. The first two are quite effective, though they can be rather expensive. The expense comes from the labor required to hand scrub every square inch of roof sheathing. Additionally, encapsulation is still necessary, as the hand sanding and brushing will not remove the underlying stain left by the mold.
Dry ice blasting is an attractive, but expensive, choice. The technique uses solidified carbon dioxide beads to blast the surface mold 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. Doing this requires several safety measures that can quickly run up the cost of the process.
Encapsulation addresses several of these problems in an efficient and cost-effective manner.
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 greater rate than plywood. As noted above, this allows mold to attack the fibers at a much quicker rate than other materials.
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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.