Mold on Attic Sheathing
Mold growth on attic / roof sheathing is a common issue in cool climates, such as the Pacific Northwest. In the vast majority of cases, the mold growth is caused by condensation. This occurs when the temperature of the sheathing drops below the dew point, creating a thin layer of moisture on the substrate. If this happens regularly, mold spores will activate and begin to grow.
What does the mold growth typically look like?
Mold growth on the 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. In fact occasionally a home inspector will misidentify the mold growth as soot from a fire. Notice the dark staining in the first picture. This type of mold growth will not smudge or wipe off. The dark discoloration will remain long after the mold has stopped growing.
Below is an image of heavy 3-dimensional mold growth on the sheathing. Beneath the fuzzy growth areas of staining can be observed as well. This type of growth is less common and typically easier to remediate. Because the staining is limited, a thinner layer of encapsulant can be applied.
Not all sheathing is equally mold resistant
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.
Skip Sheathing and Tongue & Groove.
For many years the sheathing material of choice was skip sheathing or tongue and groove. Both of these were 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. We regularly encounter attics with skip sheathing that have very poor ventilation and yet, remarkably 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.
Below is an unusual example of mold attacking the skip sheathing, while the plywood remains largely unaffected. Often this is a sign that the plywood has been replaced recently. The mold growth may have occurred years ago and has yet to attack the new plywood.
Below is a more typical example of mold growth. Note the heavy staining and growth on the plywood. The original skip sheathing has much less growth.
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.
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.
Uneven mold growth
This is where it gets weird. In some cases the mold growth will occur very prominently on one piece of sheathing, while the adjacent piece is completely clear. Two potential causes are possible here. First, the wood may have come from the factory with latent mold spores present in the wood. Once additional moisture was available, the mold starts to grow.
The second variable is the species of wood used in the plywood. Some woods are much more resistant to rot and fungal attacks. Even if conditions are conducive to mold growth (elevated moisture), plywood made from a fungal resistant wood will resist mold growth for quite a while.
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. Exceptions do exist though. In some modern homes, the insulation levels are so high that no heat reaches the roof sheathing. You could have all the ventilation in the world and you’d still have condensation problems. Why? If the sheathing is as cold as the outside air, additional ventilation doesn’t really help. We need some radiant heat to escape into the attic to slightly warm the sheathing.