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Air Leaks in Duct Systems

By: Jasna Aleksandrova

One of the most important systems in our homes is the air duct system. This system transfers heated or cooled air into every room of the house. If the air duct system is poorly insulated, or leaking, it is more likely that your energy bills will be higher. But this is only part of the problem. The other is moisture that can build in the crawl space and attic which attracts insects, creates mold and brings wood decaying fungi. The principle is very simple: depending on the season, the air duct system transfers heated or cooled air throughout the house. In the summer, cool air that leaks from the duct joints in to the warm crawl space condenses and creates moisture. The opposite happens in winter. Heated air that escapes from the duct joints meets the cold air in the crawl space which condenses and turns into moisture. This can happen even if the ducts are properly sealed, but not insulated. That condensation builds up on the outer duct wall and creates moisture. When the moisture content goes above 9% then the crawl space of your home can easily become a good environment for attracting insects. And if moisture content goes above 27% it can affect the wood framing by providing the perfect conditions for wood decaying fungi to grow. That is why periodic inspections of the air duct system, proper sealing of the joints and good insulation are so important.

Non-wood decay fungi (molds)

The non-wood decay fungi, commonly known as molds, cannot destroy wood, however that’s no reason to neglect mold if it is found on the wood in the crawl space or attic. Molds are microscopic fungi which can affect wood by discoloring the surface, but they do not break down wood fibers and weaken its structure. However molds are a good indicator of high moisture levels in the wood. Also molds can increase the capacity of wood to absorb moisture which is a good foundation for wood decay fungi to grow. Controlling and eliminating favorable conditions for mold growth can prevent wood decaying fungi from growing.

Wood decay fungi*

The wood decaying fungus (wood destroying fungus) is one of the most destructive organisms that can affect the wooden structure of your home. Research shows that wood decaying fungi creates the optimal wood conditions to attract carpenter ants, termites, and other wood destroying insects. To grow and survive, wood destroying fungus, as with every living organism on the planet, requires four basic elements: oxygen, favorable temperature, food and water. Wood decay fungi are likely to thrive when the moisture content of wood exceeds 27%, temperature reaches between the optimal temperatures 77 to 90 degrees Fahrenheit, and there is an adequate supply of nutrients and oxygen. The wood decay fungi feeds off of cells in the wood by secreting enzymes that break down fibers of the wood in to a usable food. Breaking down the fibers of the wood significantly reduces its strength, resulting in wood rot, and destruction of the structural integrity of the wood. If the condition continues over a long period of time, floor joists, sill plates and other structural damage can occur. Properly sealing and encapsulating the crawl space as well as regular air duct maintenance are necessary to control excessive moisture and to prevent insect attacks. This will also prevent molds and wood decaying fungi from growing.

*Wood Decay, http://www.nachi.org/wood-decay.htm, International Association of Certified Home Inspectors

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