Ventilation Critical Moisture Content

It is assumed, based on many observations, that cellulose having moisture content of 19% or lower will generally not support the growth of mold. Mold spores enter a kind of hibernation, able to maintain it sometimes for decades. They are practically invisible but still can make some people get allergiy sneeze, cough, and can make them feel sick. Composite deck boards rarely have moisture of their cellulose fiber above 19%, except in the very top and thin layer after a wood rain, but that layer dries fast and spores do not have time to wake up and sprout germ tubes. They dry out and go into hibernation state again. 

However, in humid, moist areas, particularly with inadequate deck ventilation (to help keep the product relatively dry), for instance, when deck boards are installed too close to the ground, or when a deck forms a box isolated underneath, or when deck boards are wet and covered, moisture content in composite deckboards can exceed 20-25% and retain as such for a long time. With a lack of proper biocides, mildewcide, or other antimicrobial agents in the formulation of the composite material, mold can be rampant. That is why installation instructions for many composite decks direct to make a deck at least 12 in., and preferably 24 in. from the grade or rooftop, or provide a wider space between boards (such as 3/16 or even 1/4 ). Some installation instruction say that failure to adhere to proper ventilation may void the warranty. 

Initially, WPG building materials were considered as being completely resistant to microbial degradation because wood hbers are completely immersed into plastic. It turned out not to be true. WPG materials are typically porous, and a 

MICROBIAL DEGRADATION OF WOOD-PLASTIC COMPOSITE MATERIALS 

Probability of such cases of microbial degradation is determined by the accessibility of the composite matrix by microflora. This in turn is determined by a degree of porosity of the composite, density of the material (specific gravity), water absorption, content of minerals in the material (minerals often play a role of a shield, blocking invasion of microbes into the matrix), and the presence of biocides or antimicrobial agents. 

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