A common practice is to pan floor joists with gypsum board, insulation, sheet metal, or even OSB which is similar to particle board. While at first glance for some this may seem like a harmless, even cost-effective alternative to installing proper air ducts, this can potentially cause damage to your lungs, your home, and your energy bill. It's just about impossible to create an airtight seal using this practice, and the constant flow of air through floor boards and joists would surely cause damage to your home's structure.
So please take the time and care to ensure that the return-air pathways and supply-air ducts in your home are air tight, made with the right materials, and planned beforehand, as opposed to just using the building cavities, like floor joists, as a substitute to correctly installed ducts. With something as important as the air supply in your home, doing it the right way will certainly pay off.