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Installing sheets of plywood over attic insulation is a popular choice, especially in storage spaces. That said, insulation works by trapping hot air—so it needs extra space to expand and be effective, or you might reduce the energy efficiency of your home. 

It’s important that you never compress the insulation by installing flooring directly on top of it. In many houses, you’ll need to install a subfloor before putting plywood over insulation in your attic.

Fiberglass is the best insulation for HVAC ducting because it's inexpensive, widely available, versatile, and long-lasting. Fiberglass also offers acoustic insulation in addition to thermal insulation, which can contribute to a more peaceful home environment. You can use other materials like mineral wool, but fiberglass is the industry standard.

Whether or not you need to insulate exposed ductwork depends on state and local building codes. There is no national requirement for duct insulation. Even if your local codes don't require it, duct insulation can be useful to increase the efficiency of old, thin, or flexible duct materials. Ducts should also be insulated in basements and ceilings to avoid condensation.

Exposed ductwork can sweat just like ductwork placed behind walls, sometimes even more so. Ductwork sweats—like a cold drink on a hot day—because of a temperature imbalance between the air in the duct and the air in the room. In a crawl space or wall, this temperature can equalize faster due to the small space but in lofts and open rooms, that wouldn’t happen as quickly. Many people insulate their ducts for this reason.