How Much Will PVC Sweat for a Duct System?

Condensation

• Condensation occurs when warm, humid air comes into contact with a surface that has a lower temperature. Since cool air holds less water than warm air, the extra water in the cooled air is deposited on the colder surface. This condensation is also known as “sweat.” There’s no way to tell how much a polyvinyl chloride, commonly referred to as PVC, pipe will sweat when it’s used in a duct system because the amount of sweating depends on several factors, such as the thickness of the pipe wall, the level of humidity and the temperature of the fluid in the duct.

Properties

• All plastic pipes have low thermal conductivity, which means they need less insulation than metal pipes do in order to control sweating. According to the American Society of Home Inspectors, PVC pipes used for condensate drains sweat less than copper ones do, and they don’t usually need to be insulated in order to keep water from dripping off them.

Chlorinated Polyvinyl Chloride

• Chlorinated polyvinyl chloride pipes are very similar to regular PVC pipes, but they’ve undergone a chlorination process that eliminates problems with condensation. In addition, chlorinated polyvinyl chloride is resistant to corrosion and can be used where temperatures reach 200 degrees Fahrenheit. Regular PVC can only be used up to 140 F. Chlorinated polyvinyl chloride can also be bent, shaped and welded, allowing it to be used in a wide range of applications.

Insulation

• Several types of insulation are available for PVC pipes that sweat. Fiberglass is the most common material used. It’s available in rigid form or as a flexible insulation that’s sold in rolls. When installing insulation on ducts, make sure the foil or vinyl side faces out to keep moisture out of the fiberglass. Don’t use insulation faced with paper because it’s flammable and doesn’t resist moisture. Several companies also sell a spray-on insulation designed to stop sweating and dripping problems on air-conditioning ducts and other items. When dry, it forms a flexible, white, water-resistant coating.

Tips

• You can reduce condensation problems even further by using pipes with a smooth interior. Keep duct runs short and straight whenever possible. Slope the duct a bit toward the exterior outlet. That way, condensation will drain to the outside. If you’re using PVC pipe to vent a high-efficiency furnace, the vent pipe connections must be sealed with PVC cement. Using high-temperature, room temperature vulcanizing sealant on the first joint of the PVC vent pipe will make it easy to remove the pipe for later servicing without having to cut it.