Controlling Condensation on Duct Systems

Anyone who takes a glass of ice water outdoors on a hot day has experienced condensation. A layer of moisture that looks like frost forms on the outside of the glass. Eventually the tiny water droplets become fatter and slide down the glass, wetting whatever’s underneath. Whenever warm moist air hits a cold surface, the moisture in the air will leave its vapor form and become liquid. This condition, condensation, is annoying when your glass sweats on your good dining room table…but it can cause severe moisture problems when it happens on ductwork in your attics.

Uninsulated duct systems traveling through attics will likely experience condensation when the weather turns hot. Because cold air-conditioned air is flowing through the ducts, the moisture in the hot attic air will condense on the exterior of the ducts and can lead to a host of issues. One problem is that water can collect in the attic insulation, which degrades the performance of the insulation. Water can also collect on the back of the ceiling wallboard, causing stains, and may drip into the house at lights and HVAC registers and grilles. Unprotected metal edges will eventually rust. In severe situations, water can trigger growth of that four-letter-word: mold.

Don’t wait for the worst to happen. Control condensation by either locating ductwork in conditioned space or insulating ducts, and you’ll reduce the risk of these moisture problems.

Get ‘Em Outta The Attic
The best way to avoid condensation, and to increase the life of the space conditioning system, is to relocate the furnace, air-conditioning units, and the ductwork into conditioned space.

Relocating ducts requires an early start in the design process and will typically require some minor changes to the architectural arrangement of the home. So what happens if you find out the homes you’ve already built have moisture problems from condensation on ductwork? Let’s look at some solutions involving ducts that are already in the attic.

Warm Your Ducts
To reduce condensation potential, you need to raise the temperature of the exterior surface of the duct system through insulation. There are two materials to accomplish this. One is an insulated duct wrap that can be used on the outside of sheet metal ducts. The other is an insulated duct system.

Using Insulated Duct Wrap on Sheet Metal Ducts
Wrapping sheet metal ducts with an insulated duct wrap will raise the temperature of the duct’s exterior surface. Duct wrap is composed of an insulating layer of fiber glass. The insulating layer is covered with a vapor retarder that limits the movement of moisture in the humid attic from passing through the insulation and reaching the cold duct surface. The insulation levels of duct wrap are typically available in R-values of 4, 6, or 8. The higher the temperature and humidity levels are in the attic, the greater the insulation level needed to control condensation. For complete installation details, consult manufacturer’s literature or National Commercial and Industrial Insulation Standards, published in 1999 by Midwest Insulation Contractors Association (MICA), available at http://www.micainsulation.org.

Using Insulated Duct Systems
Duct board and flex duct are insulated duct systems that have built-in insulation, so you don’t need to worry about fabricating a duct system and then insulating it. In addition to their insulated core, they also have a vapor retarder layer on the exterior surface. Duct board and flex duct, like insulated duct wrap, are also available in nominal insulation levels of R-4, 6, or 8. When insulated duct construction systems are selected, correct specification, assembly, and quality control is critical for successful implementation. For complete instructions, consult Sheet Metal and Air Conditioning Contractor’s National Association’s (SMACNA) Fibrous Glass Duct Construction Standards, available at www.smacna.org.

Hesitant to try fiber glass products? Most of us have heard rumors that the surface material of fiber glass duct board is conducive to mold growth, and that fibers can erode. Independent research has proven these concerns aren’t valid. Studies conducted by researchers at Duke University, the University of Nevada Las Vegas, and North American Insulation Manufacturer’s Association (NAIMA) confirm that fiber glass ducts do not support mold growth. And testing by NAIMA shows that fiber glass duct material does not break away, flake off, or show any other evidence of continued erosion or structural damage, after being subjected to a four-hour test at 2½ times the manufacturers’ maximum operating velocity.

Based on this research, as well as our own experience with the product, IBACOS believes that fiber glass duct board is a good option, provided it’s fabricated and installed according to SMACNA’s recommendations for installation. When following these recommendations, ducts will be airtight, durable under most applications, resistant to microbial growth, and quite effective at reducing noise. For more information on fiber glass duct board, download IBACOS’ white paper on the material.

Insulated duct systems should be assembled to eliminate cold air leaking from the ducts and to maintain the insulation performance by keeping the full specified thickness of the insulation throughout the entire duct surface. Cold air leaking from the duct into the insulating wrap or insulated core of the flex duct will reduce the temperature of the outer surface of the vapor retarders, where condensation can occur.

The best strategy for limiting condensation potential on air conditioning ducts in attics is to build a leak-free duct system, with proper levels of insulation and a vapor retarder on the outside of the insulation.

Whether you relocate your duct systems or keep them in the attic and insulate them, planning to control condensation on ductwork is essential to minimize the potential for moisture problems.

For information on best practices for attaching flex duct onto a duct board trunk, see the Best Practices section on Attaching Flex Duct to a Duct Board Trunk.