A few times in the past, we’ve heard the phrase “the house is the biggest duct” because of the way air moves through it. Well, we can take it a step further and see that there are similarities that go just beyond air movement. 

A building has several layers to keep things like moisture, heat, and air out. Flex ducts are no different; they’re also surrounded by materials that each have a specific purpose. These layers keep things like moisture, air, and heat out of the ductwork. However, since duct sweating and leakage are such prevalent issues in our humid climate, we need to put serious thought into duct design and especially installation.

Like a building, flex ducts have layers that protect the air inside from temperature and humidity gains and losses. We’re going to look at each one and see what they do so that we can avoid common installation mistakes that lead to sweaty ducts.

Control Layers in Buildings

Buildings have layers that control heat, air, and moisture—all of which affect indoor temperatures when we’re trying to condition the space. There are four broad categories of barriers we use in construction that we also see in ductwork.

Air barriers have one job: restrict air movement. In construction and building science, air barriers fill the space between gaps and cracks in a structure. Their main goal is to prevent air leakage and drafts, so membranes, house wrap, and closed-cell spray foam all count as air barriers. 

Vapor barriers prevent water vapor from entering or leaving a structure via diffusion. In case that’s a mouthful, it stops water vapor from seeping into a structure through the building materials. Wood, concrete, most forms of insulation, drywall, and other building materials are porous, so water vapor can slip through those pores and get into a building—unless there’s a vapor barrier. Vapor barriers may include glass, metal, metalized film, and appropriately rated polyethylene sheets.

Bulk water barriers (also called moisture barriers or water barriers) are similar to vapor barriers but a bit different. While they both prevent H₂O from getting inside, the water barrier blocks liquid water instead of water vapor, such as from weather events like rain. These are weather barriers that stop rainwater and moisture from soil from getting into a home, not just mere water vapor, and they’re often polyurethane-type material. 

Radiant barriers are reflective materials that reduce radiant heat gains. Whenever an object radiates heat, that heat can either be reflected or absorbed by another object; radiant barriers reflect most of the heat. A classic example is the metallic sheet you may see in some attics. These radiant barriers reflect incoming solar radiation back to the roof decking, which keeps the ductwork, insulation, and any possessions in the attic cooler than if there was no radiant barrier.

Insulation is usually none of these; it is a thermal barrier. The construction industry often uses the term “thermal barrier” to describe a building layer or coating that resists heat transfer, particularly to prevent fires. While we don’t use the fiberglass blanket insulation in flex ducts to prevent fires, that insulation does resist heat via conduction or direct contact. Unless the insulation also prevents air or water vapor from passing through, like closed-cell spray foam, it is NEITHER an air barrier nor a vapor barrier (and definitely not a radiant barrier).

Flex Duct Layers

Now that we have a basic understanding of the layers we use to resist the impacts of heat or the elements on the insides of our buildings, let’s take a look at the layers in flex ducts.

Outer Jacket

The outer jacket of the flex duct is a vapor barrier. It is a non-permeable material that prevents water vapor from slipping through and condensing on the inner duct surfaces.

There are two kinds of flex ducts we usually see: silver and black. They each have a slightly different effect. 

Silver flex is more common and also doubles as a radiant barrier; the metallic film reflects most of the solar radiation that enters the attic or crawl space, so the surface stays cooler (as does the air inside the ductwork). The main downside to a cooler surface is that it’s more likely to sweat when there are high dew points. 

Black flex absorbs more heat than it reflects, so the surface stays warmer, and sweating is less likely. However, because the black jacket absorbs more heat than the silver, that’s more heat that can attempt to seep into the ductwork. 

In Florida, we also occasionally see homes where the ducts are outside the building, such as underneath mobile homes. Since these ducts are exposed to the elements (especially rainwater), they need to have an additional weather barrier-type layer. These are functionally similar to a home’s external sheathing and have even lower permeability than a standard vapor barrier. This protective layer will resemble black flex in appearance, but it is more robust and actually functions as a weather barrier instead of just a vapor barrier. Notice how the material in the picture below appears to be thicker than the black flex in the image above. (It’s from THIS video where Bert worked on a duct system under a mobile home.) 

Fiberglass Blanket Insulation

The next layer is fiberglass insulation. This large blanket doesn’t do anything to prevent air or moisture from getting through the ductwork. It purely exists to minimize conductive heat transfer into or out of the ductwork. 

Attic air exceeding 120°F is a huge problem when we’re trying to cool a building and moving air that’s roughly 55°F through the ductwork. The insulation stops the heat from affecting the air in the ductwork. The strength of the insulation is determined by the R-value, and common duct insulation R-values are R4, R6, and R8. Higher R-values indicate better resistance to heat transfer.

However, the R-value is not the entire picture. For insulation to be effective, it needs to be able to expand and take up all of the space it needs for its fibers to resist heat. Compressing the insulation significantly reduces its ability to reduce heat gains into the ductwork. In fact, you’ll often see sweating where insulation has been compressed. Sags and areas where nylon duct connector straps (Panduit) have been tied down are particularly prone to sweating because the insulation has been compressed; the cool air moving through the ductwork is right up against hot, moisture-laden attic air.

Inner Lining

The innermost layer of the flex duct is the inner lining. It has a coil inside to keep its form, and it’s what attaches to the collar at connections before anything else gets strapped, taped, or sealed down.

It’s usually made of plastic-type material and is the first line of defense against air or water vapor trying to enter the ductwork. If it is pierced or otherwise compromised, the blanket insulation won’t stop air or water vapor from getting into the ductwork, and a compromised vapor barrier provides an opportunity for duct leakage. 

Duct leakage is costly and significantly reduces your system capacity. It also increases the load on the home because it can create undesigned pressure drivers when the HVAC system runs; that’s called MAD-AIR. If you want to learn more about MAD-AIR, you can read John Tooley’s 1989 study and his article, “Pressure Balancing a House and MAD-AIR.” Genry Garcia also has a great diagnostic chart for it. 

Installing Flex Duct to Minimize Sweating and Leakage

Now that we know what each layer does, it’s time to think about how we can install flex ducts to avoid compromising any of them.

Some of the most common risks with poor duct installation are sweating and duct leakage. Luckily, we can avoid both of those with the right tools and processes. 

First, make sure you stretch out the flex duct and give it enough time to expand to its proper size before trying to assemble the connection right out of the packaging. It helps to have a team or at least a helper on site so that you can pull the flex from both ends.

You can start assembling duct connections once the flex duct has been fully extended and left to decompress for a while. Before you start, make sure you have the following tools on hand:

• Denatured alcohol and a clean rag
• Silver tape
• Tape squeegee
• Mastic
• Panduit straps

Our preferred method is to use the denatured alcohol and rag to clean the collar, pull back the insulation and outer jacket, align the inner duct lining with the collar, and fit it over the lip or ridge on the collar about three rings deep. We use silver tape for the first seal—squeegeed down, of course.

The Panduit strap and mastic are applied afterward, respectively.

Let the mastic dry before you attempt to pull the outer jacket over the connection. The same process applies: tape (squeegeed) first, Panduit strap next, and then mastic last. In our experience, the inspectors we’ve dealt with like to see a lot of mastic, so we keep that in mind when we make that outer seal.

Duct connection assembly steps make more sense when we know what each layer of the duct is supposed to control. Clearly, we need to make sure the air and vapor barriers (inner liner and outer jacket) are sealed to prevent leakage or sweating at the connection; the insulation is just in the middle to prevent heat from entering or leaving the ductwork on contact.