Sweating Ductwork: A Larger HVAC Issue

During this hot, humid Louisville summer, your home’s air conditioner is likely running around the clock. While it’s definitely the season where you are sweating outside, your home’s ductwork should never be sweating inside your home. If it is, there’s a larger issue at play that needs to be addressed by a professional.

Why Do Ducts Sweat?

Sometimes in the humid, sticky summer months, condensation may start to form on your home’s HVAC ductwork. High humidity indoor can be a typical cause of sweating ducts, but there may be other underlying issues contributing to it too, namely a lack of insulation and a leaky house!

Insulation’s Role in Sweaty Ducts

If your ductwork and house are properly insulated and air-sealed, it’s doubtful that your ducts will experience “sweating.” However, if there are areas of your house or ducts which are not properly insulated and sealed, condensation is much more likely to form.

Condensation on ducts can occur when hot, humid, outdoor air is allowed to enter the home due to inadequate insulation and air sealing.  It can also occur if your A/C system gets really cold, but there isn’t enough air moving through the system and ductwork to push that cold air throughout the rest of the house.  This makes the “A-coil” and the “supply plenum” very very cold, and can cause the A-coil to actually freeze up and malfunction.  

When your air conditioner is running, the air temperature coming out will typically be in the 50’s, so the actual surface temperature of the metal duct is also very cold. Most home owners keep their A/C set between 72-76 degrees, so there could easily be a 20 degree temperature difference between the air in your ductwork versus the air inside your home.

The point at which the relative humidity of the air reaches 100% is referred to as the “dew point.” Many times the warmer, more humid air inside your home will meet the colder ductwork, and the cold metal surface is at dew point, so condensation forms, leading to the apparent “sweat” on ducts.

The hot yellow square in the image above is an uninsulated section of ceiling (attic) that is in the same ceiling/floor cavity as the supply duct above. The hot attic temp and the cold supply duct temp meet in the middle and condensation occurs!

The picture above shows a cold supply duct running between the first and second floor of a house I recently inspected. The floor cavity the duct is in is also connected directly to a small attic space about 10 feet away, seen below.

The Danger of Sweating Ducts

Although condensation in your ducts may seem like a relatively harmless annoyance, it’s actually a serious problem. Apart from being evidence of an insulation and air sealing problem, sweating ducts have the potential to:

  • Decrease the thermal resistance (R-value) of your insulation when it gets wet

  • Create moisture issues in your ceilings and walls, ruining building materials and potentially compromising the structural integrity of the building itself

  • Feed the growth of mold & mildew, endangering your home’s indoor air quality and your family’s health

Solutions for Condensation in Your Ducts

Solving any insulation issues around and on your ducts is a crucial step to resolving sweating ducts. There are also some additional steps you can take to prevent moist air from coming into contact with your ducts:

  • Install a Vapor Barrier

If your house has a crawlspace foundation, make sure there is a 6 mil thick vapor barrier over the entire ground, as well as over the foundation walls.  Close off any vents in the walls and insulate the foundation walls & rim joist.  This helps stop sweating ducts in crawlspaces.

  • Duct Sealing

Sealing your ducts is critical so that no air is allowed to enter or exit.  Ducts that leak air to attics or crawlspaces, or even inside the house, are detrimental to the health and comfort of the occupants.

  • Add Insulation

Air-sealing alone won’t stop ducts from sweating - you also need insulation on them to prevent it!  Air-seal your ductwork and your house well, then add appropriate insulation to stop ductwork from sweating.

If your ducts are sweating, Building Performance can help! Don’t wait until the problems get worse, contact our experienced team to schedule an inspection today!

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