During hot and humid Louisville summers, 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. Sweating ductwork is a sign of larger HVAC issues that need 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 indoors 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 house and ductwork 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 excessively cold and 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” extremely cold and can cause the A-coil to actually freeze up and malfunction. When this happens the A/C system will stop working and no air will come out.
When your air conditioner is running the air temperature coming out will typically be in the mid 50’s, so the actual surface temperature of the metal duct is also that temperature. Most home owners keep their A/C set between 70-75 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 or below dew point, so condensation forms and leads to the apparent “sweat” on ductwork.
Poor Airflow Through Ductwork
Another common reason ductwork may sweat is due to restrictions in the airflow through the ductwork and HVAC equipment. Oftentimes the ductwork is too small on the "intake side" (return-air) for what the A/C equipment requires, which causes poor airflow and will make the system much louder than it should be.
This is a simple problem to diagnose, as all you need to know is the tonnage of your A/C equipment (generally two to four tons) and the size of your return-air ductwork. For every ton of A/C you have, you generally need 400 CFM of airflow, so a two-ton system needs 800 CFM to function properly. Measure the length and width of your return-air ductwork closest to the equipment inside your home and then use a duct-design table to determine how much airflow your ductwork can move. Keep in mind these figures are best-case numbers, but it will quickly let you know if your ductwork is too small for your A/C equipment. Now, whether your A/C equipment is too big for your house is another question!
Other common things that restrict airflow are using "high-efficiency" or "allergen reduction" air-filters, which have high "MERV" ratings. Most HVAC systems are not designed to account for how restrictive these air filters are, so we generally don't recommend them unless you use four or five inch-wide "media filters" that provide more surface area for filtration and therefore better airflow.
The Danger of Sweating Ductwork
Although condensation on your ductwork may seem relatively harmless, 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 (minimum) vapor barrier over the entire ground, as well as over the foundation walls. The vapor barrier should overlap about a foot and be sealed together. Close off any vents in the foundation walls and insulate the foundation walls & rim joist with spray-foam or rigid insulation. This helps stop sweating ductwork in crawlspaces.
Duct Sealing
Sealing your ductwork is critical so that no air leaks out to unwanted areas. Ductwork that leaks into attics or crawlspaces, or even inside walls and floor cavities, is detrimental to the health and comfort of the occupants.
Add Insulation
Air-sealing ductwork 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.