Condensation on Ductwork – Causes and Solutions (Or In Ductwork)

The cool surface of sheet metal in a humid area causes condensation on ductwork, aka sweating. It is an issue that causes corrosion, dripping, staining, mold, wet floors or ceilings and water damage.

You probably already know that, and that’s why you want to know how to stop condensation on ductwork. Insulation on metal ductwork will prevent condensation by forming a barrier between the air and the cool AC duct or ducts.

What if it happens in winter? While uncommon, it is a possibility, so condensation on ductwork in winter and the solution are covered here too.

Let’s walk through the whole situation. If you’d prefer, jump down the page where stopping condensation on ducts with insulation is explained.

What Causes Condensation on Air Conditioning Ducts?

It’s not rocket science, though it is physics. As you know, moisture in humid air condenses on cool objects, like water on a glass of something cold.

As cooled air passes through supply ducts, the sheet metal ductwork can become 10 to 50 degrees cooler than the basement or attic air. We won’t get into dew point, but if the air is humid, that much of a temperature difference is going to cause condensation and wet ducts.

This typically happens in a crawl space or basement, though it can happen in a poorly ventilated attic or humid garage. In fact, any time an AC duct runs through untreated space, which is air that isn’t air conditioned (or heated), sweating on ductwork is possible.

How Do I Stop Condensation on My Air Ducts?

Condensation in your air ducts that results in leaking is a different problem addressed next.

Most commonly, however, you will find condensation on the outside of metal ductwork. If your AC is properly sized and the system is working as it should, the air in the ductwork will be dry – too dry to cause condensation.

Key: Wrapping/insulating ductwork to prevent condensation is the answer.

We’ve completed a guide called How to Insulate Ductwork in the Basement or Attic. One of the well-known Pick HVAC how-to guides, it gives clear, step-by-step directions on wrapping and insulating ductwork to stop condensation from forming.

Type of Insulation

You’ve got quite a few options.

Fiberglass: The most affordable type is fiberglass batt insulation or roll insulation. It is easily wrapped around or fitted over exposed ducts.

Use tough seam tape to hold the insulation seams together and the batts in place. We recommend paper-faced or foil-faced fiberglass with the paper or foil side out, so the fiberglass blanket is between it and the ducts.

When the ducts are built using floor joists overhead in a crawl space or basement, then the fiberglass batts can be stapled to the joists. We still recommend taping the seams for a super-tight fit.

Insulation expert tip: Don’t compress the fiberglass in the batts when installing or taping it. Compressing it lowers the insulation’s R-value – its ability to prevent heat from transferring through it.

Boards: Rigid foam board is another good option. Cut the boards to fit and use construction glue and/or tape to hold the foam insulation to the ductwork.

Spray: Spray foam insulation, sometimes called SPF, gives you the best insulation value, called R-value and fully explained in our How to Insulate Ductwork Guide. We also go into a lot of detail in our Guide to Spray Foam Insulation. 

Or you can compare cost differences in our Spray Foam vs Fiberglass Insulation Cost & Performance guide.

Advantage – Spray Foam: One of the benefits of spray foam insulation is that, when properly done, it seals the ductwork too. In fact, we recommend sealing ductwork to prevent air leaks and lost money before applying insulation.

Our guide called How to Seal Ductwork will be of help to you. It discusses the materials needed and details the steps to insulating ductwork.

How Do I Stop Condensation in My Air Ducts?

If your ducts are leaking water, you’ve got a problem with your AC unit or the evaporator coil in the air handler, which might be the furnace.

Causes

There are two primary causes of condensation in ductwork – A dirty evaporator coil or an AC unit that is undersized.

If the problem recently developed, then it’s probably the coil. If you’ve had leaking water from your ducts since the day the AC unit was installed, the unit is too small.

How to Prevent Ductwork Leaking Water

Depending on the problem, here are solutions.

Clean your evaporator coil: This should be done annually for peak energy efficiency regardless of whether your ductwork is dripping. When it is really dirty, it will not condense enough moisture out of the air that passes over it, and that humidity will condense on the inside of the metal ductwork. When enough sweating occurs, it will leak.

You can DIY using quality coil cleaner. Or hire a pro to give your HVAC system a thorough cleaning and tune-up. This fixes the issue and might prevent major mechanical failure.

Check sizing of the AC unit: Explain to an HVAC technician – and not from the company that installed the air conditioning system – that your ducts are leaking and dripping. The tech will likely do a load calculation for your home that will determine if the air conditioner is big enough. It likely won’t be, especially if your ducts have been leaking water since “Day 1.”

If your AC is too small, it won’t remove enough humidity from the air circulating through the ducts. That moisture can condense on or in the ductwork and create water problems.

Is it really hot out? A less likely cause, but possible on super-hot days, is that your indoor evaporator coil freezes because it is working so hard pulling heat from your home and condensing out moisture. Cold and wet equals frozen. Then, when it thaws out, it might cause water to run into the ducts where it will leak out of seams.

To fix a frozen coil, turn the HVAC system to Fan mode, and let warm air cycle over it until it thaws. And grab some towels to pick up the moisture.

What Causes Winter Time Condensation in My A/C Ductwork?

That’s a different story – condensation during a heating cycle. Or maybe it is really a different chapter of the same story, the physics of evaporation.

In winter, when you are heating your home, the ductwork in unheated spaces like the attic, basement or garage is cold – as cold as the air around it.

If your home is really humid – like your humidifier is set too high or you live in a swamp – the water molecules in moist air going through cold ducts will condense on the inside of the ductwork, and you will have major problems.

Think about it this way. A dehumidifier can remove 5-10 gallons of water per day. Essentially, cold ductwork in humid air acts like a dehumidifier. That’s a recipe for leaking, dripping and all the related problems.

How to Prevent Ductwork Leaking Water in Winter

You need to reduce the humidity in your home. A furnace naturally does this in a home that isn’t “wrapped too tight,” by pulling in cooler air from outside through gaps in your home’s envelope.

Well, cool air holds less moisture than warm air (it’s a spectrum from cold to hot), so as cool air is exchanged, the humidity in the air is gradually lowered.

But if you’re still having a problem, try these solutions:

Turn Down or Turn Off the Humidifier

If you’re using one, it is putting out too much moisture.

Start a Dehumidifier or AC with Dry Mode

It’s a rare home that needs a dehumidifier in winter, but if you’ve turned off the humidifier and your ducts are still leaking, you might have to resort to this.

If you’d like an appliance with more features than just drying the air, consider a portable air conditioner with Dry/Dehumidifier mode. Most have this mode. It won’t cool the air, but it will remove the moisture.

A good place to begin researching is the Pick HVAC Portable Air Conditioner Guide.

FAQs

Here are common questions related to condensation on and in ductwork.

I’d like to know how to get water out of ac ducts?

Here is how to dry out wet ductwork. We recommend turning off anything creating humidity, and running your furnace with the thermostat set to about 80 for several hours. This should dry them out. Or run the air handler or furnace on Fan Only mode to move dry air through the ducts.

If there is pooling/standing water in the ducts, a pro might have to disconnect a section of ductwork and use a shop vacuum or similar to suck out the water. Be sure to find out why you have condensation in your ductwork, and solve the problem.

Can you tell me how to eliminate condensation on ductwork in basement?

Yes. Insulate the ducts to prevent moisture from condensing on cold sheet metal ductwork. The process of insulating ductwork is described above.

What happens if ductwork gets wet?

Nothing good! It can rust, water can run into your furnace or air handler and cause electrical damage or corrosion, electrical shock can occur and your ductwork can become a place for mold to grow. The leaking and dripping can cause stains and water damage too.

Why would my ductwork have water in it? How do you fix condensation in ductwork?

Well, unless a tub overflowed or a pipe broke, the most common reason is that moisture from humid air traveling through the ducts is condensing onto cold sheet metal. The content above the FAQs explains ductwork condensation causes and solutions.

How do you dry flex duct?

Because of the crevices, water can get trapped more easily. As suggested, run the furnace with the thermostat set to 80 degrees for a few hours, or if the air in your home is dry, try running the furnace or air handler on Fan mode to move dry air through the flex duct.

If the problem is serious, you’ll have to disconnect the ductwork and use a shop vacuum or other wet/dry vacuum to remove the water.

Will wet ductwork insulation dry out?

If it is rigid foam or spray foam, it should. But fiberglass batts might not, at least for a while and especially where they are taped. Give it a week with a fan or heater on it. If it dries out and doesn’t smell moldy, you’re likely OK. Otherwise, you’ll have to replace it.

Can I use the AC if the ductwork is leaking?

We recommend that you do not use the HVAC system when a lot of water is present. Electric shock and damage to equipment can occur. Turn off power to the system and call an HVAC technician.

Why is water leaking from my ductwork?

We’ve covered the most common issues.

If your air handler or furnace is above ductwork, located in the attic or on the roof, the condensate drain could be plugged. If it has a pump, the pump could be plugged or it might be broken. If the issue is overhead, this could be the problem. We recommend a pro technician to solve this leaking ductwork dilemma. 

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