What Causes Condensation on Ductwork in Crawl Spaces?

You hear a drip, drip, drip and it sounds like it's coming from under the house. You take a peek, and you see it: condensation on the ductwork in your crawl space. But don’t fret—there are a few things that make your ducts sweat and some things you can do about it. Here are the most common reasons for condensation on ductwork in crawl spaces.

1. It's Too Hot Outside

You're grateful for your air conditioning, which keeps your home at a cool and comfortable 72 degrees on days when the mercury climbs to 90 or higher outdoors. But that much of a temperature difference can be what's making condensation form on the ducts.

Think back to your middle or elementary school science classes. You learned that when you fill a glass with cool water and let it sit at room temperature, condensation forms on the exterior of the glass. The same thing is happening in your ducts, except instead of cool water, cool air is flowing through them.

When that cool air meets the higher temperature inside your crawl space, condensation forms.

Over time, too much moisture on the outside walls of the ducts will encourage mold growth and may even damage the ductwork itself. You can't control the outside temperature and likely don't want to air condition your crawl space. Your best bet is to add insulation to the ductwork to keep the cooler air from coming into contact with the heat.

Contact an installer who does ductwork to find out more about your options for adding insulation. Depending on the age and condition of your ducts, they may recommend replacing everything with insulated flex ducts.

The cost to add ductwork insulation ranges between $1,000 and $2,700. While this may seem high, it’s still less than having to replace your entire HVAC system or repair damage to the structure of your home caused by excessive moisture.

2. It's Too Humid

If you live in a humid area, the odds are, your crawl space is humid too, especially in the middle of a hot summer. All that water in the air has to go somewhere, and more often than not, it forms condensation on the exterior of your ductwork.

To keep your ducts from sweating, you need to lower the humidity in the crawl space area. There are two ways to do so. Option one is a temporary fix and involves setting up a dehumidifier in your crawl space. The dehumidifier will pull moisture from the air and collect it in a reservoir. Depending on the model, you may have to empty the reservoir any time it fills up.

Option two is to encapsulate your crawl space so that moist air is less likely to get in. Oftentimes, encapsulating a crawl space involves using a dehumidifier and installing a vapor barrier along the ground, so that moisture can't get in. If the crawl space was vented originally, those vents will be sealed during the encapsulation process.

3. The Ducts Are Leaking

Small openings in your ductwork or gaps along the seams can allow warm air from the crawl space to seep in, interacting with the cool air flowing through the ducts and causing condensation. If you suspect leaky ducts are the problem, talk to a local professional duct installer about your options. 

Your best choice may be to replace damaged flexible ducts that are leaking. For sheet metal ducts leaking at the seams, having a pro seal the seams may be all you need. The total cost of duct sealing depends on the size of the project but usually ranges from $0.50 to $1.50 per square foot.