A water stain around a ceiling vent, or drops falling from a register on a hot afternoon, is alarming because it looks like a roof leak. In summer in Sacramento, it usually is not. It is condensation forming on the duct work in your attic, and it has specific causes you can fix.
Why ducts sweat
Your air conditioner sends cold air through ducts that run through an attic that can reach well above outdoor temperature on a summer afternoon. When hot, humid attic air touches a surface colder than its dew point, water condenses on it, the same way a cold glass sweats on a summer day. Properly insulated and sealed duct work keeps the outer surface warm enough that this does not happen. When something is wrong, water forms and drips onto the ceiling below.
Where it usually happens
- At the supply boots, where the duct connects to the ceiling register. Gaps between the boot and the drywall let cold air leak out and meet hot attic air right at the ceiling.
- Where insulation is torn, compressed or missing on flex duct, often where someone crawled over it or a technician pulled it back.
- At disconnected or loose joints, where cold air blows straight into the attic.
- On uninsulated metal plenums or fittings.
- At the air handler itself, if the unit is in the attic. A clogged condensate drain or a full secondary pan also causes ceiling leaks and is a different problem. See air handler leaking water.
Why it gets worse on certain days
- Humid days. Our air is usually dry, but humidity spikes after rain, during certain weather patterns, and in homes near the rivers.
- A thermostat set very low. Colder supply air means colder duct surfaces.
- Low airflow. A dirty filter or failing blower lowers the air temperature in the ducts and can freeze the coil.
- An oversized system sending very cold air in short bursts.
How to tell condensation from a roof leak
- Condensation shows up when the AC runs on hot days, and stops when it is off.
- A roof leak shows up during or after rain.
- Condensation usually stains in a ring around a vent. Roof leaks follow framing and can appear anywhere.
What to do now
- Protect the ceiling. Wet drywall sags and can collapse.
- Replace the filter and make sure all supply registers are open.
- Raise the thermostat a couple of degrees for now.
- If you can safely look in the attic, check around the boot above the wet spot for torn insulation or a loose connection. Watch your footing and stay on the framing.
The real fixes
- Seal the boot to the drywall and seal the boot seams with mastic.
- Repair or replace damaged flex duct insulation.
- Reconnect and seal loose joints.
- Replace old, degraded flex duct when the insulation is failing along whole runs.
- Correct airflow problems so supply air is not colder than it should be.
Sealing and repair are covered in duct sealing versus replacement. If the duct insulation is moldy, the EPA notes it generally cannot be cleaned effectively and should be replaced, which we explain in is air duct cleaning worth it.
Why fixing it pays off
A duct that sweats is also a duct that is leaking cold air into the attic. The same repair that stops the drip usually improves cooling in that room and lowers your bill.
What we do
We find the wet section in the attic, check insulation and seals, test airflow and the condensate system, and repair or replace what is failing. Our diagnostic is a flat $69 and it is credited toward the repair. See our duct work services.
Frequently asked questions
Why is water dripping from my AC vent in the ceiling?
Usually condensation. Cold air in the ducts meets hot, humid attic air at a gap, a torn insulation sleeve or a poorly sealed boot, and water forms and drips onto the ceiling.
Is water from a ceiling vent a roof leak?
Probably not if it happens only when the AC runs on hot days. Roof leaks show up with rain.
Can a clogged AC drain cause water from the ceiling?
Yes, if the air handler is in the attic. A clogged condensate line or full secondary pan can overflow onto the ceiling. That is a different fix from duct condensation.
How do you stop ducts from sweating?
Seal the supply boots to the drywall, repair or replace damaged duct insulation, reconnect loose joints and correct airflow problems.
Is condensation on ducts dangerous?
It damages drywall and insulation and can lead to mold. Wet drywall can sag and fall, so protect the ceiling and have it repaired.
Does duct condensation mean my ducts are leaking air?
Often yes. The gaps that let cold air meet attic air are also losing cooling into the attic.