What is a vent boot?
Random Moisture Stains on your Ceiling
The above picture shows an uninsulated vent boot attached to the HVAC duct. The vent cover connects to the vent boot on the inside of the ceiling. This boot has no insulation, and the joints are not sealed well. The sealant between the boot and the ductwork is leaking, and there is no sealant between the duct and the ceiling. Moisture accumulating on the vent boot drips down until it is absorbed by the drywall ceiling board, causing moisture stains around the vent cover.People often mistake the dark stains that sometimes form around the vent for mold. The probable cause of the dark stains is the vent boot was not sealed to the ceiling board. The dust from the attic then sticks to and stains the moist drywall board around the vent cover.
When you see the stains around your ceiling vent covers, you should not ignore them. Water damage is costly to repair, and moist conditions may encourage mold growth.
If you have random stains on the ceiling, it may be the joints that are not sufficiently insulated or sealed. Moisture stains are visible at the joints of the ductwork in the attic in the picture above. Condensation forms on the joint and may not evaporate in the attic. If the condensation continues to drip, it may saturate the insulation and cause moisture stains on your ceiling.
Main Causes of Condensation at AC Vents
Not enough insulation on the ducts themselves
The insulation on the duct above was separated from the duct. Condensation had developed in the duct vapor barrier, and overtime had stained the middle of the ceiling.
Recommendation: Ducts in the attic should be well insulated and in good repair. Damaged or old under-insulated ducts should be replaced. Consider replacing the ductwork in the attic if it is 20 years or older. It will be a good investment that will be easily recovered and will save you money on your future electric bills.
Uninsulated vent ‘boot’
The metal box in your attic that attaches to the duct and the vent cover screws to is the boot. In Florida, the uninsulated boot in the attic will generate large amounts of condensation due to the cold air in the boot and the hot air in the attic.
Dark Stains around your vent Cover
Dark stains around your vent cover may be mold, but more likely, it is dust collecting on the damp ceiling. The boot should be properly sealed to the ceiling. If not, it will allow attic dust or dirt to leak out of the crack between the ceiling and the vent cover. Sealing the duct to the boot is likewise very important. You may discover a leak where the vent boot attaches to the duct. If not properly sealed, the cold air will leak out. When it meets up with the hot attic air, it forms condensation.
Recommendation: You should find and seal the leak. If the problem vent is easy to access from your attic, you may be able to fix the problem yourself. We recommend having a local HVAC contractor to evaluate and repair your HVAC ducts.
Covering Stains around the Vent Cover
The dark or water stains around your vent cover are difficult to cover. When the repairs have been made, the ducts have been caulked, and no more condensation is present on the covers, it will be time to cover the stains.
Normal ceiling paint will not cover these stains. Moisture and dirt stains around the vent cover are resolute stains. Even with multiple coats of ceiling paint, the stains will continue to bleed through the ceiling paint.
Recommendations: We recommend you first cover the stains with two coats of KILZ 2 Multi-Surface Primer & Stainblocker then, paint with your choice of ceiling paint.
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