Your Furnace Is Heating Your Attic (The Duct Leak Nobody Checks), by Ray Dutton's Garage on YouTube. 12 min 20 s.
- Video length
- 12 min 20 s
- Time to do it
- 2 hours
- Difficulty
- Medium
- Steps
- 9
What you need
Steps
Turn the furnace off at the thermostat, then go up to the attic with a flashlight. Look along each flex duct run for sections crushed under stored boxes, pinched at sharp bends or flattened against the rafters.
Check the insulation wrap on each duct. Compare the air temperature at the supply register with the air at any bare spot using a kitchen thermometer. A big drop means the wrap is torn or missing.
Wrap bare or torn sections with new insulation sleeve and seal the seams. Ducts lying in water or touching hot roof decking need a technician.
Brush duct mastic onto every joint, elbow, collar and seam. A tub costs about twenty dollars. Press mesh tape into wide gaps first, then coat over it.
Push on every duct end to check it is still attached to the register boot or the furnace plenum. Reconnect loose ducts with a tie strap and a coat of mastic.
Look at the return ducts as well. A return leak in the attic pulls hot, dirty attic air, and in a tight house it can pull furnace fumes back inside.
Hold a tissue against the return grille and near the furnace while it runs. If it flutters the wrong way, or the burner flame looks lazy, stop and call a technician.
Test your CO alarms and fit fresh batteries. Never seal a joint close to the furnace cabinet or flue. That zone is for a qualified technician only.
If the room is still cold after sealing, check register airflow, window drafts and attic floor insulation before you blame the furnace.
Tips and common mistakes
Skip duct tape. Attic heat dries the adhesive and it lets go within a season.
Before anyone quotes you a new furnace, ask one question: did you check the ducts in the attic first?
Do the work on a cool morning, because attics get dangerously hot by midday.
Wear gloves and a dust mask. Insulation fibres and mastic are messy.
Questions
How do I know if my attic ducts are leaking?
Look for crushed sections, bare insulation and ducts that have come off their fittings. Sealing the joints and rechecking the room temperature tells you whether the leak was the cause.
Can I use duct tape instead of mastic?
No. Duct tape dries out and peels in attic heat. Use duct mastic, with mesh tape pressed into any wide gaps.
Why is one bedroom still cold after sealing the ducts?
It may be a balance problem rather than a leak: a partly closed damper, a blocked register or missing attic floor insulation above the room.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
