New ductwork: what makes the difference
Most homeowners don’t think about ductwork until something forces them to. A remodel, an addition, or a system replacement where the existing ducts won’t work. When you do need new ducts, the install quality determines whether your AC will perform for the next 20 years or fight problems from day one.
What good ductwork looks like
The basics that separate a real duct install from a quick-and-dirty job:
- Proper sizing using Manual D calculation, not eyeballing
- Sheet metal trunks where space allows, properly sealed at all joints with mastic
- Flex duct runs kept under 8 feet where possible and supported every 4 feet
- All connections sealed (mastic or UL-181 tape, not duct tape)
- Insulation R-6 or better on supply runs in unconditioned spaces
- Adequate return air sizing (this is the most commonly undersized component)
The sealing matters more than people realize. A duct system that loses 25% of its air to leaks is functionally a much smaller system than it appears on paper.
Common mistakes
What we see when we inspect bad duct installs:
- Trunk lines too small for the system tonnage
- Returns that are half the size they need to be
- Flex runs draped through trusses with multiple sharp bends
- Sealing skipped or done with cloth duct tape that fails in five years
- No isolation between adjacent runs causing crosstalk
If you’re getting bids for new ductwork, ask for the Manual D worksheet. Ask what sealing material they use. Ask to see photos of past work. More on ductwork install and what goes into it.