Stock ductwork versus in-house sheet metal fabrication — what builders, GCs, and homeowners taking on a major remodel should know about duct design before framing closes it in forever.
Ductwork is the part of an HVAC system nobody thinks about until it’s wrong — and by the time it’s obviously wrong, it’s usually behind drywall. For builders, general contractors, and homeowners managing a significant remodel or new build in the Spokane area, understanding what goes into duct design and fabrication before framing closes it in is worth the ten minutes it takes to read this.
Why Ductwork Is Where So Many HVAC Problems Actually Start
A homeowner will call about a room that’s always too hot or too cold, an upstairs that never quite cools in summer, or a system that seems to run constantly without keeping up. In a meaningful number of these cases, the equipment itself is fine — the ductwork is the actual problem. Undersized trunk lines, too many sharp turns, poor sealing at joints, or a duct layout that was never balanced for the home’s actual room-by-room heating and cooling loads. Equipment gets sized on paper using load calculations; ductwork has to physically deliver on that math, and stock or rushed duct installs are where the gap between the two shows up.
Stock Components vs. Custom Fabrication
A lot of residential HVAC work today uses pre-manufactured duct components — standard-size elbows, transitions, and boots pulled off a shelf. That’s not inherently wrong; for straightforward layouts, stock components work fine and keep costs down. But new construction and remodels frequently have situations stock parts don’t handle well:
- Tight mechanical chases that need a custom-fabricated transition to fit the actual dimensions available, not the nearest stock size.
- Commercial rooftop curb adaptations when a replacement unit’s dimensions don’t exactly match the existing roof penetration.
- Long horizontal runs that need properly sized and sealed trunk lines to avoid pressure loss over distance.
- Architectural constraints — exposed duct in a commercial space designed to look intentional, not like an afterthought.
- Additions and retrofits tying new ductwork into an existing system without shorting airflow to the rest of the house.
This is where having an in-house sheet metal shop rather than subcontracting fabrication out matters. We build custom duct, transitions, plenums, and curbs at our own shop, which means a measurement taken on-site translates to a part that actually fits — not a stock component forced into a space it wasn’t designed for. Learn more about this side of our work on our sheet metal and ductwork page.
What This Means for a New Construction Project
If you’re a builder or GC bringing in mechanical for a new home or commercial build, here’s roughly how the process should go:
- Load calculation first. Before any duct gets designed, the heating and cooling load for the specific building — square footage, insulation, window orientation, ceiling heights — determines what the equipment and ductwork actually need to deliver.
- Duct layout designed to the load, not the other way around. Trunk and branch sizing should be based on the airflow (CFM) needed at each register, not just “whatever fits in the joist bay.”
- Coordination with framing, not after it. The earlier mechanical is in the conversation with framing and structural, the fewer compromises get forced later — a duct chase that’s an inch too small because nobody flagged it during framing is a problem that follows the building for its entire life.
- Sealing, not just connecting. Duct leakage at joints is one of the most common (and most fixable, if caught early) sources of HVAC inefficiency. Properly sealed and, where required, tested ductwork keeps conditioned air going where it’s supposed to instead of into an attic or crawlspace.
- Commissioning and balancing. Once installed, a proper commissioning process checks that airflow at each register actually matches what the design called for — not just that air is coming out somewhere.
Commercial Considerations
Commercial buildouts add their own layer: rooftop unit curb work, transitions between old and new ductwork during a tenant improvement, and coordination with other trades in a tighter mechanical space than most residential jobs. For restaurant buildouts specifically, kitchen exhaust and makeup air ductwork has its own code requirements around grease duct construction and clearances — not something to hand to a fabricator who’s mostly done residential work. Our commercial and light industrial fabrication work covers both standard HVAC ductwork and these more specialized applications.
Remodels: The Trap of “It’s Already There”
The most common ductwork mistake we see in remodels isn’t a new install gone wrong — it’s reusing existing ductwork without checking whether it was sized correctly for the new layout. If a kitchen remodel changes room configuration, or an addition adds square footage tied into the existing system, the original ductwork was sized for a different house than the one that exists after the remodel. Reusing it without recalculating can leave you with a beautiful new space that never quite feels comfortable — a problem that’s far more expensive to fix after the drywall is up than during the planning phase.
Timeline: When to Bring HVAC Into the Project
A mistake we see even from experienced builders is treating mechanical as a late-stage trade instead of an early design input. Ideally, mechanical involvement looks like this:
- Design/pre-construction: Rough load calculation and system approach discussed alongside architectural plans, so chase sizes and equipment locations are accounted for before framing drawings are finalized.
- Framing: Final duct routing confirmed against actual framed chases, with any conflicts resolved while walls are still open.
- Rough-in: Ductwork, refrigerant lines, and venting installed and inspected before insulation and drywall.
- Trim-out: Equipment set, registers and grilles installed, controls wired and configured.
- Commissioning: System started up, airflow balanced room by room, and documentation handed over.
Bringing mechanical in at the design stage instead of the rough-in stage rarely adds meaningful time to a project schedule, and it avoids the far more costly alternative — a field change order after framing is already up because a duct chase turned out to be two inches too small.
Questions Worth Asking Your HVAC Contractor Before Framing Closes
- Has a proper load calculation been done for this specific building, or is duct sizing based on rule-of-thumb?
- Is duct fabrication happening in-house, or is it being subcontracted to a separate fabricator?
- How will duct joints be sealed, and will the system be tested for leakage?
- For additions or remodels: has the existing ductwork been evaluated for the new layout, or just extended?
- What’s the plan for balancing airflow room-by-room after installation?
If a contractor doesn’t have a clear answer to these, that’s worth a second opinion before the walls close in around the answer.
Working With Us on a Build
We work with builders and GCs across Spokane, Spokane Valley, Liberty Lake, and the broader service area on both residential new construction and commercial tenant improvement work, with in-house fabrication that means custom transitions and curbs get built to the actual site measurements rather than adapted from a catalog part. If you’re in early planning on a build and want mechanical involved before framing decisions get locked in, that’s exactly the right time to call.