Pick ductwork as a system, not a material sample
A useful duct specification answers five questions: how much air each room needs, what pressure the blower can deliver, how long and restrictive each path is, what material fits the route, and how leakage and insulation will be controlled.
| Type | Where it earns a look | Main field control |
|---|---|---|
| Sheet metal | Trunks, plenums, exposed or damage-prone routes, custom fittings | Seal seams and joints; add the specified insulation and vapor control |
| Flex duct | Routed branches where flexibility reduces fitting count | Pull taut, keep runs short, avoid kinks, and support correctly |
| Fibrous-glass ductboard | Fabricated insulated duct where its listed pressure and temperature range fit | Use listed closures, reinforcement, dry storage, and careful handling |
No material fixes a missing design. ACCA says Manual D provides ANSI-recognized duct-sizing principles and calculations that apply to all duct materials. (ACCA Manual D) Start with room-by-room loads and equipment airflow; our HVAC sizing and Manual J guide explains that upstream step. Then select duct type and fittings around the design airflow, available static pressure, installation space, and adopted code.
Rigid sheet metal duct
Sheet metal is useful where the layout needs fabricated trunks, plenums, transitions, and durable exposed sections. Round, oval, and rectangular shapes can all perform; the design must account for the actual shape, fittings, equivalent length, pressure class, and supports.
Metal is not automatically tight or insulated. ENERGY STAR recommends sealing accessible duct leaks with mastic sealant or metal tape and says never to use duct tape because it is not long-lasting. (ENERGY STAR duct sealing) For runs outside conditioned space, specify the insulation level and continuous vapor-control details instead of leaving wrap duct as a field decision.
Treat fittings as design components. A compact trunk with abrupt transitions and high-loss takeoffs can consume more pressure than a longer route with better fittings. Require the schedule or drawing to show fitting types and balancing dampers, not just duct dimensions.
Flexible duct: installation quality is the specification
Flex duct can solve routing problems and reduce fabricated fittings, but only when the installed shape matches the design assumption. The current Manual D specifically includes the impacts of excess length, sag, and compression in flexible ducts. (ACCA Manual D)
The NAIMA installation guide says flexible-duct connections should avoid bends greater than 90 degrees and use the shortest possible lengths. (NAIMA fibrous-glass insulated HVAC duct systems guide) The same guide limits support spacing to 5 feet and permissible sag to 1/2 inch per foot between support points. (NAIMA guide)
Turn those details into inspection points: verify the delivered diameter, pull the inner core taut, support without crushing the insulation, use smooth-radius turns, seal the core at collars, restore the insulation and jacket, and photograph concealed runs before close-in.
Fibrous-glass ductboard: listed assembly, dry handling
Ductboard is not loose insulation folded into a box. NAIMA describes fibrous-glass duct systems as modules fabricated from flat ductboard with a factory-applied reinforced aluminum exterior air-barrier finish. (NAIMA fibrous-glass insulated HVAC duct systems guide) The guide also says systems made from qualifying ductboard products are UL 181 Class 1 air ducts. (NAIMA guide)
The value is combined duct structure, thermal control, and sound absorption in one assembly. The tradeoff is that fabrication, closures, reinforcement, surface condition, and storage all matter. The NAIMA guide says jobsite materials should be protected from dust, dirt, moisture, and physical abuse. (NAIMA guide)
Reject wet, crushed, punctured, or poorly closed sections rather than hiding damage above a ceiling. Confirm the product listing, pressure, velocity, temperature, closure, and cleaning limits against the design.
Manual D sizing, sealing, and balancing
Manual D is not a diameter lookup performed before the rest of the design. ACCA says its early sections cover duct-system physics, airflow, blower airflow, and the interaction between a blower and a duct system. (ACCA Manual D) That means the designer needs the equipment airflow and blower data, room airflow targets, available static pressure, longest or most restrictive paths, fitting losses, filter and coil pressure drops, and the installed duct characteristics.
Leakage belongs in the same conversation. ENERGY STAR says a typical house loses about 20 to 30 percent of the air moving through its duct system because of leaks, holes, and poor connections. (ENERGY STAR duct sealing) It recommends mastic or metal tape for accessible leaks and insulation on accessible ducts in attics, crawlspaces, unfinished basements, and garages. (ENERGY STAR duct sealing)
Close the job with measured results: total external static pressure, delivered airflow or accepted proxy, room balance, filter condition, and duct-leakage testing where the project requires it. If equipment replacement is part of the scope, check whether you need an HVAC permit and which tests the local inspector expects.
Frequently asked questions
Which duct material is best?
There is no universal winner. Sheet metal fits durable trunks and fabricated fittings; flex can fit routed branches when installed taut and supported; ductboard combines structure and insulation where its listing fits. Design pressure, route, cleaning needs, code, labor, and field quality decide.
Why does duct sizing matter?
The blower and duct system operate together. Duct and fitting resistance affect delivered airflow, noise, room balance, and equipment performance, so dimensions should come from an accepted design method rather than a rule of thumb.
How much air can leaky ducts lose?
ENERGY STAR says a typical house loses about 20 to 30 percent of duct airflow through leaks, holes, and poor connections. The actual project needs testing rather than assuming the typical range.
How should flex duct be installed?
Keep it as short and direct as the design allows, avoid sharp bends and compression, support it at the required spacing, limit sag, seal the inner core at connections, and preserve the insulation and jacket.