Ductwork installation: what separates a system built to a standard from one that merely fits? The design comes first. A standard-grade job starts with an ACCA Manual J load, selects equipment by Manual S, sizes ducts by Manual D, seals with mastic, insulates to a stated R-value, then proves itself with a measured leakage test.
Key Takeaways
- Item 6.4 of the ENERGY STAR National Rater Field Checklist (Rev. 14) caps total duct leakage at the greater of 4 CFM25 per 100 sq. ft. of conditioned floor area or 40 CFM25 at rough-in, and 8 CFM25 per 100 sq. ft. or 80 CFM25 at final.
- Item 6.5 holds rater-measured leakage to outdoors at 4 CFM25 per 100 sq. ft. or 40 CFM25.
- Item 6.3 requires supply and return ducts in unconditioned space, trunk connections included, to be insulated to at least R-6.
- ENERGY STAR’s duct sealing guidance: “Never use duct tape, as it is not long-lasting.”
- In Lawrence Berkeley National Laboratory tests of 20 sealants, only cloth-backed rubber adhesive tapes failed.
- Moderate compression of flexible duct, the kind seen in the field, can raise pressure drop fourfold.
- Item 6.2 requires bedrooms pressure-balanced to within 3 Pa of the rest of the house.
What does a standard ductwork installation actually require?
A standard ductwork installation is documented before it is built. ENERGY STAR splits the evidence across three forms: a design report naming the load and duct design method, a rater field checklist covering the install, and a commissioning checklist recording measured airflow and static pressure. A system that merely fits skips all three.
That paper trail matters because duct faults disappear behind drywall. In a typical house, about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts, according to ENERGY STAR. On an existing system, diagnosis starts with measured static pressure, not the nameplate.
Why does ductwork installation start with Manual J, S and D?
Because duct size follows equipment airflow, and equipment size follows the room-by-room load. The ENERGY STAR National HVAC Design Report runs in that order: Item 3.1 calculates room-by-room loads, Item 4.1 selects equipment per ACCA Manual S, and Item 5.1 requires the ducts to be designed for that equipment per ACCA Manual D.
Item 3.1 accepts Unabridged ACCA Manual J v8 or 2013 ASHRAE Fundamentals. Item 5.2 records design fan airflow in each mode, Item 5.4 the design total external static pressure, and Item 5.5 the room-by-room design airflows, which must sum to the higher mode. A layout without those numbers is a guess.
Item 3a.1 of the National Rater Design Review Checklist then requires that report to comply with ANSI / RESNET / ACCA / ICC 310, no applicable items blank. Sizing from square footage, the habit that fails on every house, corrupts the duct design downstream, which is why insulation work should precede equipment selection.
Which duct material should a ductwork installation use?
All three common materials can hit the same leakage target, so the question is what each punishes. Rigid galvanized sheet metal, rigid fiberglass duct board and insulated flexible duct differ in where the insulation sits, how joints close, and how far an installer can degrade the design unnoticed.
| Material | Where the insulation sits | Approved closure | Where performance is lost |
|---|---|---|---|
| Rigid galvanized sheet metal | Wrapped outside to the code R-value | Mastic, or metal tape meeting UL 181, 181A or 181B | Joints. LBNL reports most leakage occurs at register, plenum and branch connections |
| Rigid fiberglass duct board | Built into the board, no separate wrap | Closures carrying a UL 181 listing | Same joint rule; rated R-value holds only where the board is intact |
| Insulated flexible duct | Factory jacket over the wire helix core | Mastic or listed tape plus the draw band UL 181B-FX assumes | Compression, sag and sharp bends the design never allowed for |
Flex gets blamed most, and the blame is misdirected. Pulled taut and supported, it performs to its published tables. LBNL tested 6 in., 8 in. and 10 in. flexible ducts under ASHRAE Standard 120-1999 and found moderate compression typical of field installations could increase pressure drop by a factor of four, with further compression approaching ten. A compressed run breaks the friction rate Manual D assumed, and resurfaces as a starved room that a zoning proposal is asked to rescue.
How much insulation do ducts in unconditioned space need?
At least R-6. Item 6.3 requires all supply and return ducts in unconditioned space, including connections to trunk ducts, to be insulated to at least R-6. DOE Building America restates the 2009 IECC section 403.2 rule, which is stricter on the supply side: supply ducts in unconditioned space insulated to at least R-8, with returns at R-6. Later code editions set the level by duct location and diameter, so confirm the edition your jurisdiction enforces.
The better answer is to stop putting ducts in unconditioned space. Building America recommends locating ducts in conditioned space and notes the DOE Zero Energy Ready Home program requires supply ducts inside it. Where that is impossible, how the attic is vented sets the duct’s surroundings.
Why does sealing use mastic instead of cloth duct tape?
Because cloth-backed duct tape is the one product that reliably fails. ENERGY STAR tells homeowners to seal with mastic or metal tape, then says plainly: “Never use duct tape, as it is not long-lasting.” The name is an accident of history.
The evidence is federal lab work. Walker and Sherman at Lawrence Berkeley National Laboratory ran accelerated longevity tests on 20 sealants: 3 UL 181B-FX tapes, 15 other tapes, a UL 181B-M mastic and an aerosol sealant. Only cloth-backed rubber adhesive tapes failed, in anything from a few days to several weeks. One sample peeled itself off a joint on a laboratory bench, before testing began.
A later phase tested eight cloth-backed tapes on round sheet metal joints; four failed after 87 days, including the one UL 181B-FX product among them. The same rule governs what a repair should put back.
The name is not a spec
“Duct tape” and “duct-rated sealant” are unrelated categories. Item 6.4 is a one-time test, so a sealant that degrades over a few summers passes on day one and fails three years later, unobserved.
What duct leakage numbers does the ENERGY STAR checklist require?
Item 6.4 offers two options. At rough-in, rater-measured total duct leakage must be the greater of 4 CFM25 per 100 sq. ft. of conditioned floor area or 40 CFM25. At final, with register grilles installed, it is the greater of 8 CFM25 per 100 sq. ft. or 80 CFM25. Item 6.5 caps leakage to outdoors at 4 CFM25 per 100 sq. ft. or 40 CFM25.
Footnote 28 relaxes the total for a system with three or more returns to 6 CFM25 per 100 sq. ft. at rough-in and 12 at final, and Footnote 27 requires testing to ANSI / RESNET / ICC 380, per system rather than per home.
How should flexible duct be supported?
With hanger strap or saddle supports at least 1.5 in. wide, spaced no more than 4 ft apart, per ACCA Manual D and the manufacturer’s instructions. The DOE Building America Solution Center caps sag between supports at 1/2 in. per foot and calls for extra supports either side of sharp bends.
Item 6.1 bans kinks, sharp bends, compressions and excessive coiled flexible ductwork outright. Footnote 24 defines the terms: a sharp bend is a turn with a radius less than one duct diameter, and compression is flexible duct in unconditioned space run through a cavity smaller than its outer diameter. Coils are permitted only for acoustical control. Each fault spends the same static pressure budget a high-MERV filter draws on.
Does every closed room need its own return?
It needs a path, not necessarily a duct. Item 6.2 requires bedrooms to be pressure-balanced using transfer grilles, jump ducts, dedicated return ducts or undercut doors, to a rater-measured differential of minus 3 Pa to plus 3 Pa against the main body of the house while all air handlers run.
Footnote 25 sets the test conditions: bedroom doors closed, doors to rooms reachable only through the bedroom open, highest design fan speed. It allows 5 Pa either way for bedrooms with design airflow of 150 CFM or more. A bedroom that pressurizes when its door shuts pushes conditioned air out through the shell, which is why closing supply registers makes the balance worse.
What documents should a ductwork installation hand over?
Three: the completed HVAC design report with room-by-room loads and design airflows, the duct leakage result in CFM25, and a commissioning record. The ENERGY STAR National HVAC Commissioning Checklist logs return and supply external static pressure at marked test holes, then requires measured fan airflow within 15% of the design value.
Air balancing sits one step beyond. Items 4.1 and 4.2 are recommended rather than required, asking for room-by-room airflows within the greater of 20% or 25 CFM of design, using balancing dampers at the trunk instead of looped duct. Those numbers give a homeowner a baseline for later claims, including a pitch for duct cleaning EPA does not routinely recommend. The same logic applies to a ducted heat pump swap or a ductless system that deletes the ducts.
How we researched this
Research completed September 2026. Cited: four EPA ENERGY STAR forms, two Lawrence Berkeley National Laboratory duct studies, three DOE Building America guides. Excluded: installed prices, since no federal series publishes duct pricing, plus all contractor and lead-generation sources. Reddit was not mined, because this topic turns on test procedure.
Frequently asked questions
Does new ductwork have to be leakage tested?
In an ENERGY STAR certified home, yes. Item 6.4 requires a rater-measured total duct leakage test, and Footnote 27 requires it to follow ANSI / RESNET / ICC 380, assessed per system rather than per home. Outside certification programs, local code decides.
Is flexible duct allowed in a code-compliant ductwork installation?
Yes, when installed to the design. Item 6.1 bars kinks, sharp bends, compressions and excessive coiling, and Building America calls for supports no more than 4 ft apart with sag held to 1/2 in. per foot.
What R-value do ducts in an attic need?
At least R-6 under ENERGY STAR Item 6.3, covering supply and return ducts and their trunk connections. The DOE Building America Solution Center puts the code figure at R-8 for supply ducts in unconditioned space.
Can ducts be sealed with duct tape?
No. ENERGY STAR states “Never use duct tape, as it is not long-lasting.” Lawrence Berkeley National Laboratory tested 20 sealants and found only cloth-backed rubber adhesive tapes failed. Use mastic, or tape listed to UL 181, 181A or 181B.
Does every bedroom need its own return duct?
No, but every bedroom needs a pressure path. Item 6.2 accepts transfer grilles, jump ducts, dedicated returns or undercut doors, provided the closed room measures within 3 Pa of the main body of the house.
What should the installer hand over at the end?
The HVAC design report with room-by-room loads and design airflows, the duct leakage result in CFM25, and a commissioning record showing measured external static pressure and fan airflow within 15% of design.
Sources
- U.S. EPA ENERGY STAR. “National Rater Field Checklist, Version 3.1 / 3.2 / 3.3 (Rev. 14).” https://www.energystar.gov/sites/default/files/2025-01/National%20Rater%20Field%20Checklist_Rev%2014.pdf. Accessed September 2026.
- U.S. EPA ENERGY STAR. “National Rater Design Review Checklist (Rev. 14).” https://www.energystar.gov/sites/default/files/2025-01/National%20Rater%20Design%20Review%20Checklist_Rev%2014.pdf. Accessed September 2026.
- U.S. EPA ENERGY STAR. “National HVAC Design Report (Rev. 10).” https://www.energystar.gov/sites/default/files/asset/document/National%20HVAC%20Design%20Report%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf. Accessed September 2026.
- U.S. EPA ENERGY STAR. “National HVAC Commissioning Checklist (Rev. 14).” https://www.energystar.gov/sites/default/files/2025-01/National%20HVAC%20Commissioning%20Checklist_Rev%2014.pdf. Accessed September 2026.
- U.S. EPA ENERGY STAR. “Duct Sealing.” https://www.energystar.gov/saveathome/heating-cooling/duct-sealing. Accessed September 2026.
- Walker, I.S. and Sherman, M.H. “Sealant Longevity for Residential Ducts.” Lawrence Berkeley National Laboratory. https://eta-publications.lbl.gov/sites/default/files/2003-astm-stp-1453.pdf. Accessed September 2026.
- Abushakra, B., Walker, I.S. and Sherman, M.H. “Compression Effects on Pressure Loss in Flexible HVAC Ducts.” Lawrence Berkeley National Laboratory. https://escholarship.org/uc/item/0d76400v. Accessed September 2026.
- DOE Building America Solution Center. “Duct Sealing and Insulation.” https://basc.pnnl.gov/home-improvement-expert/checklists/duct-sealing-and-insulation. Accessed September 2026.
- DOE Building America Solution Center. “Sealed and Insulated Flex Ducts.” https://basc.pnnl.gov/resource-guides/sealed-and-insulated-flex-ducts. Accessed September 2026.
- DOE Building America Solution Center. “Sealed and Insulated Metal Ducts.” https://basc.pnnl.gov/resource-guides/sealed-and-insulated-metal-ducts. Accessed September 2026.
Leave a Reply