Service guide
Ductwork: seal it, repair it, or replace it
The ducts are where the measured losses are, yet they are quoted last. Three decisions, one design document, and what a quote must say.
Get HVAC Options Duct sizing calculator
Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking
The short version
Most houses do not need new ducts. They need the ducts they have sealed, and insulated where they run through attics or crawl spaces.
Source fact: DOE’s review of 44 field studies found duct leakage in 90-100% of systems, and sealing improved efficiency by 16-41%.
Replace ducts when they have failed, cannot carry the airflow, or the walls are open anyway.
Why the ducts are the part that is worth measuring
A furnace or heat pump makes conditioned air. The duct system decides how much of it arrives, and where. That is an unglamorous role and it is why the delivery system is consistently the last line on a quote and the first thing cut when a budget tightens.
Source fact: The field literature does not support that ordering. In DOE’s review of 44 field studies, duct leakage was found in 90-100% of systems tested, low airflow in more than 50%, and once ducts are included in the count, some fault was present in 90-100% of systems.
Sealing the ducts improved efficiency by 16-41% and recovered up to 33% of capacity. A tune-up of the equipment itself returned approximately 5%.
HyreHVAC analysis: Put those two numbers next to each other and the ordering of a typical service conversation inverts.
The intervention with the small measured return is the one that gets sold annually; the intervention with the large measured return is the one that requires opening a crawlspace.
That is not a claim about anyone’s motives — sealing ducts is dirty, slow, hard to photograph and harder to explain to a customer who cannot see the ducts. It is a claim about where the evidence points.
The same review reports that systems in the field measured 59-84% of rated SEER. Equipment ratings are established on a test stand with the airflow the manufacturer specifies.
The gap between the sticker and the house is largely the delivery system, which is why buying a higher rating without fixing the ducts buys a fraction of what the rating promised.
Three different jobs that all get called “ductwork”
The three jobs below are not price tiers of the same job. They are different work, aimed at different faults, and being quoted the wrong one is the commonest way to spend money without fixing the complaint.
Sealing and insulating what is there
Mastic or approved tape at every joint, seam, boot and plenum connection; the air handler cabinet and the filter box sealed; insulation added or repaired where duct runs outside the heated part of the building. Nothing is removed and no layout changes.
This is the right job when the ducts are physically intact and reachable and the complaint is bills, dust, or uneven temperatures that are not extreme. It is also the job with the largest measured return in the DOE review, and the one most often skipped.
Targeted repair and partial replacement
Replacing crushed or disconnected flex runs, re-supporting sagging duct, replacing a boot that never sealed to the drywall, adding a return where a room has none, or replacing a run that cannot be sealed because of what it is made of.
This is the right job when one or two rooms are the problem and the rest of the system works. It is also the honest answer far more often than a full replacement quote implies — a single disconnected supply in an attic can account for an entire room’s complaint.
Replacement and redesign
New trunk and branches, sized and laid out to a design, usually with the plenum and the returns rebuilt and often with the equipment changed at the same time.
This is the right job when the ducts are unreachable, degraded beyond sealing, sized for equipment that no longer exists, or laid out so that no amount of sealing will move the required air.
It is also the right job when the ceilings are already open for other work, because access is most of the cost. HyreHVAC analysis: replacement without a design is the expensive way to keep your existing problem.
What actually fails, and what each fault does to the house
Duct faults are mostly not dramatic. They are a joint that was never sealed, a run that was stepped on, or a return that was never big enough — and each has a signature you can recognize.
| The fault | What it looks like in the house | Why it happens | What fixes it |
|---|---|---|---|
| Unsealed joints, seams and boots | Higher bills, dust arriving from nowhere, rooms furthest from the equipment underperforming | Sealing is slow and invisible, and cloth-backed duct tape fails within a few years in an attic | Mastic at every joint. This is the highest-return item in the DOE review and it is a labor cost, not a materials cost |
| Crushed, kinked or sagging flex duct | One room that never quite gets there, regardless of the thermostat | Flex laid over joists, pulled around a tight corner, or run longer than needed and left to droop | Replace or re-support that run. Cheap, targeted, and frequently the entire fix |
| A supply that is disconnected entirely | One room dramatically worse than the others, and an attic or crawlspace that is unusually comfortable | It was never fastened, or it was knocked off during other work in the attic | Reconnect and seal. The most satisfying repair in the trade, and one nobody finds without looking |
| Undersized or missing returns | Doors that pull shut, whistling under closed doors, bedrooms warm in summer and cold in winter | Returns are cheaper to omit than supplies and their absence is not visible | Add return path — a return in the room, a transfer grille or a jump duct. A design question, not a patch |
| Building cavities used as ducts | Leakage that cannot be sealed, and air drawn from a wall cavity or a floor joist bay into the house | Common practice in older construction: a panned joist bay used as a return | Line or replace it. Note that building cavities are not used as supply ducts, plenums or return ducts is a specific item a code official inspects (R403.3.5) |
| Uninsulated duct outside the heated space | Sweating ducts in summer, weak heat in winter, and a measurable share of the energy delivered to the attic | Insulation degrades, gets compressed, or was never adequate for the climate zone | Insulate to the required level. DOE’s Zero Energy Ready Home checklist requires a minimum of R-8 for ducts in a vented attic |
| A duct system sized for different equipment | The system runs constantly, static pressure is high, and no single component is faulty | Equipment was replaced and the ducts were not reassessed — the most common upgrade path there is | A Manual J load calculation and a Manual D duct design. This is the case where replacement is genuinely indicated |
HyreHVAC compilation of standard residential duct failure modes. The code section and the R-value cited are from the DOE documents in the sources below.
The design documents, and why a quote without them is not a specification
Three ACCA procedures run in sequence, and skipping any one of them invalidates the ones after it. This is the part of a duct quote that a homeowner can check without knowing anything about ducts.
It establishes the heating and cooling load of the dwelling, room by room, from the actual construction of the actual building.
Source fact: This is not optional best practice: DOE’s own field-study form for the 2021 IECC records it as an inspected item under sections R302.1, R403.7 — “Heating and cooling equipment is sized per ACCA Manual S based on loads calculated per ACCA Manual J or other methods approved by the code official”.
It selects equipment against the Manual J load at the design conditions.
New York State’s Clean Heat program states the requirement about as plainly as it can be stated: “Residential equipment sizing shall be based on manufacturers’ extended performance tables in accordance with ACCA Manual S, not based on nominal size or AHRI ratings.”
Nominal tonnage is a marketing category, not a capacity at your design temperature.
It sizes the duct system to deliver the airflow the equipment needs.
Duct size is not a function of airflow alone; it is a function of airflow and the friction rate available after the equipment, coil and filter have taken their share, divided across the total effective length of the run.
The duct sizing calculator works through why no chart of CFM against duct size can be correct.
EPA’s ENERGY STAR HVAC Design Report (Rev. 14, January 2025) records the duct design as four items: Design HVAC fan airflow; Design HVAC fan speed setting (e.g., low, medium, high).
Design total external static pressure (corresponding to the mode with the higher airflow in Item 5.2); Room-by-room design airflows documented below (which must sum to the mode with the higher airflow in Item 5.2).
HyreHVAC analysis: a design is airflow and pressure. If a quote gives you inches of duct with no CFM and no static pressure behind them, no design has taken place.
The most quietly important line in New York’s guidance is its list of what must not be included in a load calculation, which ends with: “Multiplicative or additive safety factors with no defined source.”
Padding the load is how oversized equipment gets specified, and oversized equipment short-cycles, dehumidifies poorly and needs ducts it will never properly use.
What a code official actually inspects on a duct system
DOE’s Building Energy Codes Program publishes the field-study form its assessors fill in against the 2021 IECC.
It is the clearest public statement of what a duct system is checked for, and it makes a useful checklist against a quote.
The code text itself is ICC’s and proprietary; we cite the sections and the inspected items, not the code language.
| Section | What is checked |
|---|---|
| R403.3.1 | Insulation R-value of every supply and return duct in unconditioned space, and separately in the attic |
| R403.3.2 | All joints and seams of ducts, of air handlers, and of filter boxes are sealed |
| R403.3.2.1 | The air handler carries a manufacturer’s designation of air leakage |
| R403.3.3 | A duct test report is provided to the code official |
| R403.3.4 | Post-construction total duct leakage, measured by the project team using the RESNET protocol, in CFM per 100 ft² of floor area at 25 Pa |
| R403.3.5 | Building cavities are not used as supply ducts, plenums or return ducts |
| R403.3.6 | Whether supply and return ducts are buried in ceiling insulation, and whether that is compliant |
Items from the Single-Family Field Study Printable Data Collection Form — 2021 IECC, US Department of Energy, Building Energy Codes Program, September 2024.
HyreHVAC analysis: Two of these are worth reading twice. R403.3.4 says the leakage is measured and reported as a number — so a duct job in a jurisdiction enforcing this ends with a test result, and you are entitled to a copy of it (R403.3.3).
And R403.3.5 rules out the panned joist bay: a building cavity is not a duct.
What is enforced where you live depends on which code your jurisdiction has adopted and amended, which is a question for your building department, not for us.
Reading a duct quote
Duct quotes vary more than any other line in an HVAC project, because the scope is genuinely elastic. These are the items that make two quotes comparable — and their absence is what makes the cheapest bid cheap.
Not a square-footage rule and not "same as the old one". If the contractor is replacing ducts without knowing each room’s load, the new ducts are being sized by memory.
A drawing with CFM on each branch. This is the deliverable Manual D produces, and it is the thing that makes a later dispute resolvable.
EPA’s commissioning checklist (Rev. 14, January 2025) builds the whole verification on it: test holes are created and marked, return and supply external static pressure are read separately and added, and the measured total is compared to the design figure. Two manometer readings settle more duct arguments than any amount of discussion.
Where the code requires it, this is already an inspected item. Where it does not, it is still the only way to know whether the sealing you paid for worked.
DOE’s Zero Energy Ready Home checklist sets a reference point for ducts in a vented attic: “Ducts (but not air handlers) may be located in a vented attic if minimum R-8 duct insulation is used, duct leakage to outdoors is measured ≤ 3 CFM25 per 100 ft2 of conditioned floor area”.
EPA’s checklist requires measured fan airflow within ± 15% of the design airflow, and room-by-room airflow within the greater of ± 20% or 25 CFM of design. A quote that promises comfort and specifies no tolerance has promised nothing measurable.
Boot-to-drywall connections and the plenum takeoffs are where much of the leakage is and where the work is least pleasant. A scope that says "seal accessible joints" has excluded them by wording.
Returns are the half of the system homeowners never think about and contractors most often leave alone. If any room’s complaint involves a door that pulls itself shut, the return is the fault and a supply-side quote will not fix it.
A replacement quote that reproduces the existing runs, sizes and register positions is a materials change, not a design. It will cost most of what a designed system costs and deliver the performance of the system you are replacing.
Duct size is an output of a calculation whose inputs are airflow and available pressure. A number of inches with neither behind it came from habit or from a chart that has never seen your house.
What moves the price, and why there is no price range on this page
HyreHVAC does not install, service or sell HVAC equipment, and holds no data on quotes, invoices or finished jobs. Nothing here is a price we observed. That is also why these tools can tell you to keep the system you have.
What can be set out honestly is what actually drives a duct job up or down, because the drivers are physical. Access is most of the cost.
A vented attic with a floor and standing headroom, a 30-inch crawlspace, and a slab house where the ducts are in the concrete are three different projects at the same square footage — and the third frequently is not a duct replacement at all, because the honest alternative is a different distribution system entirely.
Whether the building has to be opened: If ceilings, soffits or walls must be cut and restored, drywall and paint can rival the duct work.
This is why duct replacement is dramatically cheaper during a renovation than as a standalone project, and why "we are opening that ceiling anyway" is the single most useful sentence in this decision.
Materials and method: Fabricated sheet metal, insulated flex, and duct board are different labor as much as different material. Scope.
Adding returns, adding zoning, relocating the air handler and rebuilding the plenum are each separate work that arrives under the same heading on a quote. Testing.
A leakage test and a commissioning report cost money and are the part that makes the rest verifiable.
The useful next step: get the sealing-versus-replacing question answered by measurement rather than by quote.
A static pressure reading and a duct leakage number cost a fraction of a replacement and frequently show that the replacement is not required.
Then, if equipment is being replaced in the same job, put the whole project through the replacement cost calculator, which carries the ductwork and the electrical work as separate editable lines rather than burying them, and use the quote comparison worksheet to check three bids against the same scope.
The case where the answer is no ducts at all
Not every duct problem should be solved with ducts. Where the runs are in a slab, where an addition has no plausible route back to the air handler, or where a single room is the entire complaint, a ductless or partly ducted system removes the distribution losses rather than reducing them.
Source fact: New York’s Clean Heat load calculation guidance makes the accounting explicit — its list of components that must not appear in a load calculation includes “Duct losses/gains, where indoor equipment is ductless or where ducts are located inside conditioned space”.
Where there is no duct outside the conditioned space, there is no duct loss to calculate.
HyreHVAC analysis: This is not an argument that ductless is better. Ducted systems filter, ventilate and distribute in ways a wall cassette does not, and a well-designed duct system inside the conditioned envelope has no distribution loss worth the name either.
It is an argument that "replace the ducts" is one option among several, and that a contractor who has never raised the alternative has not costed it.
DOE’s own program rules acknowledge the same physics from the other direction, by permitting a limited exception rather than a blanket one: “Up to 10 ft. of total duct length is permitted to be outside of the home/unit’s thermal and air barrier boundary.”
Get connected
Tell us about the work
Three short steps: the job, your home, and how to reach you. Nothing goes to an HVAC company unless you agree.
A straight answer first. Matching is still being built, so this is not an instant quote. If you smell gas or a CO alarm sounds, leave the house and call 911 or your gas utility first.
Get HVAC Options
Tell us about the system and where you are. We will follow up about connecting you with heating and cooling contractors serving your area.
Questions
Do I need to replace my ductwork, or can it be sealed?
How much does new ductwork cost?
Is duct leakage testing required?
Why is one room always hotter or colder than the rest?
What insulation do attic ducts need?
Should I replace ducts at the same time as the equipment?
Can I seal my own ducts?
Written and audited by
HYRE HVAC Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the federal microdata file, the statute or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify.
The counts below are generated from the published pages themselves, last counted September 28, 2026, and they are what we have actually published rather than what we intend to.
- 13
- studies published
- 12
- federal sources read and cited
- 8
- studies published with their full dataset as CSV
- 51
- jurisdictions reproduced against EIA’s own tables
How this desk works
- Primary sources only. Federal data comes from the agency that collects it, in the file that agency publishes. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- We validate against the agency before we publish. First, we use each federal microdata file to reproduce the agency’s own published tables. Our cooling research reproduces EIA’s state estimates and standard errors for all 51 jurisdictions. That check caught a variance formula that was off by a factor of four.
- Every estimate carries its uncertainty. These are survey figures, not counts. Standard errors are computed from the replicate weights the federal file supplies and printed beside the estimate. An estimate too imprecise to publish is reported as such rather than printed.
- Nothing is typed by hand. Prose, tables and charts all read from one dataset built by script, so a number in a sentence and the same number in the table below it cannot disagree.
- We publish the data, not just the conclusion. 8 of our 13 studies offer the full computed table as a CSV download on the page, so you can check the analysis or disagree with it. A study without a row-level dataset gets no download link and claims none in its structured data.
- We correct in public. Where we have published a figure wrongly we fix the figure, rewrite any analysis that rested on it rather than patching the number underneath it, and leave a dated correction note on the page.
- We do not install or sell HVAC equipment, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Data as of DOE BTO literature review (2018); ENERGY STAR Rev. 14 checklists (January 2025); DOE ZERH V2 (December 2022). Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
HyreHVAC is a research desk and a referral site. We do not install, service or sell HVAC equipment, we do not perform electrical work, we do not dispatch technicians, and we hold no dataset of quotes, invoices or completed jobs.
Nothing on these pages is a price we have observed. That is also why they are free to conclude that the right answer is to do nothing.