Somewhere in a kitchen drawer or a downloads folder, there is a home energy audit report that a homeowner paid several hundred dollars for and never fully read. It has a cover page, a summary paragraph, and then two or three pages of numbers — ACH50, HERS Index, CFM25, delta-T — that look like they belong on a mechanical engineer's desk, not in a homeowner's hands. That report gets filed away, the recommendations get skimmed at best, and the money those numbers were supposed to unlock never gets claimed. This guide exists to fix that. By the end of it, you'll be able to look at your own audit report and know exactly what every core number means, which rebate or tax-credit program it feeds into, and how to turn a page of diagnostic data into a ranked action plan instead of a filed-away PDF.
The reason this matters more than it used to: federal and utility incentive programs have gotten stricter about requiring actual measured or modeled data, not just a contractor's word that a project improved efficiency. That's good for homeowners in the long run — it means the programs are harder to game — but it also means your audit report isn't just a diagnostic summary anymore. It's an application document. Understanding it is the difference between claiming every dollar you're eligible for and leaving money on the table because a form asked for a number you didn't know how to find.
There's also a practical reason to learn this material even if you never file a single rebate application: these numbers tell you, in objective terms, where your house is actually losing money every month. A high ACH50 score means conditioned air — air you paid to heat or cool — is escaping through gaps you can't see. A high CFM25 duct leakage number means a meaningful share of that same conditioned air never even reaches your living space before it leaks into an attic or crawlspace. Once you can read these numbers, you stop having to trust a contractor's sales pitch about which upgrade you "need" and start being able to verify it yourself against your own report.
This guide is organized the way you'd actually use it against your own report: first the four core metrics in plain English, then exactly which incentive program each one feeds into, then how to turn a flat recommendation list into a ranked action plan using real payback-period math, a worked composite example showing the whole process start to finish, and finally a short list of questions to ask before you ever pay an auditor a deposit. If you already have a report in hand, feel free to jump straight to the metric-to-money crosswalk in Section 2 — that's usually the part that turns a confusing document into an actionable plan the fastest.
Section 1 — The Four Core Metrics, in Plain English
Almost every diagnostic-grade audit report is built around four numbers. Once you can translate these four, the rest of the report — the narrative sections, the photos, the recommendation list — becomes much easier to interpret, because everything else in the report is really just supporting detail for these core scores.
ACH50 (Air Changes per Hour at 50 Pascals) comes from the blower door test — a calibrated fan mounted in an exterior doorway that depressurizes your home to a standardized pressure so the total air leakage can be measured consistently regardless of outdoor wind or weather. The result is expressed as how many times the entire volume of air in your house would be replaced per hour at that pressure. Lower means a tighter building envelope and less conditioned air escaping through gaps, cracks, and penetrations.
HERS Index is a modeled whole-house score, not a raw field measurement — it's calculated by a RESNET-certified rater using software that factors in your building envelope, HVAC equipment, water heating, air leakage, and duct performance. A score of 100 represents a home built exactly to the 2006 International Energy Conservation Code reference home; a score of 0 represents net-zero energy use. Every point below 100 represents roughly a 1% reduction in modeled energy use relative to that reference home.
CFM25 (Duct Leakage at 25 Pascals) comes from a duct blaster test, a smaller version of the blower door test applied directly to your ductwork. It measures how much conditioned air is escaping the duct system in cubic feet per minute before it ever reaches your living space — reported both as total leakage and leakage specifically to outside the conditioned envelope (attics, crawlspaces, garages).
Thermal Delta-T comes from an infrared camera scan, usually performed while the blower door is running so depressurization pulls outside air through any gaps and makes them visible as temperature anomalies on camera. Delta-T is simply the temperature difference, in degrees, between a suspect area and the surrounding surface — the larger the delta-T, the more significant the insulation void or air leak.
It helps to understand how these four numbers relate to each other, because a report that only shows one of them in isolation is telling you an incomplete story. ACH50 tells you how leaky the building envelope is as a whole, but it doesn't tell you where the leaks are. Thermal delta-T pinpoints the where. HERS Index rolls ACH50, your HVAC equipment efficiency, your water heating, and your duct performance into a single comparative score, which is why two homes with identical ACH50 numbers can still have very different HERS scores if one has a newer furnace or tighter ducts. And CFM25 is really its own mini blower door test focused specifically on the duct system, which matters because a house can have an excellent envelope ACH50 and still be losing a large share of its conditioned air through leaky ductwork before it ever reaches a supply register. A complete audit report uses all four together — envelope leakage, targeted leak locations, whole-house modeled performance, and duct-specific performance — because acting on just one in isolation usually means missing a meaningful chunk of the total energy loss.
One common misreading is treating the blower door number as a pass/fail grade rather than a baseline. There is no universal "good" ACH50 in a vacuum — a 6.0 on a 1975 ranch home is a real improvement worth celebrating, while the same 6.0 on a home built to current energy code would be considered underperforming. Always read your number against the benchmark table for your home's construction era, not against an arbitrary target you saw online.
| Home Age / Era | Good ACH50 | Average ACH50 | Poor ACH50 |
|---|---|---|---|
| Pre-1980 | Under 8 | 10-15 | Above 18 |
| 1980-2000 | Under 6 | 7-10 | Above 12 |
| 2000-2015 | Under 4 | 5-7 | Above 9 |
| 2015-Present (code-built) | Under 3 | 3-5 | Above 6 |
Section 2 — The Metric-to-Money Crosswalk
Here's the part most homeowners never connect: each of those four numbers isn't just diagnostic trivia — it's the specific evidence a rebate program or tax credit asks for. Programs don't take your word that a project improved efficiency; they want the measured or modeled number that proves it.
| Metric | Program It Feeds | Why It's Required |
|---|---|---|
| ACH50 (before/after) | Federal 25C, utility air-sealing rebates | Proves the air-sealing work actually reduced leakage |
| HERS Index | HOMES Rebate Program | HOMES calculates rebate tier directly from modeled savings |
| CFM25 duct leakage | Utility HVAC rebates, new-construction code sign-off | Required for duct-sealing and system-replacement incentives |
| Delta-T / insulation findings | HEEHRA, utility insulation rebates | Documents the specific insulation deficiency being corrected |
The Federal 25C Energy Efficient Home Improvement Credit rewards documented envelope and equipment upgrades identified by your audit, up to annual category caps. The HOMES Rebate Program calculates its rebate tier directly from your modeled or measured whole-house savings — meaning your pre- and post-upgrade HERS scores are essentially the application itself. HEEHRA (the High-Efficiency Electric Home Rebate Act) layers income-qualified incentives on top, often keyed to specific measures your audit identifies. And local utility programs almost always want the raw test numbers, not a narrative description, as the proof line item on their rebate form.
In practice, most homeowners are working with two separate audit reports over the life of a project: the initial diagnostic report that identifies the opportunity, and a post-upgrade verification report that documents the result. Rebate programs almost universally want to see both, because the delta between them is the actual proof of savings — a single report showing "R-38 insulation installed" is a receipt, but a before/after ACH50 or HERS comparison is evidence. This is also why it's worth budgeting for the re-test as part of the total project cost from the start rather than treating it as an optional add-on after the fact — without it, several of the larger rebate tiers simply aren't reachable no matter how much work was actually done.
It's also worth knowing that these programs are not mutually exclusive. A single air-sealing and insulation project can often qualify for a federal 25C credit on the material and labor cost, a HOMES or HEEHRA rebate based on the modeled or measured whole- house savings, and a local utility incentive on top of both — provided the documentation for each program is assembled correctly and the applications are filed in the right order. Stacking rules and caps change program to program and year to year, so treat any specific dollar figures you see online as a starting estimate to confirm against current program guidelines, not a guarantee.
Section 3 — Ranking Your Recommendations by ROI, Not Urgency
Most audit reports list recommendations in the order the auditor walked through the house, not in the order that makes financial sense. That's backwards. The right way to work through a recommendation list is by payback period — the amount of time it takes for the energy savings to pay back the project cost, net of any rebate or credit dollars.
Here's a worked example using plausible numbers for a mid-size single-family home. Suppose air sealing costs $1,800, is expected to save $420 per year in energy costs, and qualifies for a $600 federal 25C credit. Net cost after credit is $1,200, and the simple payback period is 1,200 ÷ 420 = roughly 2.9 years. Compare that to an attic insulation upgrade costing $3,600, saving $380 per year, with a $900 utility rebate — net cost of $2,700, payback of 2,700 ÷ 380 = roughly 7.1 years. Both projects are worth doing eventually, but the air sealing project should be sequenced first — not because it's urgent, but because the dollars come back faster and, critically, sealing first improves how effectively the insulation project performs on its own re-test.
Ask your auditor to rank your specific recommendation list this way if the report doesn't already do it. A good diagnostic-grade report will show projected cost, projected annual savings, applicable rebate/credit amount, and calculated payback period side by side for every recommendation — not just a checklist of things that could be fixed.
A second example illustrates why sequencing matters as much as the individual numbers. Say your report also lists a duct-sealing project: $1,200 in cost, reducing CFM25 leakage enough to save an estimated $260 per year, with no rebate currently available in your utility territory for that specific measure. Net cost stays at the full $1,200, and payback is 1,200 ÷ 260 = roughly 4.6 years — worse than the air sealing project but better than the insulation project on a pure-math basis. Ranked purely by payback period, the order becomes air sealing, then duct sealing, then attic insulation. But a diagnostic-grade auditor should also flag interaction effects: if your ductwork runs through an unconditioned attic that is about to be re-insulated anyway, it may be worth doing the duct sealing and insulation work in the same site visit even though the pure payback math would sequence them separately, simply to avoid paying for two separate attic access trips. This is the kind of judgment call a ranked-but-not-rigid list gives you room to make; a flat checklist doesn't.
One more nuance worth flagging: payback period math changes if your household is planning to sell within a few years. A HERS Index improvement has a documented, appraisal-relevant value in many markets even if the simple energy-cost payback period is longer than your expected time in the home — ask your auditor whether your local MLS or appraisal process recognizes HERS scores, since that can shift a project from "skip it" to "worth doing" even on a compressed timeline.
Section 4 — A Composite Before/After Example
The following is a hypothetical, composite illustration — not a specific real project — built to show how the numbers connect from initial test through final documentation.
A 1988-built, 2,100-square-foot home starts with an ACH50 of 11.4 (average-to-leaky for its era) and a modeled HERS Index of 92. The audit identifies rim joist gaps, an undersealed attic hatch, and roughly R-19 attic insulation where current code targets R-49. After air sealing and an attic insulation top-up, the home re-tests at an ACH50 of 5.6 — a 51% reduction — and the updated HERS model lands at 71, a 21-point improvement. That improvement translates to an estimated $540 in annual energy savings, a $600 federal 25C credit for the air sealing, and an $850 utility rebate for the insulation upgrade tied directly to the documented R-value increase and the post-sealing blower door number. Total project cost of $5,400 minus $1,450 in combined credits and rebates nets to $3,950 out-of-pocket, against $540 per year in savings — a payback period of roughly 7.3 years, with the first two-plus years of that timeline already offset by the incentive dollars claimed using the audit's own before/after numbers as documentation.
Notice what made this example work from a documentation standpoint: the pre-upgrade report established a baseline (11.4 ACH50, HERS 92), the scope of work was chosen specifically to move those two numbers, and the post-upgrade re-test produced a second data point that could be directly compared to the first. Every rebate and credit dollar in this example traces back to that before/after pair — not to a contractor's invoice describing what was installed, and not to a photo of new insulation in an attic. If the homeowner had skipped the re-test to save a few hundred dollars, the utility rebate portion of this example (the larger of the two incentives) would likely have been unreachable, since it was calculated from the documented R-value and airtightness improvement rather than a flat per-project amount.
It's also worth noting what this composite example doesn't include: combustion safety re-testing. Any time air sealing meaningfully tightens a home's envelope, diagnostic-grade practice calls for a worst-case depressurization and combustion safety check on any fuel-burning appliances before the project is considered complete — not because the numbers above are wrong, but because a tighter envelope changes how those appliances draft. A responsible auditor folds this into the project scope rather than treating it as a separate upsell, and a complete report will show a passing combustion safety result alongside the final ACH50 and HERS figures.
Section 5 — Questions to Ask Your Auditor Before You Pay for the Report
- Will the final report include a ROI-ranked recommendation list, or just a checklist?
- Is the blower door test performed to BPI or RESNET standard, and will I get the raw CFM50/ACH50 numbers, not just a pass/fail summary?
- Does the report include a HERS Index, or only a qualitative "good/fair/poor" rating?
- Will you provide documentation formatted for the specific rebate or tax-credit programs I plan to apply for?
- Do you offer post-retrofit re-testing, and is that priced separately or bundled?
A diagnostic-grade audit should answer every one of these with specifics, not general reassurance. If an auditor can't tell you whether their report will include a modeled HERS Index or raw ACH50 figures, you're likely looking at a walkthrough-style inspection rather than the instrument-based testing that rebate programs actually require.
It's worth pressing on each of these before you book, not after — a report that lacks raw test numbers can't be retroactively upgraded once the technician has already left your house, and a rebate application built on a qualitative "good/fair/poor" rating instead of an actual ACH50 or HERS figure will very likely bounce back from the reviewing program asking for the number it was missing in the first place. Five minutes of questions before you pay a deposit is cheaper than a second site visit to collect data the first report should have included.
The numbers on your audit report aren't decoration — they're the paperwork. Once you know how to read them, the report stops being something you file away and starts being the tool that tells you exactly which upgrade to do first, and exactly how much of it someone else is going to pay for.