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Home Energy Scope · Glossary
Energy retrofit terms, and how to use them when reading a proposal
This glossary defines the terms that appear in energy assessments, insulation bids, window labels and solar proposals: blower door, ACH50, R-value, U-factor, SHGC, air barrier, stack effect, rim joist, combustion safety, HERS Index, net metering and interconnection. Each definition is short, and the sections below explain how to use the terms to compare proposals and ask better questions.
12 terms you will see in home energy improvements proposals
- ACH50
- Air changes per hour at 50 pascals of pressure difference, a size-normalized leakage figure; older homes often exceed 10, tight existing homes reach 3 to 5.
- Air barrier
- The continuous layer that stops air movement through an assembly, distinct from insulation, which slows heat conduction; sealing the air barrier comes before adding insulation.
- Blower door test
- A calibrated fan mounted in an exterior door that depressurizes the house and measures air leakage, reported as CFM50 or ACH50; the baseline and target for air sealing.
- Combustion safety test
- Checks that gas appliances draft properly and do not spill exhaust or carbon monoxide indoors, performed before and after a house is tightened.
- HERS Index
- The Home Energy Rating System score from a RESNET-certified rater, where 100 is a reference new home and lower is better; the 2024 average rated home scored 55.
- Interconnection
- The utility's approval process for connecting a solar system to the grid, including permits, inspection and permission to operate, often taking weeks after installation.
- Net metering
- A utility rule crediting exported solar electricity at the retail rate; where replaced by lower export rates, self-consumption and batteries become more valuable.
- R-value
- Resistance to heat flow through a material or assembly; higher is better. ENERGY STAR attic targets for an uninsulated attic run from R-30 in climate zone 1 to R-49 in zones 2–3 and R-60 in zones 4–8.
- Rim joist
- The perimeter framing where the floor meets the foundation, usually uninsulated and leaky in older homes and a prime target for closed-cell spray foam.
- Solar heat gain coefficient (SHGC)
- The share of solar heat a window admits; low values suit hot climates and west exposures, higher values help south-facing glass in cold climates.
- Stack effect
- Warm air rising and leaving through the top of the house while cold air is drawn in low, which is why attic and rim joist sealing matter more than caulking windows.
- U-factor
- Rate of heat transfer through a whole window or door unit; lower is better, with about 0.27 or below a strong retrofit target in cold climates.
Energy proposals are full of numbers and abbreviations that look precise and are easy to misread. A contractor who promises R-49 without mentioning air sealing, or a window quote that lists a U-factor without the solar heat gain coefficient, is giving you half the picture. Knowing the terms lets you notice what is missing. That matters most when two bids use different terms for the same thing, or the same term for different scopes.
The glossary's term list defines each item. The sections here explain how to use the terms when reading a proposal, which ones get confused, and which questions they unlock. For the tests themselves, see the energy audit page. None of these terms is complicated once defined, but each one is a lever a contractor can use to make a proposal look more complete than it is.
How do you use these terms when reading a proposal?
Start by looking for measurable targets. An insulation proposal should state an R-value and a settled depth, plus a before-and-after blower door number in CFM50 or ACH50. A window proposal should list U-factor and SHGC for each unit, taken from the NFRC label. A solar proposal should state system size in kilowatts, estimated annual production in kilowatt-hours, and how the utility's net metering or export rules were applied.
When a term appears without a number, ask for the number. When a number appears without a term, ask what it measures. A proposal that says the house will be much tighter without a blower door target cannot be verified, and leakage is the norm: Pearl's registry analysis finds air leakage problems in ~90% of homes. The cost guide shows how these terms map to line items in a bid.
Which terms are most often confused?
R-value and U-factor measure related things in opposite directions. R-value is resistance to heat flow, used for insulation, and higher is better. U-factor is the rate of heat flow through a whole window or door, and lower is better. They are not direct inverses when comparing a window label to an insulation spec, because U-factor includes the frame, glass and edge effects. U-factor matters most where glass is weakest, and our analysis of the EIA's 2024 household survey found 33.1% of homes still have mostly single-pane windows.
Air barrier and insulation are also confused. Insulation slows heat conduction; an air barrier stops air movement. Fluffy insulation is not an air barrier, which is why air sealing comes first, and EPA's 15% saving estimate assumes both. ACH50 and CFM50 both come from a blower door test, but ACH50 normalizes for house size, making it easier to compare homes, while CFM50 is the raw leakage flow a contractor reduces.
- R-value versus U-factor: higher R is better; lower U is better.
- Air barrier versus insulation: one stops air, the other slows heat.
- ACH50 versus CFM50: size-normalized leakage versus raw leakage flow.
- Net metering versus interconnection: how exports are credited versus how the system gets approved to connect.
What questions do the terms unlock?
Each term suggests a question. Blower door: what is the current number, and what target will you reach? Combustion safety: will you test gas appliances before and after sealing? Stack effect: have you sealed the top and bottom of the house, not just the windows? It is a common cause of the constant drafts our EIA analysis finds in 13.8% of homes. Rim joist: is it included, and with what material? SHGC: is this value right for the west-facing windows?
For solar, net metering and interconnection prompt questions about how exports are credited, how long approval takes, and whether the production estimate uses your actual rate plan. For resale, the HERS Index prompts a question about whether a rating is worth getting. RESNET reports 436,798 homes were rated in 2024, a sign that more buyers and agents recognize the score.
Where do these terms show up in the rest of the site?
The audit comparison uses blower door, ACH50 and combustion safety. The window frame comparison leans on U-factor and SHGC. The insulation pages rely on R-value, air barrier, stack effect and rim joist, terms that matter because households describe 17.6% of homes as poorly insulated and 2.6% as uninsulated, by our analysis of EIA 2024 data. The solar pages rely on net metering and interconnection. Knowing which terms belong to which decision makes each page easier to use.
If you see a term in a proposal that is not listed here, ask the contractor to define it in writing. A clear explanation is a good sign; a vague one suggests the term is being used for marketing rather than measurement. The retrofit quiz tests several of these terms if you want to check your understanding before collecting bids.
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Glossary questions
What is a good ACH50 number for an older house?
Older homes often exceed 10 ACH50 before any work, and a well-executed air sealing retrofit on an existing home often reaches somewhere in the 3 to 5 range, as the glossary notes. The right target depends on the house and its ventilation. Ask the contractor what reduction they expect from the planned scope rather than aiming for a fixed number.
Is CFM50 or ACH50 more useful on a proposal?
Both are useful. CFM50 is the raw leakage the contractor is reducing, so it is the easiest number to track before and after. ACH50 adjusts for house volume, which makes it better for comparing your house with others or with a program target. Ask for both, and make sure the before and after tests use the same setup.
Why do window labels list SHGC as well as U-factor?
U-factor tells you how much heat passes through the whole unit by conduction, while SHGC tells you how much solar heat the glass admits. In hot climates, SHGC often matters more for comfort and cooling bills. In cold climates, a moderate SHGC on south-facing glass can help. Both appear on the NFRC label for a reason.
Does a higher R-value always mean better insulation?
A higher R-value means more resistance to heat flow for that material or layer, but installed performance depends on air sealing, gaps, compression and moisture. An R-49 attic over unsealed top plates can underperform a lower R-value over a sealed floor. Treat R-value as necessary but not sufficient, and pair it with a blower door result.
What is the difference between a HERS rating and an energy audit?
A HERS rating is a standardized score produced by a RESNET-certified rater using a defined protocol, useful for comparing homes and for some programs. An energy audit is a broader diagnostic that may or may not produce a HERS score, focused on finding problems and ranking fixes. Some auditors provide both.
Why does net metering matter so much for solar payback?
Net metering determines how much the utility credits you for power you export. Under full retail net metering, the grid effectively stores your surplus at full value. Where export credits are lower, self-consumption and batteries become more valuable. The same system can have very different payback under different rules.
Is stack effect the same in every house?
It is present in every heated house, but its strength depends on height and the temperature difference between inside and outside. Tall, multi-story homes in cold climates have the strongest stack effect. That is why sealing the attic floor and the basement rim joist usually does more for drafts than sealing around windows.
See the terms in context
Service guideHome Energy AuditsA home energy audit connects comfort complaints and utility bills to the measured condition of the house through a blower door test, thermal imaging, duct leakage testing and combustion-safety checks, then delivers written, prioritized recommendations.
Service guideInsulation & Air SealingAir sealing closes the gaps that move air through the attic plane, rim joist and penetrations; insulation slows heat conduction through the assemblies around them.
Service guideWindow & Door ReplacementWindow and door replacement addresses openings with a defined problem: damage, failed seals, poor operation, drafts, single-pane glass in a cold climate, or a door that leaks and does not latch.
Service guideSolar Panel InstallationSolar panel installation covers the roof and shading assessment, equipment selection, electrical and structural permits, utility interconnection and the production and financial assumptions in the proposal.
Free planning tools
Free toolEnergy audit cost calculator: size the assessment before the retrofitSize, material tier and complexity in, a realistic budget range out.
Free toolEnergy retrofit comparisons: the five decisions inside a retrofitSide-by-side tables for the decisions that change the price most.
Free toolEnergy retrofit readiness quiz: do you know the sequence and the tests?10 questions that show what you already know and what to check next.
Free toolEnergy retrofit polls: how other homeowners weighed the same decisionsSee what other planners chose and add your vote.
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