Heating and cooling make up 52% of a home’s total energy use, EIA found, so those leaks show up directly on your bill every month. Space heating alone is the largest single piece of that, 46% of total energy consumption in single-family homes in 2015. Every leak below cuts into that slice of your bill.

Air leaks let conditioned air escape

Warm air rises and pushes out through gaps at the top of a house, an attic hatch, a recessed can light, a gap around a flue. Cold air gets pulled in low down, around sill plates and rim joists. Building scientists call this the stack effect, and it pulls hardest on the coldest days, exactly when a house needs to hold on to its heat.

The EPA models the fix on a typical 1,700-square-foot house built between 1970 and 1989. Its target is a 25% cut in total air infiltration from sealing gaps around the attic and foundation, plus around windows and doors, on a modeled house that starts out with 23% of its duct system air already leaking.

Combined with adding insulation, sealing air leaks can cut 15% off your heating and cooling costs, or 11% off your whole bill. How much of that lands in your pocket depends on your climate zone, from 5% in the warmest zones up to 16% in the coldest.

The attic and walls lose heat without enough insulation

Walls and rim joists make up more than 40% of a house’s total exterior envelope area, the Department of Energy notes, so thin or gapped insulation there covers a lot of surface. The trade group representing insulation manufacturers estimates that 90% of US homes are under-insulated, though that figure comes from an industry source with a stake in insulation sales.

How much insulation a house needs depends on climate zone. Under the 2021 building energy code, ENERGY STAR’s retrofit guidance calls for R30 to R60 in the attic depending on the zone. Find your own zone in the table below and see whether your attic already clears it.

Source: ENERGY STAR, “Insulation R-Values”, based on the 2021 International Energy Conservation Code.

Windows lose heat through the glass and the frame

A window loses heat straight through the glass by conduction, and around a loose sash or frame by air leakage. The glass side is rated by U-factor, which for windows on the market ranges from about 0.15 to 1.1. The lower the U-factor, the less heat moves through the glass, and a single pane of old glass sits at the leaky end of that scale.

Standing next to an old single-pane window in winter feels colder than the room’s thermostat setting would suggest. That’s radiant heat loss, the cold glass pulling warmth from a body nearby even before any air moves.

Swapping out a window like that for an ENERGY STAR certified one can cut your household energy bills by an average of up to 13% nationwide. A separate Department of Energy resource guide puts the range at 6% to 13% in utility bill savings for the same switch. More than 85% of windows sold in the US are ENERGY STAR certified, so a lot of the easy replacement gains are already banked into homes built in the last decade or so.

Ducts lose conditioned air before it reaches a room

In a typical house, 20% to 30% of duct system air leaks out before it heats or cools a room, through loose connections and gaps at the plenum. That leakage alone can cut a system’s efficiency by as much as 20%. Even a newly installed furnace or heat pump still loses 10% to 30% of its output to duct leakage, so the fix is in the ducts, not the equipment.

If your ducts run through an unconditioned attic or crawlspace, that lost air is heating or cooling a space you never use, on your dime. A 2.5-ton air conditioner moves 1,000 cubic feet of air a minute through the ducts, easy math for what leakage costs in cooling terms. With 20% duct leakage, it misplaces about 6,000 Btu an hour, roughly half a ton of cooling, before that air ever reaches a room.

Technicians test duct leakage against ANSI/RESNET/ICC Standard 380, and size a full duct replacement to the industry’s Manual D standard, which now has its own section on how leaks change a system’s performance.

Aerosol duct sealing, a technology Lawrence Berkeley National Lab licensed out for commercial use, seals 70% to 90% of duct leaks with a 10-year warranty without opening up walls or ceilings.

Floors and basement walls lose heat too

An uninsulated basement wall can be responsible for up to a third of an average home’s heat loss on its own, according to a Department of Energy-funded Building America study from 2003, revised in 2007. Basement walls sit behind the finished part of a house, so they’re an easy place to skip. A basement or crawlspace with bare concrete or block walls in your own house fits that same up-to-a-third estimate for heat leaking out.

A cold floor over an unheated crawlspace works the same way a cold window does, pulling heat out of the room above it long before anyone touches the thermostat. The floor above an unconditioned crawlspace or basement needs the same climate-zone range ENERGY STAR gives for attics, from R13 up to R38, shown in the table above.

What sealing and insulating are worth on a real bill

The federal Weatherization Assistance Program has run these fixes on real, low-income households since 1976, a useful check on the estimates above. Participating households save $372 or more a year on average, and the program now reaches about 32,000 homes annually. It has helped more than 7.2 million families since it started, real households whose bills back up the modeled numbers above. Skip the sealing and insulating in your own house, and you’re leaving something close to that $372 a year on the table.

A thermostat setback costs nothing and works before any sealing or insulation happens. Lowering the thermostat 7 to 10 degrees for eight hours a day can save up to 10% on annual heating and cooling costs. Sealing and insulating are the same fixes behind Energy Collective’s look at where household energy goes to waste. Once the easy leaks are sealed, a geothermal heat pump is the next lever worth pricing out for the equipment itself.

The US wasted 65.8% of the primary energy it consumed in 2024, or 62.27 of 94.6 quadrillion Btu, according to Lawrence Livermore National Laboratory’s annual energy flow chart. Most of that loss happens before a fuel or a kilowatt reaches your meter, as heat that never turns a wheel or lights a room. That share hasn’t moved much in years, even after a decade of cheaper LED bulbs and tighter appliance rules.

How much energy the US wastes

Total US energy use splits into two buckets on that LLNL chart. One bucket is energy that does something, like running a motor or heating a room. The other is energy that’s rejected along the way, mostly as waste heat from burning fuel and generating power. In 2024, only 32.34 quadrillion Btu counted as useful energy services, the other 34.2% of the total.

A year earlier, in 2023, the chart put total consumption at 93.6 quadrillion Btu with 65.7% rejected, almost identical to 2024’s share. Whatever you paid for gas, electricity or heating oil that year, roughly two out of every three Btu never reached a light, a motor or a furnace vent as useful heat.

Behind that flat total, the generation mix shifted. Natural gas supplied a record 34.3 quadrillion Btu in 2024, and renewables supplied a record 8.7 quadrillion Btu. Coal fell to a record low of 7.9 quadrillion Btu. None of that moved the rejected share.

Where the wasted energy goes, sector by sector

Power plants and vehicles reject more energy than anywhere else in the system. Converting fuel to electricity lost 19.21 of 32.77 quadrillion Btu in 2024, a 58.6% loss rate. That works out to roughly 41% conversion efficiency for the average US power plant, a different number than a plant’s capacity factor, which tracks how much of its yearly potential it delivers. See the capacity factor breakdown for that number by source.

Picture the heat and exhaust pouring out of a tailpipe, gasoline you paid for at the pump that never turned a wheel. Transportation runs almost entirely on combustion, which is why LLNL attributes its loss to an assumed 21% end-use efficiency for engines and drivetrains. That leaves 22.35 of 28.29 quadrillion Btu rejected in 2024, a 79% loss rate, the highest of any sector.

Industry and buildings reject a smaller share of what they use. The table below breaks out each sector’s 2024 loss rate, from industry down through homes and businesses. At home, less of what you pay for in electricity and gas leaks away as heat than what leaves as exhaust from your car.

Source: Lawrence Livermore National Laboratory, 2024 US Energy Flow Chart

Is US energy efficiency improving

By some measures, yes. US energy intensity, the amount of energy it takes to produce a dollar of GDP, fell from 5.03 to 4.05 thousand Btu per dollar between 2015 and 2024, a drop of about 19.5%, according to EIA. Energy use per person fell too, from 295 to 278 thousand Btu over the same years. If your home has swapped in an LED bulb, a more efficient furnace or an electric heat pump, that’s part of the same trend, even though the national rejected-energy share hasn’t followed it down.

Why bigger efficiency gains haven’t moved the total

LED bulbs and better appliances deliver real, measured savings, but they’re small next to the two categories that dominate rejected energy, transportation and power generation. LEDs saved an estimated 1.1 quadrillion Btu in 2017, about $12 billion in avoided energy costs that year, according to the Department of Energy’s solid-state lighting forecast. Transportation alone rejected 22.35 quadrillion Btu in 2024. Those numbers come from different years and aren’t a direct comparison, but lighting is a small slice of a total that transportation fuel and power-plant conversion losses dominate.

Even DOE’s most optimistic case falls short of that total. That scenario projects cumulative LED savings of 78 quadrillion Btu and about $890 billion in avoided costs from 2017 to 2035. Rejected energy alone topped 62.27 quadrillion Btu in 2024, more than DOE’s entire 18-year projection combined. Whatever you’ve swapped to LEDs at home, the national total barely moves, because lighting is a sliver of a system where cars and power plants do the rejecting.

A 2013 estimate from Energy Collective, citing a 2008 study, put US efficiency at 42%. LLNL’s 2024 chart puts useful energy at 34.2% of total consumption, lower than that decade-old figure.

LED lighting is the easy win worth taking

At the bulb level, the payoff is real. ENERGY STAR-certified LEDs use 90% less energy than incandescent bulbs, a gap wide enough that EPA retired the dedicated ENERGY STAR light bulb label because it’s no longer possible to buy an inefficient bulb for most household uses. More than 3 billion ENERGY STAR certified bulbs have sold in the US since the label launched in 1997.

If you still have an old incandescent bulb somewhere in the house, swapping it for one of those LEDs is the way to use less power for the same light.

The federal floor is rising too. A new Department of Energy standard raises minimum bulb efficacy from 45 to more than 120 lumens per watt, a separate action from the incandescent phase-out already in force, and takes effect in 2028. DOE projects it will save $1.6 billion a year and $27 billion cumulatively over 30 years.

Over that same span, the agency expects it to cut 70 million metric tons of CO2. That’s a floor every bulb sold after 2028 has to clear, so the swap in your house eventually happens automatically, whether you shop for it or not.

Adoption still has room to run. As of 2020, LEDs made up about 48% of installed lighting units in the US, meaning the other 52% of fixtures were still due for the swap, the most recent figure DOE has published. If your own home hasn’t swapped every fixture yet, the data says you have company.

Buildings already run more efficiently than transportation or industry

Residential and commercial buildings together used 27.6% of US end-use energy in 2023, or 36.9% of total US energy once you count the electricity-generation losses baked into every kilowatt-hour the grid delivers to them, according to EIA. Even so, homes and businesses reject a smaller share of what they use than transportation or industry does. Residential and commercial use both rejected 35% of their energy in 2024, well below transportation’s 79% and industry’s 51%.

That 35% figure is one of LLNL’s fixed modeling assumptions for how efficiently buildings use the energy delivered to them. The lab doesn’t remeasure it every year, so it can’t say on its own how fast the US building stock is improving. A lot of what a building wastes leaves as heat. If you want to know where that heat escapes a house, there’s a page on that too.

What the lightbulb standard means for your own bulbs

Transportation and power plants remain the two places most of the waste happens, since transportation rejects more energy than any other sector and runs almost entirely on combustion. Moving cars from combustion to electric drivetrains does not eliminate that loss. It relocates the loss from the tailpipe to the power plant, where average conversion efficiency is only about 41%.

The lightbulb standard is the one policy here with clear, sourced numbers on both sides of the change. It takes effect in 2028 and applies to every bulb sold after that date, so the upgrade happens the next time an old bulb burns out, with no separate purchase decision required. DOE projects that single change will save $1.6 billion a year once every bulb sold meets the new floor. You’ll see your share of that saving in your own electric bill, starting with the next bulb you replace.