High heating bill in winter? Start with the therms

A high winter bill feels like one number, but it's two multiplied together: how much gas the house used and what each unit cost. Only one of those is in your hands. Before anyone talks about insulation, it's worth ten minutes to find out which one moved.

The rate part, in one paragraph

Gas rates for nearly every Bay Area home are set by PG&E, under rules approved by the state; Palo Alto is the exception, where the City of Palo Alto Utilities supplies both gas and electricity. Your community choice provider, such as MCE, Ava, SVCE, WestLight, San José Clean Energy, CleanPowerSF or Sonoma Clean Power, supplies electricity generation, not gas. If the price per therm went up, the bill went up with it, and no attic work changes that. The rest of this page is about the other half.

Pull the numbers from your account

PG&E's online account lets you compare the current month with the same month of the previous year, view usage by bill, month or hour, and compare your usage with weather and temperature. CPAU customers have their own account tools. What you're after:

1. Therms used this billing period and the same period last year. Therms are on the gas line of the bill.
2. Days in each billing period. Periods vary by a few days, and a 33-day bill isn't comparable to a 28-day one.
3. The weather. Use the account's weather overlay. A cold snap in one January and a mild one the next tells you most of the story.
4. Whether a bill was estimated. PG&E notes that a gas meter that stops communicating can lead to an estimated bill, or a delayed one covering more than a month.

Divide therms by days to get therms per day. That's the number to compare.

A worked example

These numbers are made up to show the math, not taken from any real bill.

The bill is 28 percent higher in therms, but per day the house used about 20 percent more gas. With similar weather, that's a real change in the house, worth chasing. If this January had been markedly colder, most of the difference would be weather and there might be nothing to fix.

What burns gas in a Bay Area winter

EIA's 2020 housing survey found natural gas was the main heating fuel in 64 percent of California homes. In a typical gas-heated Bay Area house the winter bill is mostly:

• The furnace, the biggest swing with the weather.
• The water heater, which works harder when the incoming water is colder.
• The stove and dryer, usually small and steady.

The climate shapes the furnace's share. In the fog belt (San Francisco, Daly City, the Oakland and Alameda flats, all climate zone 3) winters are mild but long, and the heat runs on and off for months. Inland valley cities such as Walnut Creek, Concord and Livermore (climate zone 12) get colder winter nights, so the furnace works harder for a shorter season.

Which attic problems actually touch the furnace load

Three things in an attic add to the heat a furnace has to make:

Leaky ducts. If the furnace's ducts run through the attic, every leak heats the attic instead of the room. ENERGY STAR estimates that about 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes and poorly connected ducts, and lists higher heating bills and rooms that are hard to heat among the signs. Chewed or disconnected ducts are a common find in attics where rodents have been. See ductwork.

Air leaks through the ceiling. Warm air rises and escapes into the attic through gaps around fixtures, pipes, wires and the hatch, and cold outdoor air is drawn in lower down to replace it. The furnace heats that replacement air. See air sealing.

Thin or disturbed insulation. Insulation that has settled, been trampled or been pushed aside around a repair lets heat through the ceiling. Topping it up is blown-in insulation.

ENERGY STAR's estimate for sealing and insulating together is up to 10 percent savings on annual energy bills. We don't promise a figure for any particular house.

If what you notice most is cold rooms and drafts rather than the bill, a cold, drafty house in winter covers the comfort side.

When usage jumped, look for what changed

A per-day increase on similar weather has a cause. In rough order of how often it's the answer:

• A furnace or water heater problem. A failing part, a dirty filter or a thermostat setting can raise use without any change upstairs. That's a heating technician's call, not ours.
• Duct damage. A duct that came apart in the attic dumps heated air there. If rooms at the end of a run went cold this winter, suspect this.
• More people at home, or a new routine. Working from home through the winter shows up on a gas bill.
• A change to the attic, such as insulation disturbed by a repair, a cable install or a pest problem.

Safety before sealing

Tightening a house changes how air moves through it. Older gas appliances that draw combustion air from the room and vent by natural draft can be affected. California's 2025 Energy Code recognizes this: homes with atmospherically vented combustion appliances inside the pressure boundary are exempt from its attic air-sealing requirement. Before any sealing near those appliances, they and their venting should be checked.

CPSC's carbon monoxide advice is to have the heating system professionally inspected and serviced annually, and to install CO alarms on every level of the home and outside sleeping areas. Its fact sheet says that inspection should check chimneys and flues for "blockages, corrosion, partial and complete disconnections, and loose connections."

Rebates and the order of work

Utility and city programs change often. What's open in your city, and what it requires, is on our rebates page. The order that makes sense for a gas-heated house is usually: fix the ducts, seal the ceiling, then top up the insulation.

Getting the attic side checked

Book a free inspection or send photos and a short video from the hatch for an estimate. Bring the two bills you compared; they help us see whether the attic is a likely part of the answer. You get a written quote, and nothing is due before you sign.