Heat pump insulation: get the attic done before the heat pump

A heat pump is a big purchase that gets sized once. Whoever installs it measures your house, works out how much heating and cooling it needs, and picks equipment to match. If the attic gets sealed and insulated a year later, the house changes and the heat pump doesn't.

That's the whole argument of this page: the order of work for heat pump insulation, meaning the house around the heat pump, not the wrap on its refrigerant lines, which belongs to the HVAC installation. It doesn't cover which heat pump to buy, and it doesn't repeat rebate amounts. Those live on our rebates page, which is kept current. What follows is the sequence that avoids paying for the same attic twice, and the code checks you'll meet along the way.

Why the attic comes first

Two federal sources make the case plainly.

The Department of Energy's Building America checklist for heat pump replacement says a "room-by-room load calculation shall be performed in accordance with" ACCA Manual J, and that the heat pump is then sized "in accordance with ACCA Manual S based on ACCA Manual J load calculation results." In other words, the equipment follows the numbers, and the numbers follow the house.

A weatherization guide from the Consortium for Energy Efficiency, hosted on the Department of Energy's building-science site, finishes the thought. Its advice to homeowners: before installing a heat pump, make sure the home is properly air sealed and insulated. And when the timelines don't line up, it says to "always size the heat pump for the reduced loads after weatherizing upgrades are complete," so the heat pump isn't oversized once the envelope has been upgraded.

Notice what neither source promises. They don't say you'll save a particular amount, and we won't either. A load calculation on your house is the only real answer to "how big?" The point of doing the attic first is that the calculation then measures the house you'll actually be living in.

The order of work

This is the sequence we plan around. Steps 1 to 5 are attic work; steps 6 and 7 belong to your HVAC contractor.

1. Look at the attic and the ducts together. One inspection, with the insulation depth, the air leaks and the duct runs all on the same notes. The duct question matters because a heat pump usually reuses them.
2. Clear the blockers. Rodent damage, wet insulation, knob-and-tube wiring or pebbly vermiculite all stop the job until they're dealt with, and each has its own guide. Better to find them now than on install day with an HVAC crew waiting.
3. Air seal the attic floor. Top plates, wiring holes, plumbing chases and gaps around flues and recessed lights. Sealing has to come before insulation, because once the fill is down you can't reach the holes. See air sealing.
4. Seal, repair or replace the attic ducts. Leaky or crushed ducts in a hot or cold attic waste whatever the heat pump produces before it reaches a room. If the ducts are being replaced, do it now, while the attic is open. See ductwork.
5. Insulate. To the level your climate zone asks for when an attic is altered, which in most Bay Area zones is R-49. See blown-in insulation.
6. Load calculation. Your HVAC contractor runs Manual J on the finished envelope and picks equipment with Manual S.
7. Install and verify. The heat pump goes in, and the code's duct checks (below) happen on ducts that are already in good shape.

Not every project can wait. The same CEE guide says that if upgrades won't be finished before heating season, keep the existing heating system or add supplementary heat, and size the heat pump for the house as it will be.

What the Energy Code checks when the heat pump goes in

California's 2025 Energy Code took effect on January 1, 2026, and the Energy Commission's compliance manual answers the question most people don't know to ask. In Example 9-43, a homeowner replacing only the heat pump equipment asks what applies to the ducts. The answer: altering a space-conditioning system triggers duct sealing. If the ducts stay, they must be tested by the installer and an ECC rater (the role formerly called HERS rater) to no more than 10 percent total leakage or 7 percent leakage to outside. If they can't meet that, an ECC rater can verify by smoke test and visual inspection that all accessible ducts have been sealed. Example 9-35 adds that replacing just the outdoor unit triggers the same requirement.

The manual lists exceptions, including ducts a rater has already sealed and tested, systems with less than 40 feet of duct, and duct systems that contain asbestos.

Two more rules bear on the order:

• New ducts in a vented attic pull the attic in. The manual says the attic insulation and air-sealing requirements apply "when an entirely new duct system is installed in the vented attic," not only when insulation is added. Planning the attic and the ducts as one job means meeting those rules once, on purpose.
• Duct insulation depends on your zone. New ducts must be R-8 in climate zones 2, 4 and 12, and R-6 in zone 3. And under the 2025 code, more than 25 feet of new or replacement duct (the 2022 code's threshold was 40) brings in rater testing.

If the ducts were sealed and the attic finished beforehand, the test at install day is a formality instead of a surprise.

Fog side or valley side

The order of work is the same everywhere in the Bay Area. Why it matters shifts with the weather.

In climate zone 3, the bayside belt from San Francisco and Daly City through Oakland, Berkeley, Hayward and Redwood City, the heat pump will spend most of its life heating. Cool, damp air finds the gaps in the attic floor, and ducts running through an uninsulated attic lose heat on the way. The code doesn't require attic air sealing in zone 3 when insulation is added, but sealing still makes the heating load smaller.

In climate zones 4 and 12, San Jose and the Santa Clara Valley, Concord, Walnut Creek and the Tri-Valley, summer cooling matters as much as winter heating, and ducts in a vented attic sit in the hottest part of the house. When those attics start below R-19, the code asks for air sealing as well as R-49. The Title 24 page has the details for each zone, and our climate zone 12 notes cover the inland valleys.

In climate zone 2, Napa, San Rafael and Santa Rosa, both seasons count, and the zone 2 rules mirror zone 4 for the attic.

If a gas appliance is staying

Many heat pump projects replace the furnace but leave a gas water heater, at least for now. That changes one detail. The Energy Commission's manual exempts homes with atmospherically vented space- or water-heating appliances inside the building's pressure boundary from the attic air-sealing requirement. The exemption exists because tightening a house around an appliance that relies on natural draft needs care. It doesn't cover appliances in a vented attic, garage or crawlspace.

If you have one of these, tell everyone involved. Air sealing around it should come with a combustion-safety check, and if that appliance is due to be replaced too, it's worth deciding the timing before the attic is sealed.

Rebates, in one line

The three attic insulation rebates we could confirm open in the Bay Area today all pair insulation with a heat pump or another electrification project, and they differ on timing, so read the details before you book anything: San José Clean Energy, City of Palo Alto Utilities and Silicon Valley Power. Each wants paperwork in place before or right after the work, which can decide which job goes first. Silicon Valley Clean Energy customers: we couldn't confirm an insulation rebate on SVCE's own page, so ask SVCE directly (details).

Who does what

The heat pump itself, the load calculation and the refrigerant work are your HVAC contractor's job. Attic WISE handles the attic: inspection, air sealing, insulation, and duct sealing or replacement in the attic. The useful thing we can do for the heat pump is finish before it's sized.

Next steps

If a heat pump is on your calendar, book a free inspection before the HVAC contractor's visit, or send photos of the attic and the ducts if you'd rather start that way. Tell us your city, which equipment is staying and whether you're applying for a program, and we'll plan the attic so the load calculation can follow it.