energy
Air sealing before insulation
Why you seal the attic floor before you blow cellulose, what Angi prices that line at, which holes actually matter, and how to read a bid that tries to skip it.
Insulation slows heat through drywall. It does not stop air. Warm air still leaves at the top of the house through the plumbing stack, the can lights, the chase around the chimney, and the attic hatch you never weatherstripped. If you blow new cellulose over those holes, you insulated a chimney. The air still leaves. You just paid for fluff.
Weatherize before the heat pump is why this work belongs before a bigger outdoor unit. Attic insulation payback is the depth and the R-value once the floor is closed. Stay here when the live question is the sealing line on that same bid.
Doors and windows you can finish without becoming a contractor live on the weatherize kit. Recessed lights, top plates, and the chimney chase are hire. Foam-in-a-can around a live can light is how attics smell like hot dust.
What does air sealing actually cost?
Angi’s 2026 attic-insulation page budgets air sealing at $250 to $750. Their blown-in page is wider: $300 to $1,500 as an add-on, because some attics only need a few stacks foamed and some need the old fluff pulled so the crew can see the drywall. Both numbers are the sealing line, not the blow.
Angi: blown-in insulation cost
If the bid is $2,100 all-in and there is no sealing line, they are probably blowing over the leaks. That can still be a honest cheap top-up when the attic is already tight. It is not ENERGY STAR’s job.
Ducts in the attic are a different invoice. Angi puts professional duct sealing at $500 to $4,000, averaging about $2,250. Do not let “we foamed the cans” stand in for mastic on the trunks.
What savings number is even real?
ENERGY STAR’s modeled estimate is that air sealing plus insulation in attics, floors over crawl spaces, and rim joists saves an average of 15% on heating and cooling, or about 11% on the whole energy bill, in a typical existing U.S. home. The model assumes a 25% cut in total air infiltration, plus insulation brought toward 2012 code in the cheap places. It is sealing plus insulation, modeled, average. It is not a promise on your January bill, and it is not insulation alone.
ENERGY STAR: estimated savings methodology
DOE’s Energy Saver guide puts sealing uncontrolled leaks at 10% to 20% on heating and cooling bills, and says insulation comes after air sealing, because fluff does not stop air rushing through gaps.
Take last winter’s heating dollars and 15% of that number. Compare it to Angi’s sealing band plus the attic blow. If heat cost $1,200, 15% is $180, and a $2,500 combined job is a long payback unless comfort, ice dams, or a smaller heat-pump size is why you are up there. If heat cost $3,500, the same percentage is $525. Use your bills.
The federal 25C credit covered qualifying insulation and air sealing through December 31, 2025. A 2026 start does not get 30% or $1,200 off because last year’s flyer still has the IRA logo. State and utility rebates may still exist.
IRS: Energy Efficient Home Improvement Credit
IRS: FAQs on energy credit terminations
Which holes actually matter?
Start at the attic floor, because that is the lid of the house. Building America (DOE) treats the hatch like an exterior door: weatherstrip, latch, and insulation on the lid, because an open hole in a new R-49 ceiling is a joke you paid for.
Building America: air seal and insulate attic access
Walk the rest with a flashlight before anyone brings a hopper:
Plumbing stacks and bath fans. The drywall was cut with a bigger hole than the pipe. Foam or caulk the gap, not the pipe threads.
Top plates. The top of every interior wall is a slot into the attic. A line of caulk or low-expansion foam along those plates is tedious and it is most of a real air-seal job.
Recessed lights. A can that was never IC-rated and airtight cannot be buried. The honest fixes are an airtight IC-rated retrofit, a sealed box the electrician builds, or leaving a dam and not covering it. Foam around a hot can is a fire.
Chimney and flue chases. The gap around masonry or a metal flue is a two-story leak. Metal flashing and fire-rated sealant, not a grocery-store can shoved against a hot flue.
Wire and cable penetrations. Every hole the cable guy drilled. Small, many, and worth a tube of caulk.
Rim joist. The wood band where the floor sits on the foundation, visible in the basement or crawl. Rigid foam cut to the bay plus sealant at the edges. This is the other cheap ENERGY STAR location, and it is not the attic, so it needs its own afternoon or its own line on the bid.
Do not seal the soffit vents. Those cool the roof. Baffles keep new insulation from stuffing them. Sealing the eaves is how you grow moisture and ice dams.
What should three bids mean?
Bid A, doors and hatch only. Sweeps, jamb weatherstrip, an insulated hatch lid. HomeGuide’s whole-house weatherstripping band is $130 to $470 if you hire it, or the parts on the weatherize kit if the frames are sound. This is not attic air sealing. It is the leaks you walk past. Do it anyway, then decide whether the attic still needs a crew.
HomeGuide: weather stripping installation cost
Bid B, attic air seal, then blow. Angi’s $250 to $750 sealing line, or $300 to $1,500 if they have to move existing fluff, plus the insulation depth from the payback guide. Top plates, stacks, cans, chase, hatch, baffles. This is the job that matches ENERGY STAR’s 15% model. If Bid B is $800 over a blow-only quote, that extra is not automatically a upsell.
Bid C, blower door and the whole house. A contractor runs a fan in the front door, finds the CFM50 number, seals, then tests again. Ask for before and after. If you have a natural-draft furnace or water heater, a tighter house can change how those appliances vent. That is a combustion-safety check, not a foam hobby. Price it when the bid is “we will get you to ENERGY STAR tightness,” not when you are only foaming three stacks.
If ducts sit in the attic, get a duct number next to the envelope number. Mixing them into one “seal the attic” lump is how you pay for cans and still lose air at the boots.
What has to be on the quote?
- Air sealing as its own line, in dollars, not buried inside “insulation.”
- The list they will actually seal: top plates, stacks, fans, cans, chases, hatch, rim joist. If a line is missing, it is not included.
- What they will do with recessed lights, in writing, including IC and airtight ratings.
- Fire-rated materials at flues, and a stop if they find knob-and-tube.
- Hatch: weatherstrip, latch, insulated lid.
- Baffles at the eaves, so sealing the floor does not become stuffing the soffits.
- Whether a blower-door test and a combustion check are in or out.
- Insulation depth and R-value as a second line, using your climate zone from the payback guide.
If they want to foam the roof deck and leave the soffits open, they do not have a ventilation plan. That is a different building.
How does this go wrong?
They blow first because the hopper is already on the truck, and now every leak is under 14 inches of cellulose. The next sealing job starts with a vacuum.
They foam a can light that was never rated to be covered.
They seal the soffits and call it air sealing. The roof cannot dry, and you buy ice dams.
The house gets tighter and the water heater spills combustion gas because nobody checked the draft. Hire people who will say that out loud if you have old gas appliances.
A 2026 bid subtracts 25C.
You sealed the front door and skipped the attic, then wondered why ENERGY STAR’s 15% never showed up. The model was the lid of the house, not a vinyl sweep.
How do I decide this week?
- Open the hatch with a flashlight and look for daylight at stacks, cans, and the chimney chase before you shop for bags of cellulose.
- Get a blow-only bid and an air-seal-then-blow bid on the same attic, and make the sealing line show dollars and a hole list.
- Put the hatch, the exterior doors, and the rim joist on that same week so you are not sealing the attic and leaving the lid open.
- If recessed lights are in the ceiling below, ask what they will do to each can, and stop the job if the answer is “we foam around them.”
- Cross out 25C on a 2026 start, then ask which utility rebate still pays for sealing.
- If a heat-pump quote is live, freeze the outdoor-unit size until this work and a new load exist, using weatherize before the heat pump as the order.