Air sealing is closing the holes that let outside air move through your house. It is usually the cheapest real fix in the building, it is the step most crews skip before they insulate, and in a lot of Seattle homes it does more for comfort than another six inches of anything. If that is your house, we will tell you so, even though it is the smaller job.
That is not a line we put on a page to sound humble. Air sealing an attic is often a few hundred dollars of materials and labour. Spray foam is a project. Selling the expensive one when the cheap one was right gets us paid once, and you never call us again.
The short version
Insulation slows heat moving through material. Air sealing stops air moving through holes. Those are two different problems, and insulation does not solve the second one. Seal first, insulate after. In some houses the sealing alone delivers most of the comfort you were after, and we will say so before you spend anything.
Air leakage and conduction are not the same problem
Heat leaves your house two ways. It conducts through solid material, straight through the drywall, the studs, the sheathing, the batt. R-value measures resistance to that, and insulation is the product that fixes it. Then there is air leakage: actual air, moving through gaps, carrying heat with it. R-value does not measure that at all, and nothing on an insulation bag does.
Why insulation alone does not fix drafts
Most of the insulation in Seattle houses is air permeable. Fiberglass batts and blown-in cellulose slow conduction, but air moves through them, around them and along their edges. So you can blow fourteen inches over an attic floor and still feel cold air at the baseboards in February, because the leak path is still open and you just covered it up. There is a second effect that is less obvious: air washing through loose insulation reduces what that insulation delivers. The label says R-49, the assembly performs lower, and you paid for the label. If you added insulation and the bills did not move, that is normally what happened. Longer version: why are my heating bills so high.
The stack effect in a two-storey Seattle house
Warm air is lighter than cold air, so it rises and pushes out through the top of the house. Air cannot leave without being replaced, so the house pulls cold damp air in low down: crawl space vents, the band joist, gaps around the bottom plate, the basement stair. Your house is a chimney and your heating system is the fire.
The taller the column of warm air, the harder it pulls, so a two-storey house with an attic above it is why the upstairs bedrooms in a Craftsman or a Bellevue split-level are the rooms people complain about. It also tells you where to work: leaks at the top and bottom of the house matter most, leaks in the middle matter least. A crew that starts by caulking your window trim is working on the least important third of the problem, in the one place you can watch them do it.
Our climate runs this engine gently and without a break. Zone 4C is marine: not forty below, just a modest temperature difference for eight or nine months straight. Thousands of hours of slow leakage costs more than a few days of extreme cold ever will.
Where the leaks actually are
After fifteen years of crawling around houses in this area, the list barely changes. In rough order of how much air moves through them:
- The attic hatch or pull-down stair. A thin panel on a trim ledge, no gasket, no insulation on top. The biggest deliberate hole in most ceilings.
- Top plates. The gap between drywall and the top of every interior wall, running the length of every partition. Individually small, collectively enormous, invisible once blown-in covers them.
- Recessed lights. Older non-IC cans are ceiling vents with bulbs in them, and even airtight-rated housings need sealing where they meet the drywall.
- Plumbing and wiring penetrations. The vent stack, the bath fan housing, every wire drop, every hole cut oversize because it was faster.
- Chimney and flue chases. A framed shaft with a clear path from basement to roof. Sealed with sheet metal and high temperature sealant, never ordinary foam against a hot flue.
- The rim joist and band joist. The perimeter of the floor framing where it sits on the foundation. The bottom half of the stack effect, and the reason your floors are cold.
- Ductwork in unconditioned space. Leaky ducts in a vented crawl space or a cold attic are not losing heat slowly. They blow heated air straight outside and pull crawl space air back in on the return.
- Knee wall backs. Bonus room over the garage, finished attic, anywhere with a short wall and a sloped ceiling. The back of that wall is usually bare batt facing an open attic void, with the joist bays running out to the eaves.
- Crawl space vents. Put there on purpose, so a design decision rather than a defect, but still where the cold damp air comes in. What to do about them is its own conversation: crawl space insulation.
Why this matters more in Seattle than the R-value argument suggests
Moving air carries water vapour as well as heat, and that is what the R-value conversation leaves out.
Your house makes moisture: showers, cooking, laundry, breathing. Warm indoor air holds it and the stack effect pushes it up into the attic, where it meets cold roof sheathing and condenses. At the same time the house pulls replacement air in at the bottom, out of a vented crawl space that is damp here for most of the year, and the musty smell, the spores and the rodent mess ride up with it. Over a long mild wet heating season, air sealing is a moisture control measure at least as much as an energy one. That is a different argument from adding thickness, and it is why we put sealing ahead of R-value in the Puget Sound. If your crawl space is already showing symptoms, here is why Seattle crawl spaces get moldy.
How we find the leaks
We read the house first
Age, framing, heating system, where the ducts run, where an addition was joined on. A 1920s bungalow, a 1960s rambler and a 1995 subdivision build leak in different places.
We depressurize and feel for it
A blower door pulls the house negative, so every leak becomes an inward draft you can find by hand or with a smoke pencil. On a cold morning an infrared camera shows the same paths.
We look at the dirt
Insulation that air moves through is a filter. Grey streaks around a light can, soot at a top plate. It shows you where the air has been going for thirty years, for free.
What we seal with
There is no single product. Matching the material to the gap is most of the craft, and the wrong choice is how sealing work fails or, at a flue, becomes dangerous.
| Material | Where it goes |
|---|---|
| Gun foam, single component | Penetrations, top plates, gaps up to roughly three inches. The workhorse of attic work. |
| Two-part closed-cell foam | Rim joists, band joists, knee wall backs. Seals and insulates in one pass, which is what makes the setup worth it there. See closed-cell spray foam. |
| Caulk and elastomeric sealant | Tight joints under about a quarter inch, trim, sill plates, duct boot edges. |
| Sheet metal and high temperature sealant | Chimney chases, flues, anything that gets hot. Code driven and not optional. |
| Rigid foam board, foamed or taped in | Large openings: chases, soffit drops, knee wall backs, dropped ceilings over cabinets. |
| Weatherstripping and an insulated hatch cover | Attic hatches and pull-down stairs. Cheap, quick, and one of the highest return items here. |
| Duct mastic and foil tape | Duct seams and boots in crawl spaces and attics. Never cloth duct tape, which fails within a few years. |
The honest limit: a house can be sealed tight enough that ventilation needs attention
This is the part of the trade that gets either ignored or turned into a scare story, so here it is plainly. An old leaky house ventilates itself by accident: badly, unevenly and expensively, but it happens. Seal it properly and that accidental ventilation goes away, which is the entire point, and from then on the ventilation has to be deliberate.
For most houses we work on, deliberate means something modest: bath fans that actually move air and are ducted outside rather than dumped into the attic, a range hood vented outside, and the habit of using them. That is genuinely it on the majority of retrofit jobs, because a Seattle house built before 2000 is nowhere near tight enough to need more.
Two situations need a conversation first. If the house has an atmospherically vented gas appliance, an older water heater or furnace taking combustion air from the room, sealing the house harder can affect how it drafts. And if we are taking a house to genuinely low leakage, continuous mechanical ventilation replaces spot fans as the right answer. Neither is a reason to skip air sealing. Both are reasons to do it with someone who treats the house as one system, and we raise them at the estimate rather than afterwards.
When sealing is the whole job, and when it is not
Sealing alone is often the right stopping point when the attic already has a reasonable depth of intact insulation, when the complaint is drafts and uneven rooms rather than a house that is cold everywhere, and when the crawl space is dry. In that house a focused day of sealing is the buy. We will write that on the estimate and not quote you foam you do not need.
Sealing is only the first step when the insulation is wet, flattened, rodent damaged or missing and has to come out (see insulation replacement), when the assembly has no room for the depth it needs, which is where closed-cell earns its price, or when the crawl space has a moisture problem that no amount of sealing upstairs will fix. Those jobs are real and we do them. We just do not start there.
What air sealing costs in the Seattle area
It is priced by the work rather than by the square foot, because the variable is how many holes there are and how hard they are to reach. Skilled trades here run roughly $95 to $175 per hour, and most attic sealing packages we do are a day or less for a two-person crew plus modest materials. Crawl space and rim joist work runs longer, because working on your back in an eighteen inch space is slower. Knob-and-tube wiring, opening a knee wall, or ducts that need sealing all add time.
Set that against closed-cell spray foam at roughly $1.00 to $2.00 per board foot installed, or about $2,200 to $5,400 for a thousand square feet at three inches, and the size difference is the whole argument for sealing first and seeing where you land. The payback arithmetic is in how much can spray foam cut my energy bill. We will not price your house over the phone either. Someone has to get into the attic and under the floor to see what is there, that visit is free, and there is no closer attached to it: request an estimate.
Common questions about air sealing
Should I air seal before adding insulation?
Yes, and the order is not a preference, it is the only sequence that works. Once blown-in covers the attic floor, the top plates, wire penetrations and light housings under it are buried. Sealing them later means raking the insulation back, sealing, then redistributing it, which costs more than doing it in the right order would have. A contractor who wants to blow insulation over an unsealed ceiling plane is choosing the fast version.
Will air sealing on its own lower my heating bill?
Usually yes, and in a leaky house it can be the largest improvement per dollar you will ever make there. We will not give you a percentage, because the honest answer depends on how leaky the house is now, how you heat it and how you live in it. Comfort also improves faster than the bill does. People stop feeling the draft within a day, and the bill tells its story over a full winter.
Can I air seal my attic myself?
Some of it, honestly. Weatherstripping the hatch and building an insulated cover over it is a good weekend job and one of the highest value items on the list. Caulking gaps you can reach is fine too. What we would not take on yourself: anything near a flue or chimney chase, which needs fire-rated materials, anything involving knob-and-tube wiring, and top plates buried under old blown-in, because that means moving a lot of dust in a space with no room to work. You do the hatch and we do the rest is a reasonable split.
Is spray foam the same thing as air sealing?
Spray foam air seals wherever it is sprayed, so it does both jobs at once in the assembly it covers. That is not the same as sealing the house. Foam in the attic floor cavities still leaves the hatch, the chases and the ductwork doing exactly what they were doing. A good foam job includes deliberate sealing of everything the foam does not touch, which is a fair question to put to any bidder. Full list: spray foam insulation services.