Your attic is where a Seattle house loses the most heat, and it is usually the repair with the fastest payback. Warm air rises, finds every hole in your ceiling, and leaves through the roof, and here it does that for eight or nine months a year.
There are two genuinely different ways to fix it: insulate at the attic floor and keep the attic vented, or insulate at the roof deck and bring the attic inside the heated house. Most contractors quote whichever they prefer to install. For most Puget Sound homes, the floor is the cheaper and the better buy.
The short version
Air seal the ceiling plane first. Then, in most Seattle houses, insulate the attic floor and leave the attic vented. Closed-cell foam on the roof deck is right when there is ductwork or living space up there, and it costs considerably more. Anyone quoting the expensive option before looking at your ceiling plane is selling, not diagnosing.
Why the attic comes first here
Our problem is not extreme cold. WA climate zone 4C is marine, so we get a long, damp, mild heating season: not a brutal temperature difference, a modest one for thousands of hours in a row. That is a leak problem more than a thickness problem.
- Electric heat and heat pumps. A heat pump runs longer and at lower output on a cold damp morning, and baseboard is the priciest heat most of our customers own.
- Stack effect. Air escaping the ceiling gets replaced by cold damp air pulled in at the crawl space. Cold floors and a cold upstairs are usually one problem.
- Moisture riding the air. That escaping air is humid and condenses on cold sheathing. Dark staining on north-facing plywood comes from exactly this.
In most older Seattle attics we find a settled blanket of blown-in from decades ago, thin at the eaves, over a ceiling full of unsealed holes. The insulation is not the main failure. The holes are.
The two real approaches
Insulate the attic floor, keep the attic vented
The boundary between inside and outside sits at your ceiling. We seal every penetration through it, then build depth on top with blown-in, using foam only where an air seal is needed. No cavity limits the depth, so you are not squeezing R-value into a 2×6 space.
If your attic is open and vented, has no ducts in it, and you are not planning to use the space, this is almost always the honest recommendation. It stops being right when the attic holds ductwork, when part of it is finished living space, or when the roof is low slope. For the head to head, read spray foam vs fiberglass batts for attics.
Insulate the roof deck, bring the attic inside
Closed-cell foam goes on the underside of the roof sheathing, the eave vents get closed off, and the attic joins the conditioned house. It costs considerably more, and it buys specific things: ducts out of a cold space, an attic or bonus room you can use, no wind washing at the eaves.
The downsides first. It is a whole-attic commitment, so you cannot do half of it. Foam bonded to sheathing means a future roof leak will not show up as a ceiling stain, so the roof gets dealt with first or not at all. More on our closed-cell spray foam page.
| Attic floor, vented | Roof deck, conditioned | |
|---|---|---|
| Boundary sits at | Your ceiling | The roof sheathing |
| Attic after the job | Cold and vented, as before | Close to house temperature |
| Relative cost | Lower | Considerably higher |
| Depth available | As much as you want | Limited by rafter depth |
| Ducts in the attic | Still in a cold space | Inside the envelope |
| Roof leaks stay visible | Yes | No, this is the real tradeoff |
| Best for | Most Seattle houses | Ducts up there, finished attics, bonus rooms |
Air sealing the ceiling comes first, and most crews skip it
Insulation slows heat that moves by conduction. It does almost nothing to heat that rides on moving air. If your ceiling has holes, the air goes through the insulation rather than around it, and the rating on the bag stops describing your house. Sealing gets skipped because it is slow, dirty, and invisible once the insulation goes back down. What we hunt for:
- Top plates of interior walls, usually an open slot the full length of the wall
- Plumbing stacks, wiring holes, and old chimney or flue chases
- Recessed can lights, which leak by design unless sealed and rated to be covered
- Bath fans, including the common Seattle find of a duct that just ends in the attic
- Dropped soffits over kitchen cabinets, often wide open from above
- The attic hatch or knee wall door, frequently bare plywood with no weatherstrip
Sealing costs a fraction of the insulation, and it is the difference between R-49 on the invoice and R-49 in the house. If sealing alone would fix your complaint, we will say so and quote that instead. See air sealing.
What to do with the insulation already up there
Settled, clean, dry
It has compressed and lost some rated value, but it is not doing harm. We air seal around it and add depth on top instead of selling you a removal you do not need.
Rodent damaged
Rats and squirrels are a normal Seattle attic problem, not a rare one. Tunnelled, soiled insulation has lost its loft. It comes out, the space gets cleaned, entry points get closed, then we re-insulate.
Wet or mouldy
Wet insulation is a symptom. Something is leaking, or warm moist air is condensing up there. Insulating over it just hides the problem for a couple of winters.
Removal and cleanup is its own line item, never a hidden extra. See insulation removal and insulation replacement.
One caution on older houses. If your attic holds loose grey pellets that look like tiny popcorn or gravel, that may be vermiculite, and some vermiculite contains asbestos. Do not disturb it, do not let anyone vacuum it, and have it tested before work starts.
Knee walls, bonus rooms, and wind washing
Knee walls show up constantly in Craftsman and Cape Cod style houses with a finished upstairs. The short wall behind the sloped ceiling usually has batts stuffed in with nothing holding them against the attic side, and the joists under that wall are open to the attic void. Cold air pours through the batt and under the floor, which is why that room is never warm. The fix is an air barrier on the attic face plus blocking in the joist bays below.
Bonus rooms over the garage have that on three sides plus a floor over an unheated garage. The sloped ceiling rarely has depth for blown-in to work, so this is one place closed-cell earns its price.
Wind washing at the eaves is the quiet one. Wind drives cold air in through the soffit vents and scours the blown-in where it is already thinnest. Baffles keep the vent path open, but you also want a wind dam at the top plate so the perimeter holds depth. Most crews fit the baffle and stop.
What attic insulation costs around Seattle
A blown-in top-up over a sealed ceiling is the cheap end, and it is what most houses need. Spray foam is priced per board foot, one square foot at one inch thick. Closed-cell commonly lands near $1.00 to $2.00 per board foot installed and open-cell near $0.50 to $0.70, so 1,000 square feet at three inches often works out around $2,200 to $5,400. Trade labour here runs roughly $95 to $175 per hour, and removal carries its own cost.
What moves the number: how much ceiling needs sealing, whether old material comes out, how tight the hatch is, and floor versus roof deck. Anybody quoting a firm price over the phone is guessing. Full ranges are on the spray foam insulation cost in Seattle page, and inside the city limits see attic insulation in Seattle.
How we handle an attic job
Get in the attic
What is up there, how deep it really is, and whether anything has been living in it.
Seal the ceiling plane
Top plates, penetrations, cans, fan ducts, the hatch. Before any insulation, every time.
Insulate to the right depth
Floor or roof deck, with baffles and wind dams so the eaves hold their depth.
Check it and stand behind it
We verify depth across the attic and photograph what you cannot climb up to see.
Common questions about attic insulation
Should I insulate the attic floor or the roof deck?
The floor, in most Seattle houses. It costs less, you can build as much depth as you want, and it leaves the attic vented and easy to inspect. The roof deck is right when the attic holds ductwork, when part of it is finished living space like a bonus room, or when there is no real attic volume.
Do I have to remove the old insulation first?
Not always. If the existing material is dry, clean, and simply settled, we air seal around it and add new depth on top. It has to come out when it is rodent damaged, soiled, wet, mouldy, or so compressed it has lost its loft. If someone quotes a full removal without going up to look, ask why.
How much insulation should a Seattle attic have?
For our climate zone the commonly cited target for an attic floor is around R-49, and going deeper is cheap once a crew is already up there. Depth only counts if the ceiling under it is sealed, because air moving through insulation defeats the rating. Code minimums differ by jurisdiction and get updated, so on permitted work we confirm the current requirement with your city.
Does attic insulation help in the summer here?
Yes, and more than it used to. Seattle summers now get hot enough that upstairs bedrooms in houses without air conditioning get genuinely uncomfortable, and a shallow attic over them is a big part of why. Depth on the attic floor plus a sealed ceiling takes the edge off the worst week of July. It will not replace air conditioning, but you feel it.