For most Seattle attics, fiberglass wins. Air seal the ceiling plane properly, then blow insulation in at a real depth, and you will get the same comfort as foam for a fraction of the money.
That is an odd thing for a spray foam contractor to publish, so here is the reasoning in full. Spray foam is a genuinely better product in specific assemblies, and in those assemblies we will argue for it hard. A wide open vented attic floor usually is not one of them. Fiberglass and cellulose are perfectly good insulation when they are installed well and the air leaks underneath them have been sealed first.
The short version
Fiberglass insulates but does not stop air movement. Spray foam does both. In an attic with plenty of headroom, that air sealing job can be done separately and cheaply, and then you just add more inches of cheap material on top, which is why foam usually loses the value argument there. Foam earns its price where depth is scarce, where the geometry is complicated, or where moisture is part of the problem.
How each one actually works
Fiberglass slows heat. It does not stop air.
Fiberglass works by trapping still air in a tangle of glass fibres. Still air is a decent insulator, and that is the whole mechanism. It is cheap, it is inert, it does not burn, and it has insulated most of the houses in this country for seventy years for good reason.
What it cannot do is stop air from moving through it. Fiberglass is air permeable by design. Warm indoor air carrying moisture goes straight through a batt on its way up through your ceiling, and cold outside air washes through the perimeter of the attic floor from the eave vents. That air movement is the reason a house can have code level insulation on paper and still feel cold and still cost a fortune to heat. The R-value on the bag assumes still air, and the eaves of a windy Seattle attic in November are not still air.
Spray foam insulates and seals in the same pass
Foam is sprayed as a liquid, expands, and cures against the surface. It fills the gaps it is sprayed into and it is not air permeable, so it insulates and air seals at the same time, in one operation, including around the awkward penetrations that batts never fit around. That is the real advantage, and it is bigger than the R-value difference people usually focus on. If you want the difference between the two foams themselves, we cover that on open-cell vs closed-cell spray foam.
Here is the important caveat, though, and it is the thing that decides this whole comparison: the air sealing is the valuable part, and you can buy air sealing on its own. It does not have to come bundled with expensive insulation.
R-value per inch, side by side
| Material | Typical R per inch | Stops air? | Relative cost |
|---|---|---|---|
| Closed-cell spray foam | About R-6 to R-7 | Yes | Highest |
| Open-cell spray foam | About R-3.6 to R-4 | Yes | High |
| Fiberglass batts | Typically published around R-3.0 to R-4.3 | No | Lowest |
| Blown fiberglass | Typically published around R-2.2 to R-2.7 | No | Low |
| Blown cellulose | Typically published around R-3.2 to R-3.8 | No, but it resists air better than fiberglass at depth | Low |
Those fiberglass and cellulose figures are typical published ranges rather than one fixed number, because density and product vary. Read them as a ballpark, not a spec. For what those numbers need to add up to in our climate zone, see the R-value guide for Seattle climate zone 4C.
Notice that open-cell foam and a decent batt are in roughly the same neighbourhood per inch. The gap people imagine between foam and fiberglass is mostly a closed-cell gap, and closed-cell is not usually what goes on an attic floor anyway.
Depth is why foam loses the attic argument
This is the part that decides most jobs, and almost nobody selling foam will say it out loud.
R-value per inch only matters when inches are scarce. In a vented attic with a walkable amount of headroom above the joists, inches are not scarce. They are free. If you need more R-value, you blow in another four inches of the cheap stuff, and the cheap stuff is cheap enough that going deep barely moves the invoice. Doubling the depth of blown-in is a modest cost increase. Doubling the depth of spray foam is a very large one.
So in a generous attic, the premium you would pay for a high R per inch product is buying you something you did not need. That is the value argument, and fiberglass wins it cleanly. Where it flips is the moment depth runs out: a cathedral ceiling, a low heel at the eaves, a sloped bonus room ceiling, a knee wall. Then R per inch stops being a spec sheet comparison and becomes the entire decision.
Installation quality, honestly, on both sides
This is where the published R-values stop describing reality.
Batts are routinely installed badly
In fifteen years of opening up Seattle attics we have seen very few batt installs that were done to the standard the rating assumes. The common failures:
- Compressed. An R-19 batt squashed into a shallower space is not R-19 any more. Loft is the product.
- Gapped. Small voids at the ends of bays and between batts leak far more heat than their area suggests.
- Cut short or stuffed around obstructions. Wiring, junction boxes, bracing, can lights. Batts get folded over them instead of split around them.
- Thin at the eaves. Where the roof meets the top plate there is barely any room, so the coldest part of your ceiling gets the least insulation.
A badly installed batt underperforms its rating substantially. That is a real and fair criticism of fiberglass, and it is the main reason blown-in often beats batts in an attic: blown material fills around obstructions on its own, so it is much harder to install badly than a batt is.
Foam has its own failure modes, and they are worse
We are not going to pretend our product is foolproof. Off-ratio foam that never fully cures and keeps off-gassing for months. Passes sprayed too thick, which traps heat in the curing foam and can scorch the core. Spraying onto a cold substrate so it never bonds properly. Thin spots that are sold as three inches and measure one and a half.
The difference in consequence is what matters. A bad batt job can be pulled out and redone in a day for very little money. A bad foam job is bonded to the structure, and the honest fix is usually removal, which is slow, unpleasant, and expensive. That asymmetry is why we are so particular, and it is a legitimate reason to be cautious about foam. If you already have foam and you are wondering about it, we wrote how to tell if spray foam was installed correctly.
Moisture, which matters more here than most places
We are in a marine climate with a long damp heating season, so this is not a footnote.
Fiberglass does not absorb much water into the fibres themselves, but it holds water between them, and wet fiberglass loses most of its R-value while it is damp, gets heavy, sags, and falls out of the bays. We pull soaked batts out of Seattle crawl spaces constantly. In a vented attic with a working roof it is less of an issue, because the vents dry things out, and a vented attic is a fairly forgiving assembly. That forgiveness is a genuine argument for leaving a vented attic vented.
Cellulose absorbs and releases moisture, which can buffer humidity but which also means it stays wet longer after a leak.
Closed-cell foam resists bulk water and acts as a vapor retarder at sufficient thickness, which is why we use it on crawl space walls and rim joists. Open-cell does not. The real moisture question with foam on a roof deck is different: once foam is bonded to the underside of your sheathing, a roof leak stops showing up on your ceiling and can sit against the wood unnoticed. Deal with the roof before you insulate under it.
What it costs
Spray foam is priced per board foot, which is one square foot at one inch thick. Around here, closed-cell commonly lands about $1.00 to $2.00 per board foot installed and open-cell about $0.50 to $0.70. A 1,000 square foot area at three inches often works out somewhere around $2,200 to $5,400. Seattle trade labour generally runs about $95 to $175 per hour, which is a big part of why access drives the price.
We are not going to publish fiberglass numbers we do not quote, so take this as relative framing: blown-in insulation is a small fraction of foam per square foot of attic floor, and the material is cheap enough that adding depth costs comparatively little. Air sealing the ceiling plane is its own line item and it is not expensive relative to either. Put together, air sealing plus blown-in at depth usually costs a great deal less than foaming the same attic. A firm price for either needs someone to actually get up there and look. Full detail on spray foam insulation cost in Seattle.
Where each one belongs
| Assembly | Usually the better buy | Why |
|---|---|---|
| Vented attic floor | Air sealing plus blown-in | Depth is free, so paying for R per inch buys you nothing |
| Knee walls | Foam, or foam plus a blocked and sealed batt | Batts here are usually open to attic air on the back side, which ruins them |
| Roof deck, conditioned attic | Foam | It is the practical way to bring the attic inside the envelope |
| Rim joists | Closed-cell foam | Small area, huge leak, complicated geometry, moisture exposure |
| Cathedral ceilings | Foam | No room for depth and no room for error on moisture |
| Bonus room over a garage | Foam on the hard parts | Sloped ceilings, knee walls, and a cantilevered floor all at once |
| Open attic with good headroom | Blown-in, every time | Same result, much less money |
The hybrid approach is usually the right answer
In the real world, most Seattle attics are not one assembly. They are a flat area that is easy, plus a collection of awkward geometry that is hard. So we mix.
Seal the ceiling plane
Top plates, plumbing and wiring penetrations, the chimney chase, can lights, and the attic hatch. This is the step that actually changes the bill.
Foam the hard geometry
Knee walls, the eave detail, a sloped section, a bonus room floor. Small areas where foam is worth it and nothing else fits.
Baffle and dam the edges
Keep the soffit vents clear and keep loose material from washing out at the perimeter, which is where most attics fail.
Blow the flat areas deep
Cheap material, generous depth, over everything easy. This is where most of your R-value comes from and it costs the least.
That combination typically gets more comfort per dollar than either product alone. More on the sequence in our Seattle attic insulation guide.
When we tell people to skip the foam
We turn this work down regularly. Not as a sales move, just because it is the truth.
- You have a vented attic with room to work. Air seal it, then blow it deep. Foaming the roof deck of a perfectly good vented attic is a lot of money for a change you may not feel.
- Your existing insulation is dry, intact, and just shallow. Then you do not have an insulation problem, you have a depth problem, and depth is the cheapest thing in this trade to buy.
- Nobody has air sealed anything yet. Sealing first is the highest value work in the house. Sometimes it is the only work you need, and if that is your house we will quote you air sealing and stop there.
- The roof is near the end of its life. Do the roof first. Foam under old sheathing makes a future leak much harder to find.
- The real problem is leaky ducts or a failing heat system. Fix the delivery before you upgrade the container.
If you want the broader version of that argument across the whole house, read is spray foam insulation worth it in Seattle.
Common questions
Is spray foam really better than fiberglass?
It is a different product rather than a strictly better one. Foam insulates and air seals in one step and handles complicated geometry and moisture that fiberglass cannot, so in a crawl space, at a rim joist, on a cathedral ceiling, or in a shallow cavity it is genuinely better. On an open vented attic floor with headroom to spare, fiberglass or cellulose installed well over a properly air sealed ceiling gets you the same comfort for much less money. The assembly decides, not the product.
Can I just add more fiberglass on top of what is already up there?
Often yes, and it is a reasonable plan, with one condition: air seal the ceiling plane first. Adding insulation over unsealed top plates, penetrations, and can lights buries the leaks instead of fixing them, and you will be disappointed by the result. If the existing material is dry and intact, new blown-in can go straight over it. If it is wet, matted, or has rodent damage, it has to come out first.
How deep does blown-in need to be in a Seattle attic?
Current guidance for our climate zone puts attics in the R-38 to R-60 range, which is a lot of inches of loose fill. The practical constraints are the depth you actually have at the eaves and keeping the soffit vents clear with baffles. We measure what is there, tell you what depth gets you to target, and are honest when the perimeter simply cannot hold it.
Does spray foam in an attic cause moisture problems?
It can, if it is used in the wrong place or installed poorly. Foaming a roof deck converts a vented attic into an unvented one, which is a legitimate assembly but it changes how the roof dries and it hides a leak from you. In our marine climate we want the roof in good condition, the right foam for the assembly, and the right thickness before we do that. On an attic floor, a properly air sealed ceiling plane with blown-in above it is a forgiving assembly and rarely causes trouble.
If fiberglass is fine, why would I hire a spray foam contractor at all?
Because plenty of houses have assemblies where fiberglass cannot work: damp vented crawl spaces, rim joists, knee walls open to attic air, sloped bonus room ceilings, shallow 2×4 walls. And because the air sealing that makes cheap insulation perform is skilled work in its own right. We would rather sell you the right small job and be right than the wrong big one and be wrong.