Open-Cell Spray Foam vs. Fiberglass in Seattle, WA: Which Provides Better Air Sealing?

Open-cell spray foam beats fiberglass on air sealing in nearly every Seattle, WA wall, ceiling, and crawl space application. When you compare open-cell spray foam vs. fiberglass, the difference comes down to physics: open-cell foam expands up to 30 to 60 times its liquid volume to fill cracks, gaps, and irregular cavities, forming a continuous insulation-and-air-barrier layer in a single pass. Fiberglass batts deliver a similar R-value per inch, but they only slow heat flow. They do not stop air from moving through and around them, and gaps between batts become sites of air infiltration and condensation. That matters in a wet, windy marine climate where Department of Energy research attributes roughly 40 percent of a home’s energy loss to air infiltration through walls, windows, and doorways. Foam costs more upfront, typically $0.28 to $0.75 per board foot for our open-cell material, yet it eliminates the need for a separate air sealing pass and holds open-cell spray foam performance for the life of the installation. Fiberglass remains a smart pick for open attic floors and tight budgets, especially when paired with a dedicated air sealing project.

TL;DR

• Air sealing is the deciding factor. Open-cell foam acts as insulation and air barrier in one application; fiberglass only resists heat flow and allows air to pass through it.
• R-value per inch is nearly identical. Open-cell foam runs about R-3.8 per inch, while fiberglass falls between R-3 and R-4 per inch, so the real gap is air movement, not thermal resistance.
• Thickness determines air barrier status. Open-cell foam needs roughly 5.5 inches of installed depth to function as a true air barrier, which fits a standard 2×6 wall cavity.
• The payoff is measurable. ENERGY STAR reports that sealing air leaks and adding insulation can deliver up to 10% savings on annual energy bills, and DOE research credits air infiltration with up to 40% of home energy loss.
• Our open-cell pricing runs $0.28 per board foot on the low end, $0.32 on average, and $0.75 for thick, high-R jobs. Dedicated air sealing projects range from $1,500 to $10,000 with a $4,500 average.
• Larger projects cost less per board foot, and easier site access plus flexible timelines reduce pricing further.
• The Seattle Energy Code regulates building envelope air leakage, so choosing a material that seals as it insulates simplifies compliance for new construction and major remodels.
• Fiberglass still wins on upfront material cost and suits open, accessible attic floors, provided someone seals the leaks separately.

Why Air Sealing Separates Open-Cell Spray Foam From Fiberglass

Air sealing and insulation are two different jobs. Insulation resists conductive heat flow. Air sealing stops the convective heat transfer that happens when heated air physically leaks out of the building and outside air pushes in. Both matter in Seattle, where as much as half of all household energy use goes toward heating and cooling, according to ENERGY STAR’s seal and insulate program, which also notes that air sealing plus insulation can cut up to 10% from annual energy bills.

Open-cell spray foam does both jobs at once. Applied as a liquid, it expands into cracks, crevices, and voids, adheres to irregular framing, and cures into a semi-rigid, sponge-like layer that forms an air sealing insulation. Once it reaches about 5.5 inches of depth, it functions as an air barrier. Fiberglass batts do none of this. Per Wikipedia’s technical overview of building insulation materials, even batts installed carefully leave gaps that become sites of air infiltration or condensation, and cutting batts to fit around electrical boxes and other obstructions gives air a free path straight through the cavity.

Head-to-Head: Performance Comparison

Buildings insulated with spray foam can insulate as much as 50% better than traditional insulation products, according to Department of Energy studies referenced by Wikipedia’s spray foam entry, precisely because foam blocks the convective losses that fiber insulations ignore.

Cost Breakdown for Seattle, WA Projects

A board foot is one square foot at one inch thick. The average home we work on runs 1,500 to 2,500 square feet, and most Seattle walls call for 5.5 inches of open-cell foam in a 2×6 cavity. Fiberglass is the budget material, but Wikipedia’s building insulation research notes that foam’s higher cost buys air sealing performance batts cannot match, and any savings on batts disappears if a separate air sealing crew has to follow behind them.

Seattle, WA Climate and Code Considerations

Seattle’s building environment makes air sealing a bigger deal than in milder regions. Wind-driven rain, long heating seasons, and strong stack effect in multi-story homes push moist air through wall assemblies all winter. Air that moves through wet fiberglass carries moisture into cold cavities, which is how batts end up as the wet, sagging material we regularly remove from Seattle crawl spaces.

The Seattle Energy Code explicitly regulates the building envelope, including requirements for roofs, walls, and windows to control heat loss, solar heat gain, and air leakage. The adopted 2021 Seattle Energy Code largely matches the Washington State Energy Code, so open-cell spray foam contractor in Seattle, WA pulling permits need envelope assemblies that actually control air movement, not just assemblies with the right number on the label. Open-cell foam at 5.5 inches in walls simplifies that compliance path because the insulation itself is the air barrier.

Real-World Seattle Scenarios

Factors That Influence the Decision

• Property age and framing: Older Seattle homes with irregular framing favor foam because it fills whatever cavity it finds. Straight, modern framing accepts batts more predictably.
• Cavity depth: A 2×6 wall holds the 5.5 inches open-cell foam needs to act as an air barrier. Shallow 2×4 cavities change the math.
• Moisture exposure: Crawl spaces and walls near wind-driven rain benefit from assemblies with fewer air paths.
• Project scope: Full envelope projects get better per-board-foot pricing, while easy site access and flexible timelines lower our cost further.
• Ventilation planning: Foam tightens the envelope, so some homes need ventilation adjustments to keep fresh air moving.
• Budget timeline: Fiberglass spreads cost out, foam concentrates it upfront and pays back through reduced heating load.

Who Open-Cell Spray Foam Is For (and Who Should Stick With Fiberglass)

Open-cell spray foam is the right call when:

• You want the air sealing and insulation handled in one trade visit
• Drafts, cold rooms, or high heating bills point to air leakage
• You have cathedral ceilings, irregular framing, or sound control goals
• You are building or remodeling in Seattle, WA and need envelope compliance
• You plan to own the property long enough to collect the energy savings

Fiberglass is the better fit when:

• The job is an open, accessible attic floor with straight joist bays
• Upfront material cost is the primary constraint
• A separate air sealing scope is already budgeted
• The space may need frequent access or rework

Request a Free Quote From Spray-On Foam & Coatings

If you are weighing open-cell spray foam vs. fiberglass for a Seattle, WA project, our team at Spray-On Foam & Coatings will assess your walls, ceilings, and crawl spaces and tell you honestly which material earns its place. We install open-cell and closed-cell spray foam, crawl space insulation, and dedicated air sealing across Seattle and the surrounding region. Call us at (360) 667-1993 or email [email protected] to talk through your building with our crew.

Request a Free Quote

Schedule an Air Sealing Assessment

Stop paying to heat the outdoors. Let our team seal it right the first time.

Frequently Asked Questions About Air Sealing With Open-Cell Spray Foam vs. Fiberglass

Does open-cell spray foam really stop air leaks better than fiberglass?

Yes. Open-cell foam expands to fill cracks and voids and acts as an air barrier at 5.5 inches or more of installed depth. Fiberglass slows heat flow but allows air to pass through and around the batts, which is where most energy loss happens.

How much does open-cell spray foam cost in Seattle, WA?

Our open-cell spray foam typically runs $0.28 to $0.75 per board foot, with an average around $0.32. Larger projects lower the per-board-foot rate, while greater thickness and higher R-value targets raise it.

Is open-cell spray foam safe once it cures?

The exposure concerns are limited to application time, when isocyanates in the liquid chemicals can irritate skin, eyes, and lungs, which is why EPA guidance on spray polyurethane foam stresses professional handling and proper protective equipment. Once the foam cures, it is inert and occupants can return.

Can I combine fiberglass and spray foam in the same home?

Absolutely. A common hybrid uses open-cell foam in walls and cathedral ceilings where air sealing matters most, and fiberglass batts on open attic floors where the air barrier comes from sealing penetrations below. Dedicated air sealing projects start at $1,500 and average $4,500.

Why does Seattle, WA make fiberglass air sealing problems worse?

Seattle’s wind-driven rain and long heating season drive air and moisture through wall cavities for much of the year. Air moving through wet fiberglass carries moisture into cold assemblies, and wet batts lose effectiveness, sag, and stop sealing anything.

Sources

• Seattle Department of Construction & Inspections – Energy Code – The 2021 Seattle Energy Code’s envelope requirements controlling heat loss and air leakage for new and remodeled buildings in Seattle.
• ENERGY STAR – Seal and Insulate – Federal program guidance on air sealing and insulation benefits, including up to 10% annual energy bill savings and heating and cooling energy use data.
• EPA – Chemicals and Production of Spray Polyurethane Foam – Environmental Protection Agency resource on SPF chemistry, isocyanate exposure concerns during application, and safe installation practices.