How Does Vapor Permeance Affect Closed-Cell Spray Foam Performance in Vancouver, WA?

Vapor permeance measures how much water vapor passes through a building material, rated in perms, where a lower perm rating means less vapor gets through. For closed-cell spray foam in Vancouver, WA, permeance works like a performance dial: the perm rating falls as installed thickness increases, moving the foam from a Class III vapor retarder at about 1 inch to a Class II vapor retarder near 2.5 inches, based on ASHRAE data published by the ICC. That number matters here because Clark County sits in IECC Climate Zone 4C, a marine zone with wet winters, so permeance determines whether your wall, crawl space, or roof assembly meets code, resists condensation, and still has a drying path. The right specification depends on the assembly, the cladding, and the moisture load, which is exactly what the sections below sort out.

TL;DR

• Perms are the language of moisture control. Class I is 0.1 perm or less, Class II is 0.1 to 1.0 perm, Class III is 1.0 to 10 perms, and materials above 10 perms are considered vapor permeable.
• Thickness sets the class. Closed-cell spray foam measures about 2 perms at 1 inch (R-6) and roughly 0.8 perm at 2.5 inches (R-15), a shift from Class III to Class II.
• Vancouver sits in Marine Zone 4C. The IRC calls for a Class I or II interior vapor retarder in Marine 4, with Class III permitted when vented cladding or minimum continuous exterior insulation is used.
• Air leakage, not diffusion, is the main moisture carrier. Government building-science research attributes more than 98 percent of water vapor movement in cavities to air currents, which closed-cell foam stops as both air barrier and vapor retarder.
• Open-cell needs a partner for vapor. At 3.5 inches, open-cell foam runs about 25 perms, permeable enough to require separate vapor control in our climate DOE EERE. 
• Cost scales with thickness. Our closed-cell work currently prices at $1.20 to $3.00 per board foot, averaging $1.50, and larger projects bring the per-board-foot rate down.

What Vapor Permeance Means in Real Numbers

Permeance is tested under ASTM E96 and reported as the rate of water vapor diffusion through a material. Building codes group the results into three classes, plus a fully permeable category, and each class plays a different role in a wall or crawl space assembly. Understanding closed-cell spray foam performance also requires looking at how air sealing, thermal resistance, and moisture control work together, not just at the perm rating.

Two details from the code guidance deserve attention. Permeance drops as thickness rises, so the same foam serves different roles at different depths. And air leakage usually moves far more moisture than diffusion, which is why foam that seals air while controlling vapor does double duty.

Why Vancouver’s Marine Climate Changes the Foam Decision

The DOE Building America program defines a marine climate by cool summers (a warmest-month mean under 72°F), mild winters, and a wet cold season followed by a dry summer, and its county-by-county guide places Clark County, home to Vancouver, in Zone 4C marine. In practice that means months of rain-heavy air, cool wall cavities, and outward vapor drive each winter.

In this zone, the IRC requires a Class I or II vapor retarder on the interior side of above-grade frame walls, with a Class III layer allowed where the wall has vented cladding or the code’s minimum continuous exterior insulation (R-2.5 on a 2×4 wall, R-3.75 on a 2×6). ICC guidance for Marine 4 recommends Class II as the general choice and cautions against Class I layers where air conditioning runs in summer, because they block inward drying. DOE EERE sheets for the Seattle and Portland zone make the same point another way: assemblies need a way to dry, and the most common failure mode is stacking vapor-impermeable layers so moisture that gets in cannot get out.

How Thickness Changes Closed-Cell Spray Foam Performance

Because permeance decreases with thickness, your installed depth is a moisture decision, not just an R-value decision.

Two notes from the research: high-density closed-cell foam can stand in for insulating sheathing in flash-and-batt walls, and spray foam in Vancouver, WA applied at the interior of exterior sheathing serves the continuous-insulation function when it measures 1.5 perm or less at the specified R-value. Thicker passes add board feet, and our pricing runs $1.20 to $3.00 per board foot with bigger projects at the lower end of that range.

Closed-Cell vs. Open-Cell: Permeance Is the Real Difference

Budget follows the same split: our open-cell installs average $0.32 per board foot ($0.28 to $0.75) while closed-cell averages $1.50 ($1.20 to $3.00). The closed-cell premium buys the built-in vapor retarder, which in a marine climate often eliminates the need for a separate membrane.

Application Recommendations for Vancouver Properties

Crawl spaces. Ground moisture and humid winter air make this the highest-risk zone in most Clark County homes. We insulate crawl walls with closed-cell foam and seal the rim joist, and because below-grade walls are exempt from the IRC’s interior-retarder rule, air sealing and moisture management carry the load. Our crawl space insulation projects average $5,500, ranging from $1,500 to $35,000.

Above-grade walls. Specify about 2.5 inches of closed-cell foam to reach Class II, or use a thinner Class III layer where the home has vented cladding or exterior insulation. Skip Class I products here, they fight the assembly’s ability to dry.

Attics, rooflines, and pole barns. Condensation forms on cold roof decks and metal panels during damp spells. Closed-cell foam at the deck or added thickness on metal structures controls it, and our pole barn insulation projects range from $3,500 to $50,000 with an $8,500 average.

Common Mistakes That Trap Moisture

• Treating a thin flash coat as a vapor barrier. At 1 inch, closed-cell foam is Class III, not a Class II retarder.
• Stacking impermeable layers. Poly sheeting over thick foam creates a double vapor barrier with no drying path.
• Foaming over wet materials. Enclosing damp sheathing or framing before it dries locks construction moisture inside.
• Ignoring air sealing. Diffusion moves little moisture compared to air leaks, so gaps defeat the whole strategy.
• Swapping materials mid-project. Substituting a lower-perm product into a designed assembly causes moisture problems in the near or long term.

Matching the Approach to Your Project

Signs You’ve Found the Right Approach

• The scope names a perm target and a thickness, not just an R-value.
• The plan identifies which direction the assembly dries and avoids impermeable layers on both sides.
• Whoever quotes the job asks about cladding type, vented versus non-vented, before recommending a class.
• Ground moisture, bulk water, and indoor humidity get discussed before any foam is sprayed.
• Substrate dryness and application conditions are part of the plan, because wet substrates cause failures.

Get the Vapor Numbers Right for Your Vancouver Property

Vapor permeance is where spray foam projects succeed or fail in the Pacific Northwest, and our team at Spray-On Foam & Coatings specs every closed-cell application with Climate Zone 4C, code class, and drying path in mind. Call us at (360) 667-1993 or email [email protected] and we will review your assembly, recommend the thickness that hits the right perm class, and price it plainly by the board foot. Talk with our crew before the next wet season turns small moisture issues into expensive repairs.

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FAQs

Is closed-cell spray foam a vapor barrier or a vapor retarder?

Technically a retarder at common residential thicknesses. At roughly 2.5 inches it measures about 0.8 perm, a Class II retarder, and only reaches Class I barrier territory as thickness increases further Wikipedia. 

How many inches of closed-cell foam do Vancouver walls need?

About 2.5 inches gets you to roughly 0.8 perm, the Class II level the IRC expects in Marine Zone 4. With vented cladding or qualifying exterior insulation, a thinner Class III layer is also code-permitted.

Do I still need plastic sheeting with closed-cell foam?

Usually no, and adding poly over thick foam often creates a double vapor barrier that traps moisture. Most Zone 4C assemblies perform better with the foam alone plus a defined drying path.

Which foam belongs in a crawl space in this climate?

Closed cell is the safer call here. Open-cell foam measures about 25 perms at 3.5 inches, so ground moisture would diffuse straight through it, while closed cell insulates, air-seals, and retards vapor in one layer.

Does humid weather change foam’s perm rating?

Not meaningfully. Plastic materials like spray foam hold their permeance steady at high humidity, unlike kraft paper or wood products that become more vapor-open as relative humidity climbs.

Sources

• ICC TechNote: Vapor Retarders, Reducing Moisture Risk in Frame Walls – ICC guidance on vapor retarder classes, IRC Marine 4 requirements, and ASHRAE permeance data for closed-cell and open-cell spray foam by thickness.
• DOE EERE: Vapor Control Layer Recommendations for All Climates – Building America guidance for Zone 4C covering Class III exceptions, flash-and-batt closed-cell assemblies, and the dominance of air leakage over diffusion.
• DOE EERE: Vapor Open Assemblies for All Climates – Building America research on drying, double vapor barrier risks, wet-cup versus dry-cup testing, and material substitution failures.
• Wikipedia: Vapor Barrier – Reference overview of perm classifications, ASTM E96 testing, the finding that air movement carries over 98 percent of vapor in cavities, and climate-based retarder placement.