Open Cell vs. Closed Cell Spray Foam in Tampa, FL: Which Do You Need?

Density, R-Value and Vapor Permeance Compared

• Density: open-cell is about half a pound per cubic foot, closed-cell about 2 pounds
• R-value: roughly R-3.6 per inch for open-cell and R-6.1 for closed-cell (aged values; products vary)
• Vapor permeance: about 10 perms at 5 inches of open-cell, under 1 perm at 2 inches of closed-cell
• Air sealing: both stop air leaks, and open-cell counts as air-impermeable at typical thicknesses

Those numbers come from EPA product data and a Department of Energy guide to choosing spray foam. Picture open-cell as a window screen for water vapor and closed-cell as the glass: both stop the breeze, only one holds back the vapor. Depth is the other big difference. R-20 takes roughly 5.4 to 5.7 inches of open-cell but only about 3.3 inches of closed-cell. Our spray foam cost page shows what each runs installed.

Why Tampa Humidity Usually Decides It

Hillsborough and Pasco sit in Climate Zone 2A, the code's warm-humid zone. Florida summer dew points typically run 65 to 75, Tampa's can climb past 80, and the local record is 86°F, set in August 2000. Any surface colder than the dew point collects water, and in an air-conditioned house the usual suspect is a supply duct. Open-cell is vapor-open and can't act as a vapor control layer. Closed-cell can be the insulation and the vapor control layer at once.

Sealed Attics: How Each Foam Behaves

Foam sprayed under the roof deck brings the attic inside the thermal envelope. A 2024 FSEC review for the Florida Building Commission found most Florida studies didn't see roof sheathing go above 20 percent moisture with air-impermeable insulation at the deck. Either way, the wood should be under 19 percent moisture before we spray.

Open Cell Under the Roof Deck

DOE guidance rates open-cell acceptable on sloped rafters in hot-humid climates, and it's the budget choice. The catch: Building Science Corporation found open-cell attics can get very humid, with damp air pooling near the peak, and concluded there has to be a moisture removal mechanism, such as conditioned supply air at about 50 cfm per 1,000 square feet of ceiling, or a dehumidifier. Florida Building Commission staff concluded in 2026 that code doesn't require dedicated supply and return vents in a foam-sealed attic. That's a code minimum, not a moisture plan, and on open-cell jobs we still want one.

Closed Cell Under the Roof Deck

The same guidance lists closed-cell as preferred on sloped rafters, and Building Science Corporation notes it decouples the wood sheathing from interior air. You pay more per square foot, but you're buying vapor control along with R-value.

The R-20 Path in Florida Law

Section 553.9065, Florida Statutes, deems an unvented attic insulated and air sealed with at least R-20 of air-impermeable insulation to meet the energy code's R402 section, if all four conditions hold:

• A blower door result below 3 ACH50
• A positive-input, balanced or hybrid whole-house mechanical ventilation system
• At least R-3 of air-impermeable insulation on exposed rafters the R-20 doesn't cover, unless a finished ceiling covers them
• All HVAC equipment and ductwork inside the thermal envelope

It's a way to comply, not a savings promise. When FSEC modeled the path for the state, the worst case, R-20 at the roof with no ceiling insulation, used 18 percent more HVAC energy; the best case saved 1 percent.

Block Walls, Metal Buildings and Wind

Concrete Block Walls

For new work and additions, Florida's prescriptive code wants R-6 when most of a block wall's insulation is inside, or R-4 when it's outside. The old foil-between-furring-strips method is only claimed to equal about R-4.2. About an inch of closed-cell on the inside face of the block reaches R-6, and it won't soak up water the way open-cell can. We won't pump foam into block cores, though; the Department of Energy notes core filling saves little because heat travels through the solid parts of the wall.

Metal Buildings and Pole Barns

Bare metal panels get closed-cell from us. Humid Tampa air that reaches a surface colder than its dew point turns to water, and closed-cell bonds tight to the steel and holds vapor back. Open-cell would pass that vapor straight through to the metal.

Closed Cell and Wind Uplift

University of Florida researchers tested 186 roof panels and found closed-cell retrofits raised the uplift capacity of pre-1994 code-minimum wood roof decks by as much as 250 to 300 percent. That's an upper bound, not a typical result, and we found no comparable research for open-cell. Florida's wind mitigation form (OIR-B1-1802, Rev. 04/26) now lists spray foam products rated at 110 psf uplift (FOS 1.5) for roof deck attachment and sealed roof deck, but any credit depends on the product, the install pattern and the inspector. We don't promise insurance savings.

Where Each Foam Is the Wrong Choice

• Open-cell below grade or anywhere it can sit in water, since it can absorb it
• Open-cell on bare metal, or in thin spaces like block-wall furring where every inch must count
• Open-cell in a sealed attic with no plan for removing moisture
• Closed-cell across a big roof deck when open-cell plus a moisture plan would do the job for less
• Either foam on a wet deck or under a roof that leaks today, since a foamed deck can't dry toward the inside

We'd rather talk you out of the pricier foam than oversell it. For the attic question specifically, our attic insulation cost page weighs foam against blown-in. You can also see every spray foam service we offer or book a free estimate.