Closed Cell vs Open Cell Spray Foam Insulation in a Wisconsin Climate

The biggest difference between the two foams in this part of the state is not R-value. It is what happens to indoor moisture in January. Eau Claire County sits in climate zone 6A on the ASHRAE map that energy codes use, with the state's northern tier in zone 7, and in that climate warm, damp house air spends five months pushing outward toward sheathing that sits far below its dew point. The model code answer, IRC Section R702.7, is a Class I or Class II vapor retarder on the interior side of frame walls in zones 5 through 8.

Wisconsin writes its own version into the Uniform Dwelling Code at SPS 322.38. Every frame wall, floor, and ceiling in the thermal envelope gets a vapor retarder on the warm-in-winter side, and the code defines a retarder by performance: 1.0 perm or less under ASTM E 96, Procedure A. Two exceptions matter here. No retarder is required in the box sill. And no retarder is required over a cavity where at least half the required R-value comes from spray foam rated at 1.0 perm or less, unless the foam's manufacturer says otherwise.

What SPS 322.38 Means for Each Spray Foam

Closed cell at roughly 1.5 to 2 inches, depending on the product, tests at or under 1.0 perm, so it is the vapor retarder. In a wall that needs R-20, 2 inches of closed cell supplies about R-14, comfortably over half, and clears the perm threshold on any product we spray, which is why 2 inches is the working spec you will see on our estimates. No poly sheet goes over it. Open cell never qualifies for that exception, because it is vapor-open at any practical thickness. An open cell wall is still a perfectly good wall; it just carries a separate warm-side retarder like any batt wall does, a sheet retarder or a rated vapor-retarder primer on the drywall where the inspector accepts one, and the estimate says which.

Open Cell Spray Foam on a Cold Roof Deck

The roof deck is where the vapor difference turns into rot or no rot. Foam the underside of the deck and the attic becomes unvented and conditioned, which the IRC allows under Section R806.5. In zones 5 through 8, that section requires the air-impermeable insulation against the deck to be a Class II vapor retarder itself, or to carry a vapor retarder coating or covering in direct contact with its underside. Closed cell at full thickness meets that as sprayed. Open cell does not, and without a retarder, indoor moisture diffuses through the foam all winter and collects in the sheathing, the coldest surface in the assembly.

Building Science Corporation's interior spray foam guidance goes further than the code minimum: in zone 5 and colder it recommends only high-density closed cell against the deck, and it notes that vapor retarder paints over open cell have had only limited success because the foam's rough surface is hard to coat evenly. That is our default too. On Eau Claire roof decks we spray closed cell, or a hybrid with enough closed cell against the deck to keep it above the dew point and open cell building the rest of the depth. Whether the deck is even the right place to insulate, versus the attic floor, is a separate decision covered on the attic insulation page.

Where Closed Cell Insulation Is Overkill

Filling a 2x6 wall to full depth with closed cell buys about R-38 of foam where open cell gets about R-20 for well under half the board-foot price, and the wall's real loss paths, studs, plates, and windows, cap what that extra R can return. Closed cell earns its premium where depth is scarce or moisture and structure are in play; in a dry, deep, framed bay it is mostly a bigger invoice. The same goes for interior walls sprayed for sound: closed cell is the worse acoustic product at twice the price.

Where Open Cell Foam Is the Wrong Product

Anywhere it touches concrete, steel, or ground moisture: basement walls, crawl space walls, rim joists, metal buildings. Anywhere floodwater could reach it, since FEMA keeps it off the flood damage-resistant list. And on an unvented roof deck in this climate without a vapor retarder detail. A lower bid that puts open cell in those places is not the cheap version of the same job; it is a different job with a moisture problem written into it.

Where Neither Spray Foam Is the Best Buy

An empty, vented attic over a simple ceiling is the clearest case. Foam the top plates, chases, and penetrations to stop the air, and the depth above that is usually cheapest as a deep layer of loose-fill insulation, which is not something this crew installs, and we will say so on the walkthrough. Flash-and-batt walls, a thin closed cell lift against the sheathing with batts filling the rest of the bay, are another budget option we price when asked. And a basement nobody plans to use is often better served by a rim seal alone this year than by foamed walls.

Both foams are foam plastics under IRC Section R316, and neither one is allowed exposed to living space. The standard thermal barrier is half-inch gypsum wallboard, which most finished walls and ceilings already include. Attics and crawl spaces entered only for repairs or maintenance can use an ignition barrier instead, from a code list that includes 1.5 inches of mineral fiber, quarter-inch wood structural panel, and 3/8-inch gypsum board, or an intumescent coating on a foam tested for that use. The barrier is the same question for open and closed cell, and the estimate names the treatment for each surface.

Cure and re-occupancy also run the same for both. The Consumer Product Safety Commission tells residents to stay out for at least 24 hours after two-component spray foam goes in, EPA points to the manufacturer's stated re-entry time for the specific product, and that time can run longer for sensitive occupants. Spray day gets scheduled around that window with ventilation running throughout.