Open-Cell vs Closed-Cell Spray Foam in Michigan: What Zone 5B Homes Need for Brutal Winters

It happens every February in Grosse Pointe and Grandville alike. A homeowner cranks the DTE Energy furnace past 70°F while wind chills outside drop to -20°F. The house holds temperature well enough, but somewhere in the rim joist above the foundation wall, condensation is forming inside the wall cavity — invisible damage that won’t show up until the floor starts to bounce or the framing smells musty in the spring. The homeowner never sees it coming because someone told them years ago that “spray foam is spray foam,” and they assumed any product would do the same job.

In Michigan’s climate zone, that assumption is expensive. Open-cell and closed-cell spray foam are not interchangeable. They behave differently under cold stress, they address different failure modes, and Michigan’s building code specifies which one belongs in which application. Getting this wrong doesn’t just leave money on the table — it can create moisture damage that voids warranties, fails inspections, and costs far more to remediate than the original installation.

Here is a clear, location-specific breakdown of how these two materials perform in Zone 5B and where each one belongs in a Michigan home.

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What Zone 5B Actually Means for Your Insulation Strategy

The International Energy Conservation Code divides North America into climate zones based on heating and cooling demands. Michigan sits in Zone 5B — a heating-dominated zone with moderate humidity. This distinction matters because it shapes the entire moisture physics of your building envelope.

In a Zone 5B home during winter, warm, moist interior air is constantly trying to push outward through your walls, floors, and roof assembly. This is called the moisture drive, and in Michigan it runs almost exclusively inward-to-outward from November through March. During that window, if warm humid air reaches a surface cold enough for condensation — the dew point — you get liquid water inside your wall assembly. Do that repeatedly across several winters and you get rot, mold, and structural failure.

The other pressure your home faces is pure thermal load. Detroit Metro averages roughly 6,200 heating degree days per year. Grand Rapids, which sits in the lake-effect snow belt east of Lake Michigan, can see conditions that are even more punishing in localized cold snaps. Your insulation system has to maintain meaningful R-values across that entire temperature range, not just at mild temperatures.

These two facts — outward moisture drive and extreme heating load — are why the IRC’s Zone 5 requirements for minimum R-values and vapor retarder placement exist, and why they differ from what you’d see in a Zone 3 home in Georgia or Tennessee.

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Closed-Cell Spray Foam: Where Michigan Code and Physics Agree

Closed-cell spray foam (ccSPF) expands to about 1 inch of thickness per pass and cures into a rigid, dense matrix of cells that are sealed off from each other. It delivers approximately R-6 to R-7 per inch, it acts as its own vapor retarder (at 2 inches or more, it reaches Class II vapor retarder performance), and it does not absorb liquid water. In a Zone 5B climate, those properties make it the correct choice in several specific locations.

Rim Joists

The rim joist is the band of framing that sits on top of your foundation wall and closes off the floor joist bays. It is one of the most thermally vulnerable locations in any cold-climate home, and Michigan’s building code addresses it directly. IRC Section N1102.2.1 (as adopted in Michigan) requires minimum R-20 at rim joists in Zone 5. Open-cell foam at 3.5 inches delivers roughly R-13. Closed-cell foam at 3 inches delivers R-18 to R-21 — enough to hit or exceed the code minimum in the space actually available between the sill plate and the subfloor. In most existing homes in Royal Oak, Warren, or Kentwood, that cavity is exactly 3 to 3.5 inches deep. Closed-cell is the only practical material that gets you to R-20 in that space.

Beyond R-value, the rim joist is an exterior exposure. It faces outside air and ground-level cold simultaneously. In that position, you want a vapor retarder on the interior face — which closed-cell provides intrinsically. Open-cell foam at the rim joist would leave you with inadequate R-value, a permeable vapor profile, and the possibility of condensation forming at the exterior face of the foam.

Basement Walls Below Grade

Below-grade concrete walls in Michigan basements are cold, they are in contact with soil that carries moisture, and they have no meaningful drying path to the exterior. The moisture drive at a below-grade wall is complex — it can come from groundwater vapor migrating inward as well as from interior air moving outward. Closed-cell foam applied directly to the interior face of the concrete addresses both directions: it resists liquid water and bulk moisture from the soil side, and it provides the vapor retarder function on the interior side.

Open-cell foam on a below-grade wall creates a trap. The foam will hold any moisture that enters it — from either direction — and it has nowhere to dry. In a below-grade application in a West Michigan or Metro Detroit home, that is a setup for a persistent moisture problem inside the foam itself.

Exterior Wall Retrofits and Unvented Crawlspaces

When spray foam is being applied to the interior face of an exterior wall as a retrofit — common in older brick bungalows in Dearborn, Hamtramck, or East Grand Rapids — closed-cell is almost always the correct choice. It provides high R-value per inch, which matters in walls where cavity depth is limited, and it places the vapor control layer correctly on the warm-in-winter side of the assembly.

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Michigan’s Brick Homes: A Special Case That Requires Careful Detailing

A significant portion of Detroit Metro’s housing stock is single-wythe or double-wythe brick construction from the 1920s through 1950s. These homes in neighborhoods like Palmer Park, Indian Village, Rosedale Park, and Ferndale were built with brick that was intended to absorb rain water and then dry out — both inward toward the interior and outward to the exterior. This drying capacity is what has kept those brick walls intact for 80 or 100 years.

When you add spray foam to the interior face of a historic brick wall, you are changing that drying equation. Closed-cell foam, applied correctly, can work in this application — but only if the exterior drying path remains intact. That means:

– No interior vapor barriers on top of the foam (the foam itself is the vapor retarder; adding poly sheeting traps moisture in the brick) – No exterior waterproofing coatings that eliminate the outward drying path – Maintaining weep holes and flashing at window sills and base courses – Keeping the foam thickness at 2 to 3 inches — enough to keep the brick above the dew point in winter without completely eliminating the assembly’s ability to manage incidental moisture

Open-cell foam on the interior face of a brick wall in Zone 5B is not recommended. It is permeable enough to allow interior moisture vapor to reach the brick, but it does not provide the vapor retarder function that would limit how much vapor gets there. You end up with the worst of both configurations.

If you own a pre-war brick home in one of these Metro Detroit neighborhoods, insist that your contractor explain their moisture management strategy for the brick wythe before any foam is applied.

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Open-Cell Spray Foam: Where It Belongs in a Michigan Home

Open-cell foam (ocSPF) expands to a much lighter, softer matrix. It delivers approximately R-3.5 to R-3.8 per inch, it is vapor permeable, and it is significantly less expensive per square foot than closed-cell. Those characteristics make it the wrong material for cold exterior surfaces and below-grade applications — but they make it the right material in one important Michigan application.

Conditioned Attic Assemblies

Michigan’s energy code, in line with IRC Section N1102.2.3, permits open-cell spray foam for unvented attic assemblies in Zone 5 — but with a minimum R-value of R-38 at the roof deck, and with a requirement that the interior side of the ocSPF receive a vapor retarder. At R-3.7 per inch, reaching R-38 requires roughly 10 to 11 inches of open-cell foam applied directly to the underside of the roof sheathing.

In this application, the foam’s permeability is actually an advantage. The roof sheathing needs the ability to dry inward if it picks up incidental moisture. Closed-cell foam against the sheathing would eliminate that drying path entirely, which is why some attic assemblies using closed-cell have experienced sheathing moisture problems in cold climates even when the foam was correctly installed.

The critical detail in a Zone 5B conditioned attic using open-cell foam is the vapor retarder on the interior face. Without it — typically a variable-permeance smart membrane or a Class II vapor retarder — the assembly does not meet code and does not adequately control moisture. This is not optional; it is the difference between a durable assembly and a problem waiting to happen.

Open-cell foam is also used inside conditioned wall cavities in some applications, but in Zone 5B exterior walls, the vapor permeability of open-cell foam means the assembly needs careful evaluation. If the wall is being filled to the full stud depth, a vapor retarder on the interior side is required by Michigan code. If the wall cavity is shallow or the exterior cladding traps moisture, open-cell is not the right choice.

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DTE Energy and Consumers Energy Rebates: What Insulation Qualifies

Both of Michigan’s major utilities offer residential energy efficiency incentive programs that include insulation upgrades, and it is worth understanding which products and applications qualify before your project is installed.

DTE Energy’s home efficiency programs have included rebates for air sealing and insulation upgrades in the building envelope. Consumers Energy’s Home Energy Efficiency Program has covered insulation in attics, walls, and basements. Both programs typically require installation by an approved contractor and may require a pre-installation energy assessment to verify baseline conditions.

For spray foam specifically, rebate eligibility often depends on the R-value achieved and the location of the installation — not just the product type. A rim joist project that brings an attic floor or basement band from uninsulated to R-20 may qualify for different incentive tiers than an attic conversion project.

The rebate landscape changes from year to year, and both utilities have updated their program structures as federal Inflation Reduction Act funding has been deployed through state programs. Before your project is finalized, ask your contractor to confirm current program availability directly with DTE or Consumers Energy, or visit each utility’s residential efficiency portal to check current eligible measures and application deadlines.

Federal tax credits under the Inflation Reduction Act also remain available through 2032 for insulation improvements in existing homes — currently up to 30% of the project cost, with an annual cap of $1,200 for insulation and air sealing combined. A qualified spray foam project that improves your thermal envelope can be applied toward this credit.

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The Michigan Winter Reality: Why Getting This Right Matters More Here

Homeowners in warmer climates can recover from insulation decisions that are suboptimal. An undersized attic in Texas wastes air conditioning dollars, but it does not create moisture damage inside the wall assembly. In Michigan, a vapor profile that is wrong for the climate can drive condensation into your framing for five or six months out of every year.

The stakes are different here. A home in Birmingham or Ada Township that goes through 30 freeze-thaw cycles in a winter, with interior humidity from a family living inside and exterior temperatures swinging from -15°F to 40°F, is subjecting its building envelope to repeated stress. Insulation that works in a more forgiving climate may fail — silently — in those conditions.

Zone 5B also means that your heating system is running at or near capacity for a substantial portion of the year. A thermal bypass at the rim joist, a cold basement wall that is bleeding heat into the soil, or a poorly sealed attic assembly does not just affect comfort — it affects how hard your furnace works every single day from October through April. The payback period on a well-executed spray foam project in Michigan is consistently shorter than the national average because the heating load is higher.

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Frequently Asked Questions

My contractor says I can use open-cell foam everywhere to save money. Is that true?

Not in Michigan. Open-cell foam cannot meet the minimum R-20 requirement at rim joists in Zone 5B because the available cavity space is too shallow. It is also not appropriate for below-grade basement walls or for the interior face of brick walls. Using open-cell where closed-cell is required by code or by moisture physics will result in a failed inspection or, worse, a moisture problem that develops over several years before it becomes visible.

How much closed-cell foam do I need at the rim joist to meet Michigan code?

IRC Zone 5 requires R-20 minimum at the rim joist. Closed-cell foam delivers approximately R-6.5 per inch, so 3 inches provides roughly R-19.5 to R-20. In most Michigan homes, the rim joist cavity runs 3 to 3.5 inches deep, so a full fill with closed-cell meets the code requirement. Your contractor should measure the actual cavity depth at your home before quoting.

I have a 1940s brick bungalow in Oak Park. Can spray foam be added to the interior without damaging the brick?

Yes, but only with closed-cell foam and only if the exterior drying path of the brick is maintained. The foam goes on the interior face of the brick, provides the vapor control layer, and must not be covered with an additional vapor barrier. The brick needs to be able to dry outward in warm weather. Do not allow any exterior waterproofing coating that would seal the brick face, and confirm that weep holes and window sill flashing are intact before the project begins.

Does the type of spray foam affect DTE Energy or Consumers Energy rebate eligibility?

Rebate programs typically specify minimum R-values and eligible locations rather than product type. Both open-cell and closed-cell can be eligible depending on where they are installed and what R-value is achieved. Confirm eligibility for your specific project with the utility’s current program before the installation is scheduled.

What should I ask a spray foam contractor before hiring them for a Michigan home?

Ask these specific questions: “What R-value will you achieve at my rim joists, and how does that meet the Zone 5 code requirement?” Ask: “For my basement walls, will you be using closed-cell, and what thickness?” Ask: “If I have brick exterior walls, what is your approach to maintaining the exterior drying path?” Ask: “What vapor retarder, if any, are you including with the assembly, and where does it go?” A contractor who cannot give direct answers to these questions — with specific product names, thicknesses, and R-values — is not ready for the nuance that Michigan’s climate demands.

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Getting the open-cell versus closed-cell decision right in a Michigan home is not a matter of preference or budget alone. It is a technical question with code minimums, moisture physics, and decades of building science behind the correct answers. If you are planning an insulation upgrade in the Detroit Metro area or Greater Grand Rapids and want to understand what your specific home needs, reach out for a free assessment. The right foam in the right location will perform for the life of your home — the wrong choice will cost you far more to fix than it would have cost to do correctly the first time.