Closed Cell Spray Foam Insulation

Closed cell spray foam is a dense, rigid foam that delivers about R-6.5 per inch, seals air at around one inch, resists water, and acts as a vapor retarder at roughly 1.5 to 2 inches. In a Climate Zone 7 house, it is the material that solves the most problems in a single pass. It is also the more expensive of the two foams, and it is not the right answer everywhere.

Call (218) 514-6594 and tell us what the assembly is.

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Why this material fits northern Minnesota

Every cold climate building assembly has to handle four things: heat, air, bulk water, and water vapor. Most insulation handles one of them. Closed cell handles all four.

Property	Typical closed cell value	Why it matters here
Density	about 2 lb per cubic foot	rigid, holds its shape, adds stiffness
R-value	about R-6 to R-7 per inch, plan on R-6.5	fits code R-values in shallow cavities
Air impermeable at	roughly 1 to 1.5 inches	stops the stack effect in old Duluth houses
Vapor permeance	Class II vapor retarder around 1.5 to 2 inches	no separate poly needed in most assemblies
Water	does not absorb bulk water	safe against concrete, block, and below grade
Expansion	roughly 30x to 40x	fills and adheres without over-pressuring framing

That combination is why closed cell shows up in almost every serious Duluth insulation job. A rim joist needs air sealing, R-value, and vapor control in about three inches of depth. There is no other material that does all three there.

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Thickness targets for Climate Zone 7

Duluth, Superior, Hermantown, Proctor, and Two Harbors are all in IECC Climate Zone 7. Here is what that means in inches.

Assembly	Code or practical target	Closed cell thickness
Attic, Minnesota code minimum	R-49	about 7.5 in
Attic, newer model code and our preference on a full redo	R-60	about 9 in
Unvented roof deck, minimum foam layer before anything else	R-30	about 5 in
Rim joist	roughly R-20	3 in
Crawl space or basement wall	roughly R-15 to R-20	2.5 to 3 in
2x4 wall, filled	roughly R-21	3.5 in
2x6 wall, flash and fill hybrid	roughly R-21 to R-25	2 to 3 in of foam plus batt
Pole barn steel, condensation control only	not an R target	1.5 to 2 in

The R-30 rule on unvented roofs, explained

This is the number that catches people, so it is worth understanding rather than memorizing.

When you insulate a roof from the inside and there is no vent channel above the insulation, the roof sheathing becomes the coldest surface in the assembly. Indoor moisture that reaches it will condense if it is below the dew point. The way you prevent that is to put enough air impermeable insulation directly against the sheathing that the sheathing itself stays warm.

How much is enough depends on how cold it gets outside, which is exactly what a climate zone describes. The code table scales with the zone: Zone 5 needs R-20 of foam against the deck, Zone 6 needs R-25, and Zone 7 needs R-30. At R-6.5 per inch that is roughly five inches of closed cell.

Once that five inches is on, you can add cheaper insulation below it to reach your total R-value target. That is the hybrid assembly, and it is usually the most cost effective way to build a conditioned attic here.

Verify the current requirement with your building official before permitting. Code tables get amended, and the AHJ has the final say.

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Where closed cell goes in a Duluth house

Rim joists

The highest return per dollar in most Minnesota basements. Three inches of closed cell in every rim joist bay, tight against the rim and the sill, seals the largest continuous air leak in the house and stops the condensation that rots sill plates under fiberglass batts. Full detail on the rim joist page.

Crawl space walls

Concrete block and poured walls wick ground moisture. Closed cell does not care. Two and a half to three inches on the walls, run down onto the footing, plus a sealed ground vapor barrier, converts a cold damp crawl space into part of the house. See the crawl space page.

Basement walls

Same physics as a crawl space, more square footage. Closed cell directly on the foundation wall, then framing over it, avoids the classic Duluth basement failure of a stud wall with fiberglass batts pressed against damp concrete.

Roof decks and conditioned attics

Either full depth closed cell to R-49 or R-60, or the hybrid described above. This is the assembly for a finished attic, a house with ductwork or air handlers in the attic, or a roofline with no practical way to vent. See the attic page.

Cathedral ceilings without a vent channel

A vaulted ceiling framed with 2x10s and no room for a real vent path is a closed cell assembly. Nine and a quarter inches of cavity, five inches of closed cell against the deck for condensation control, then fill.

Steel buildings and pole barns

Closed cell sprayed directly to the underside of steel roofing stops condensation, which is the actual complaint in most unheated shops around here. Two inches also stiffens the panel and cuts the drumming in a rainstorm. See the pole barn page.

Exterior walls in a retrofit

In a 2x4 wall on an old Duluth house, closed cell is often the only way to get meaningful R-value into the cavity depth available. There are real cautions around old wiring and around plaster, covered on the retrofit page.

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The vapor barrier question

Short answer: closed cell at code thickness is your vapor retarder, and you should not add a second one on the interior side.

Minnesota’s residential code calls for a vapor retarder on the warm-in-winter side of framed exterior walls in this climate. For decades that meant six mil poly stapled over the studs. It works, and it is also unforgiving, because it means the wall can only dry to the outside.

Closed cell changes the picture. Most products reach Class II vapor retarder status, under 1 perm, somewhere around 1.5 to 2 inches. Check the data sheet for the specific product, because the thickness varies by manufacturer and blowing agent. At that thickness the foam is satisfying the vapor retarder requirement itself.

The failure to avoid is doubling up. Closed cell on the outside of the cavity plus poly on the inside creates a sandwich with no drying path in either direction. Any water that gets in from a roof leak, a plumbing leak, or wind driven rain has nowhere to go. Pick one vapor control layer and let the wall dry the other direction.

If you have an existing wall with poly in it and you are adding foam, tell us. The order of the layers determines what is safe.

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Cold weather application

We spray through Duluth winters. Here is what actually governs it.

Substrate temperature, not air temperature. The chemical reaction is exothermic and the foam generates its own heat, but it has to bond to whatever it lands on. A cold substrate pulls heat out of the reacting foam, which hurts adhesion, hurts yield, and can leave the foam shrinking away from the framing later.

Product formulation matters. Manufacturers make winter grade closed cell formulations specifically for cold substrates. Depending on the product these are rated down into the 20s and sometimes lower. Standard formulations generally want a substrate above about 50 degrees.

No frost, no ice, no moisture. Foam does not bond to frost. If the sheathing has a rime on it or the concrete is damp, we heat and dry it before we spray or we come back.

Temporary heat is normal. Bringing a crawl space or an attic into range with construction heat is a routine part of a Minnesota winter job. It is also a line item, and it will be in the scope rather than a surprise.

Lift thickness is not negotiable. Closed cell goes on in passes of roughly two inches with cooling time between them. A crew that tries to hit three inches in one shot to save time is risking scorched foam in the middle of the lift, and in the worst case a fire. This is a real hazard in this trade and the cure is patience.

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What closed cell costs around Duluth

Installed, closed cell generally runs $1.00 to $1.75 per board foot in this market. A board foot is one square foot at one inch thick.

Job	Math	Typical installed range
Rim joist, 150 linear ft, 3 in	about 450 board ft	$1,200 to $2,500
Crawl space walls, 520 sq ft, 3 in	1,560 board ft	$2,000 to $3,200 plus vapor barrier
Basement walls, 1,000 sq ft, 2.5 in	2,500 board ft	$3,000 to $5,000
Unvented roof deck, 1,800 sq ft, 5 in	9,000 board ft	$10,000 to $16,000
Pole barn roof, 2,400 sq ft, 2 in	4,800 board ft	$5,500 to $8,500

Note that most of these rows land above the raw board foot math, and the rim joist row is well above it. Setting up a rig, running hose, masking, and cleaning up costs roughly the same on a small job as a large one, so every job carries a floor and small ones carry it hardest. Prep and masking scale with how finished the space is, not with how much foam goes in. Combining a rim joist with a crawl space or a basement in one visit is usually cheaper than doing them in two.

What else moves the number: existing insulation removal, access, prep and masking, whether a thermal or ignition barrier is required over the foam, and drive distance. Nobody can price this accurately over the phone.

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Talk to someone who will tell you when not to buy it

There are jobs where closed cell is overkill and we will say so. A vented attic floor with good access does not need a foam roof assembly at $14,000 to $23,000 when air sealing plus blown cellulose reaches the same R-49 for {{PRICE_ATTIC_R49}}. We would rather do the smaller job right and be the call you make next time.

Call (218) 514-6594 or send the project details through the form.

See also: open cell spray foam, rim joist insulation, crawl space insulation.