Open Cell Spray Foam Insulation

Open cell spray foam is a light, soft, vapor open foam that runs about R-3.7 per inch and expands roughly 100 times its liquid volume. It is excellent at sealing air and quieting a house. It is the right material in a handful of specific places in a Duluth home and the wrong one in several others, and the difference matters more here than it does in Missouri.

Call (218) 514-6594 if you want a straight read on which one your project needs.

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What open cell actually is

Polyurethane foam sprayed as two liquid components that react on contact. In open cell foam, the cell walls rupture as the foam rises, so the finished material is a matrix of connected air pockets. That structure is what gives it its properties.

Property	Typical open cell value
Density	about 0.5 lb per cubic foot
R-value	about R-3.5 to R-3.8 per inch
Expansion	roughly 100x
Air impermeable at	about 3.5 inches
Vapor permeance	high, generally in the double digits at 3 inches, so vapor open
Water	absorbs and holds bulk water
Structural contribution	none

Two of those lines drive every decision about where it goes. It is vapor open, which means water vapor moves through it. And it absorbs water, which means it cannot be near the ground or a surface where condensation is expected.

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Where open cell is the right call in a Duluth house

Interior walls and floors, for sound

This is the use nobody argues with. Open cell is soft and full of air, and it kills airborne sound. Bedroom walls, a bathroom that shares a wall with a living room, a laundry on the main floor, a bonus room over a garage, a home office. Three inches in an interior 2x4 cavity makes a noticeable difference.

There is no moisture question at all in an interior partition, because both sides of the wall are the same temperature and humidity. This is open cell doing exactly what it is best at with none of its weaknesses in play.

2x6 exterior walls with a proper interior vapor retarder

Five and a half inches of open cell fills a 2x6 cavity completely and lands around R-20, close to the wall target for this climate. It seals every gap, top plate, and electrical box on the way.

The catch in Minnesota is the vapor retarder. Minnesota’s code calls for a vapor retarder on the warm side of framed exterior walls in this climate zone, and open cell does not provide one. So an open cell wall needs something else to do that job, usually a Class II vapor retarder such as a vapor retarder primer or an approved membrane over the studs before drywall. That is a real added step and a real added cost, and it is one reason closed cell or a flash and batt hybrid often wins the wall on a Duluth job.

We will lay out both options and the price difference rather than picking for you.

Cathedral ceilings that are built vented

If a vaulted ceiling has a real vent channel above the insulation, running clear from the soffit to the ridge, then the roof deck is being washed with outdoor air and stays dry. In that assembly the insulation below the channel does not have to be vapor closed, and open cell works.

The condition is that the vent channel is continuous and unobstructed. A lot of vaulted ceilings around here were built with two inches of nominal air space that got squashed flat by a fiberglass batt in 1978. If the channel is not real, this is not the assembly you have, and open cell is the wrong material.

The second layer of a hybrid roof assembly

On an unvented conditioned attic, the code approach in Climate Zone 7 is at least R-30 of air impermeable insulation in direct contact with the roof sheathing. That is roughly five inches of closed cell. Once that layer is in place and the sheathing is protected, you can add open cell below it to build the rest of the R-value at a lower cost per inch.

That combination is often the most economical path to a properly conditioned attic in a Duluth house. Closed cell where the physics demands it, open cell where it does not. Details on the attic insulation page.

Pole barn and shop ceilings under a vented roof

If the building has a vented attic space or a vented ridge above the ceiling plane, open cell on the ceiling is a reasonable, economical way to insulate a heated shop. Covered on the pole barn page.

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Where open cell does not belong in Climate Zone 7

We would rather lose the sale than spray something that causes a problem in three winters. These are the assemblies where we say no.

Directly against a roof deck with nothing else

This is the big one, and it is the single most common spray foam mistake in cold climates.

Here is the sequence. Interior air in a Duluth house in January is warmer and more humid than outside. Open cell lets that moisture diffuse through it. On the far side of the foam sits the roof sheathing, and in an unvented assembly that sheathing is sitting close to outdoor temperature, which for months means below freezing. The moisture reaches a cold surface and condenses, or frosts. In spring it thaws. Do that for a few winters and you get staining, then mold, then soft sheathing.

In Zone 7 the correct approach is closed cell against the deck, or a hybrid with enough closed cell against the deck to keep its surface above the dew point. Open cell alone against the deck in Zone 7 needs a Class II vapor retarder coating applied to its interior face to be defensible, and even then we would rather do the hybrid.

Rim joists

The rim joist is the coldest interior surface in your basement and it is where warm humid house air meets cold outdoor-temperature wood. A vapor open insulation there lets that moisture straight to the sill plate. Closed cell, always. See the rim joist page.

Crawl space walls, basement walls, or anything below grade

Concrete and block wick ground moisture continuously. Open cell will absorb it. Closed cell, always. See the crawl space page.

Under a slab or anywhere it can sit in water

Not a candidate. Neither foam goes under a slab. That is rigid board.

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What it costs in the Duluth area

Open cell installed generally runs $0.45 to $0.75 per board foot in this market. A board foot is one square foot sprayed one inch deep.

Some worked examples so the number means something:

Job	Math	Typical installed range
2x6 exterior wall, 1,200 sq ft of wall, 5.5 in	6,600 board ft	$3,000 to $5,000
Interior sound walls, 400 sq ft, 3 in	1,200 board ft	$600 to $1,000
Pole barn ceiling, 1,500 sq ft, 6 in	9,000 board ft	$4,000 to $6,800
Second layer of a hybrid attic, 1,800 sq ft, 5 in	9,000 board ft	$4,000 to $6,800

What moves the price: how hard the space is to get a hose into, how much prep and masking there is, whether existing insulation has to come out, whether the job needs a thermal or ignition barrier over the foam afterward, and how far the rig has to drive. Small jobs carry a minimum charge because the rig, the crew, and the setup cost the same whether we spray 300 board feet or 3,000.

These are budget ranges. A real number comes after somebody looks at it.

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Two things to know before the crew shows up

Coverings. Spray foam left exposed inside a living space needs a 15 minute thermal barrier over it, which in practice means half inch drywall. In an attic or crawl space with limited access, an ignition barrier or an approved intumescent coating may be allowed instead, and some foams carry testing that permits exposure in those spaces. Whether your job needs a covering, and which kind, is part of the written scope.

Reoccupancy. You need to be out during the spray and for a period afterward while the foam cures and the space ventilates. Typically that is 24 hours from the last pass. The exact figure is product specific and we will give it to you before the day.

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Not sure which foam your project needs?

Tell us the assembly and what it is doing. A cold floor, a noisy room, a garage you want to heat, an attic you want to finish. The right foam falls out of the assembly, not out of a preference.

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

See also: closed cell spray foam, attic insulation, pole barn insulation.