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What is the Difference Between Open-cell and Closed-cell Polyurethane Foams?

     Open-cell foam is soft - like a cushion or the packaging material molded inside a plastic bag to fit a fragile object being shipped. The cell walls, or surfaces of the bubbles, are broken and air fills all of the spaces in the material.  This makes the foam soft or weak, as if it were made of broken balloons or soft toy rubber balls.  The insulation value of this foam is related to the insulation value of the calm air inside the matrix of broken cells. The densities of open-cell foams are around 1/2 to 3/4 of a pound per cubic foot.

     Closed-cell foam has varying degrees of hardness, depending its density. A normal, closed-cell insulation or flotation polyurethane is between 2 and 3 pounds per cubic foot. It is strong enough to walk on without major distortion. Most of the cells or bubbles in the foam are not broken; they resemble inflated balloons or soccer balls, piled together in a compact configuration. This makes it strong or rigid because the bubbles are strong enough to take a lot of pressure, like the inflated tires that hold up an automobile. The cells are full of a special gas, selected to make the insulation value of the foam as high as possible.

Closed cell foam is always more expensive per “R”. So, you could expect an R-13 of closed cell to be more expensive than R-13 of open cell. This is simply due to the fact that plastic costs more than air. There is much more plastic in closed cell than in open cell. It takes three times the amount of chemicals to make a board foot of closed cell foam as it does to make open cell foam. 
Open cell foam is typically R-3.5-R-4.0 per inch as opposed to R-6.0-R-7 for closed cell foam. The “R-value” of both of these products is a high quality stable R-value that will not materially fluctuate with wind speed, moisture, or temperature.  Fibrous insulators like cellulose and fiberglass are constantly changed by these factors.

Both types of foam are commonly used in most building applications.  Some are inappropriate in specific applications. For example, you typically would not use open-cell foam below grade where it could absorb water; this would negate its thermal performance because water is a poor insulator compared to air. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch possible.  Basically, the choice depends on the conditions of each installation. We routinely select from a wide variety of foam systems with varying characteristics, depending on the particular requirements of our clients' projects.
Moisture permeability is the most controversial characteristic of these two insulators. Moisture permeability is measured in perms and represents how much moisture can move through a given material. A perm is defined as a grain of water per hour per square foot per inch of atmospheric pressure. For purposes of this discussion, let’s just say that a perm is the amount of water on a small pinhead. The higher a perm rating, the greater the amount of moisture that can pass through the material. Open cell foam typically has a perm rating of around 15 for 2'' depth. Closed cell foam typically has a perm rating around 1 perm for 2” of depth. For comparison purposes, unfaced fiberglass has a perm rating over 100 for an R-13 batt.
Any polyurethane foam will hold up well to liquid water relative to other building materials. They are plastics and when they get wet they dry. Closed cell foam has stellar performance in these situations. In most cases, no significant amount of water will even enter the cell structure of the foam. Once the conditions that cause the water problem are removed, assuming the foam isn’t under water for months on end, the foam will simply dry out and return to action no worse for wear. On the other hand, the structure of open cell foam is not nearly as strong and it will take on water if a leak is bad enough. In the case of a roof leak or wind driven rain, open cell will simply dry out and return to normal once the leak is stopped. However, in the case of a flood that lasts for days on end whereby the foam is submerged, the pressure and weight against the foam will likely damage its cell structure beyond repair. It should be noted here that open cell foam will not wick water. This is a myth that you may have seen or heard of at a trade show. Closed cell advocates often have a bowl of colored water and drop chunks of open cell foam into it so you can watch it sink. We guarantee that if you have someone spray a piece of open cell foam, cut it up, and drop it in a bowl of water, it will sit on the top of the water for days on end. If you squeeze the piece of open cell foam like the demonstrators at a home show, the cell structure will be destroyed and, yes, it will sink and take on water. What does all of this tell you? If you live near a creek or other flood prone areas, or if you have an older home with bad basement drainage, you should seriously consider paying the premium for closed cell foam.
Now to the fun part – water vapor and moisture permeance. This is an area of great debate and misunderstanding in the insulation market and construction world as a whole. Most building inspectors and other industry “specialists” have very limited knowledge of it. As discussed earlier, closed cell foam has a low vapor perm rating (1 for 2”) and Open cell foam has a higher rating of around 10 (for 5”). Using generally accepted terms in the industry, closed cell foam is referred to as “vapor semi-impermeable” and open cell foam is referred to as “vapor semi-permeable”. A few points should be made prior to this discussion. First, moisture will move from areas of high pressure to areas of low pressure, which usually means it moves from warm to cold. Second, warm air can hold more moisture than cold air. Finally, this country has many different climates, so if you hear an expert from Texas tell you the “correct” way to insulate your home, you may want to consult a local expert. A good insulation solution in Minneapolis isn’t necessarily good for New York and a good solution for Miami isn’t the same as Seattle. The people you consult for building and insulation advice should have a thorough understanding of the local climate and building science. Unfortunately, most people in the industry don’t have this expertise. This is another example of why you should be very careful about drawing conclusions from something you see on the Internet. Every opinion you see on the Internet likely has an opposing opinion somewhere else on the Internet. Gather as much info as you can.
Every material used in building has a vapor permeance rating. Let’s discuss walls. In this region we typically see a wood framed wall cavity filled with insulation. It has painted drywall on one side and plywood or oriented strand board (OSB) sheathing on the other. On the outside of the sheathing we typically see a Tyvek type house wrap. Outside of that we see either brick or vinyl. Without discussing insulation yet, every one of these building materials (as installed on your home or building) is vapor semi-permeable or permeable meaning that moisture can flow or diffuse through them with relative ease. To add another dimension, most of these materials have the capacity to hold various amounts of moisture, sometimes referred to as the hygric buffer capacity. For example, the wood used to build an average home is capable of holding 50 gallons of water without causing a moisture problem. Vinyl siding and Tyvek have no storage capacity. Drywall has a little storage capacity. Brick and block have tons of storage capacity – like ten times the amount of wood! Steel studs have no storage capacity.
Insulation moisture permeability varies significantly. Here are the most popular in our region:
Vapor Permeable
Much Greater Than 20 perms

Vapor Permeable
Much Greater Than 20 perms

Open Cell Foam
Vapor Semi Permeable
10 perms at 5'' depth

Closed Cell Foam
Vapor Semi-Impermeable
Less than 1 perm at 2''