Open Cell Spray Foam vs Closed-Cell, and the Other Types of Spray Foam
The short answer
Open cell spray foam is light, soft and lets water vapor pass through. Closed-cell foam is dense, rigid and, at the thickness its data sheet lists, stops water vapor. Both are polyurethane, both seal air leaks well, and neither is better in every building. They suit different jobs. PolyJacks sprays closed-cell only, so this page is an honest comparison from a firm that does not sell the other one.
For a steel building, closed-cell is the product that fits, and the reason is moisture on cold metal, covered below. For some other buildings, open-cell is the right product, and a section further down says which.
The numbers on this page come from one federal guide. That guide dates from 2009 and describes typical foams of that time, so these are typical values, and a real data sheet for a real product replaces every one of them. Our own product's sheet is on closed-cell spray foam insulation.
Open cell spray foam vs closed cell: the difference table
Source for every cell in this table: the U.S. Department of Energy Building America guide "Which Spray Foam Is Right For You?" (Steven Winter Associates, updated April 3, 2009). It is a federal guide for houses in several states. The foam's behavior (what a cell structure does to heat, air, vapor and water) is the same in a Kansas shop. What changes is which assembly it goes into.
| Property | Open-cell (half-pound foam) | Closed-cell (2-lb foam) |
|---|---|---|
| Nominal density | 0.5 lb per cubic foot | 2.0 lb per cubic foot |
| Share of closed cells | under 10 percent | over 90 percent |
| R-value per inch, aged | about 3.6 | about 6.1 |
| Vapor permeance | 10 perms at 5 in thick (the guide says 5 to 10 at typical thickness) | under 1 perm at 2 in thick |
| Is it a vapor retarder? | No. A vapor retarder is 1.0 perm or less | Yes, at typical applied thickness |
| Air permeance | Air-impermeable at typical thickness | Air-impermeable; the guide's test value is far smaller |
| Takes in liquid water? | Yes: it can hold up to a third of its volume | No: described as hydrophobic |
| Feel and strength | Soft, compressible; compressive strength under 2.0 psi | Rigid; compressive strength 22 psi |
| Expansion from liquid | up to about 150 times | about 35 to 50 times |
| Single-pass limit | full depth in one pass, generally up to 10 in | typically 2 to 3 in per pass |
| Blowing agent (2009) | mainly water | mainly hydrofluorocarbons |
| Sound, wood-stud wall test | STC 37, NRC 70 | no data given |
| Flame spread and smoke | under 25, under 450 | under 25, under 450 |
| What drives installed cost | lower, because each gallon of liquid expands further | higher, because more liquid goes into each cubic foot |
A few of those rows deserve a plain reading.
R per inch is not R of the job. Aged values drift from fresh ones, which is the subject of aged R-value of spray foam. Closed-cell reaches a given R in a little more than half the thickness of open-cell. In a deep wood cavity, thickness is cheap and that matters little. In a steel building with shallow flanges, thickness is the limit and it matters a lot.
"Air-impermeable" and "vapor-impermeable" are different words. Both foams stop moving air at typical thicknesses. Only one stops moving vapor. People buy open cell spray foam for its air sealing and then expect it to behave as a vapor barrier too, and the table shows it does not.
The blowing agents have moved on since 2009. Current products use different agents from those the guide describes. The up-to-date story is on low-GWP HFO spray foam. Treat the guide's chemistry row as history.
The cost row is direction only. We publish no prices. The guide's cost figures are from 2009 and are not used here. What changes the number today is covered on what drives spray foam cost.
Why open-cell on cold steel lets moisture reach the panel
Steel is a thin skin with almost no resistance to heat, so on a cold night its inside face can sit below the dew point of the building's air. A reflective-insulation trade group's FAQ states the rule: condensation occurs on any surface at or below the dew point, and if indoor air with water vapor reaches a cold roof deck, condensation is likely (RIMA, Condensation FAQ). RIMA is a trade group for reflective products and the physics it states is general; we use it only for that.
Now put open-cell against the steel. At 5 inches it passes 5 to 10 perms in the guide's figures, and the guide says it can take in and hold liquid water. Damp inside air moves through the foam, reaches the cold panel and condenses on it, and the foam can hold the water there against the metal. Closed-cell, continuous and thick enough to count as a vapor retarder, keeps that air off the steel. The practical test is the thickness, and the method for it is on spray foam for metal buildings.
That is a general moisture argument, not a promise about any one building.
When open-cell is the right product
Open-cell wins in some cases, and a firm that sprays only closed-cell should say which. In the DOE guide's ratings, open-cell is rated preferred for a cold-climate wood-frame wall and acceptable for a sloped roof deck. Add its sound test values and you have the strongest cases:
| Case | Why open-cell fits | What the guide says must come with it |
|---|---|---|
| Wood-frame wall cavities in a cold climate (rated preferred) | Reasonable R per inch and excellent air sealing in a deep cavity, nearly always sprayed to full depth | Interior air sealing and an interior vapor retarder (paint can serve), and an exterior that lets moisture out |
| Sloped roof decks in cold-climate houses (rated acceptable) | Gives full insulation at the roof deck and blocks air | A vapor retarder over it unless the roof surface is itself moisture-tight, such as standing seam metal, and a barrier covering per the product |
| Sound control in a stud wall | The guide lists test values for it (STC 37 in a wood stud wall) and none for closed-cell; rain on a steel roof is a separate question on rain noise on a metal roof | The values apply to that tested wall, not to your building |
Two honest conclusions. First, the guide is about low-rise houses, so its cases are wood-frame assemblies and do not carry over to a steel shop.
The other types of spray foam
The word "polyurethane foam insulation" covers all of the following. The guide says open-cell and closed-cell are chemically similar, a two-part spray from an isocyanate side and a resin side (DOE Building America guide, 2009). "Urethane foam" and "liquid foam" are other names for the same families. Beyond cell type, the split that matters is how the foam is delivered, and the EPA's page on product types lists three (EPA Safer Choice, Spray Polyurethane Foam Product Types, September 2020):
| Delivery type | How it works | What it is for | What we use |
|---|---|---|---|
| Two-component, high pressure | Both sides pumped through heated hoses to a nozzle; the EPA page gives above 150 °F and 1,200 psi | Large insulation jobs, air sealing and roofing; professional installers only | This is the class of equipment we run: a proportioner and 200 ft of heated hose, with an employee crew |
| Two-component, low pressure | Sides combined at the site and sprayed as a stream or bead from small tanks | Air sealing, adhesive uses and smaller jobs | No |
| One-component foam (cans) | Pre-reacted foam that finishes curing with air moisture | Gaps from about half an inch to 3 in around windows, doors and similar | No |
Two more terms you will meet:
- Roofing foam. Closed-cell foam sprayed on the outside of a roof. The guide names low-slope roofs among the rigorous uses of closed-cell. That is a roofing trade, not interior insulation, and we do not do it on this service.
- Injection foam. Foam put into a closed cavity through holes. We do not do it.
Density labels are used loosely. The guide calls the two main families "half-pound" foam and "2-lb" foam. "Low density" and "high density" in a quote only mean something when the data sheet's actual density is printed next to them.
FAQ
What is the difference between open cell and closed cell spray foam?
Closed-cell is denser, has more R per inch and resists water vapor; open-cell is lighter, softer and vapor-open. In the DOE guide's typical values, closed-cell has about R-6.1 per inch and open-cell about R-3.6. Open-cell also takes up water, which closed-cell does not. The table above sets out the other rows.
Is open cell or closed cell better for a metal building?
Closed-cell, because open-cell lets moist air reach the cold steel. At 5 inches, open-cell passes 5 to 10 perms in the DOE guide's figures, while closed-cell is under 1 perm at 2 inches. Condensation forms on any surface at or below the dew point (RIMA). The thickness that does the job is a data sheet question, covered on metal building spray foam.
Is open cell spray foam good for attics?
In ordinary houses it is a common choice, and the DOE guide rates it as acceptable to preferred in several roof-deck and attic cases, always with the moisture controls it names. That is education from the guide, not an offer. PolyJacks sprays closed-cell only, and inside buildings such as steel shops and barns. For a barn's roof or walls, see the options in metal building insulation compared.
Book a free on-site measurement
If you are unsure which product your building needs, the visit settles it. We measure the building, look at what is on the steel now, and you get a written estimate after the visit. If a different product suits your building better, we will say so. The visit is free; how it goes is on the free estimate visit.
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Spray foam insulation is a new PolyJacks service line run by its own crew and rig.
Who wrote this: PolyJacks spray foam team. How it was made: drafted with AI assistance from the sources below, then checked against each source.
Sources
- U.S. Department of Energy, Building America, "Which Spray Foam Is Right For You?" (Consortium for Advanced Residential Buildings / Steven Winter Associates, updated April 3, 2009): www1.eere.energy.gov. Out-of-state federal guide for houses; the cell structure and its effects on air, vapor and water are the same in Kansas.
- U.S. EPA, Safer Choice archive, "Spray Polyurethane Foam Product Types" (page dated September 2, 2020): archive.epa.gov
- Reflective Insulation Manufacturers Association International, Condensation FAQ (trade group; physics only): rimainternational.org
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