Open-cell vs Closed-cell spray foam | Everything you need to know about insulation

January 6, 2022

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Spray foam is one of the most talked about insulation products. In this article we’re going to discuss how open and closed cell spray foam insulation is made, their performance and properties. We’re going to find out if one is better than the other and where you should use them.

Manufacturing Process

Both open and closed cell polyurethane spray foam consist of two parts:

Part A: Contains highly reactive, low molecular weight chemicals known as isocyanates, typically methylene diphenyl diisocyanate (MDI) or polymeric methylene diphenyl diisocyanate (pMDI). Part A is a standard component across manufacturers.
Part B: Comprises a proprietary blend of chemicals that contribute to the foam’s distinct properties. This blend may vary between manufacturers and includes a polyol (organic compound with hydroxyl or OH- groups), initiating catalysts (for reaction initiation), curing catalysts (to ensure complete reaction), flame retardants (to slow fire growth), blowing agents (responsible for foaming), and surfactants (to reduce liquid surface tension and encourage foaming).

When Part A and Part B are mixed, they undergo an exothermic reaction, generating heat and forming polyurethane bubbles. Open cell spray foam allows these bubbles to rupture, leaving behind only the struts or points where the bubbles intersect. In contrast, closed cell foam retains the bubbles, creating interconnected closed pockets with trapped blowing agents. Notably, open cell spray foam utilizes water as a blowing agent, while closed cell spray foam requires a chemical blowing agent. The environmental impact of closed cell foam has been addressed through strict EPA regulations, now using blowing agents with a remarkably low global warming potential of just 1.

Properties

Appearance

Open cell spray foam expands over 100 times its liquid volume, making it ideal for insulating hard-to-reach spaces. It is then shaved down and remains soft and flexible, with low compressive strength, making it susceptible to punctures. Closed cell foam, on the other hand, expands about 30-40 times its liquid volume, making it suitable for tight spaces. It is not easily shaved down and is hard, with no compressibility.

Density

Open cell foam has a density of 0.4 to 1.2 pounds per cubic foot, while closed cell foam has a density of 1.75 to 2 pounds per cubic foot. The lower density of open cell foam leads to a higher yield, providing more coverage compared to the same volume of raw ingredients of closed cell foam.

R-Value

Both open and closed cell spray foams have a high R-value, which measures resistance to heat flow. Open cell foam, with no trapped air, has a lower R-value of 3.5 per inch, while closed cell foam boasts a higher R-value of 6 per inch. It is essential to note that the R-value test does not account for the critical air sealing properties of spray foam. When correctly applied, both types of spray foam can act as air barriers, preventing 98% of air loss and outperforming traditional R 3.5 or R 6 insulation.

Heat Transfer

Open cell spray foam primarily blocks heat transfer through conduction, while closed cell foam additionally hinders conduction due to its trapped gas, which is a poor conductor. Both types prevent heat transfer via convection currents or air movement, unlike traditional fiberglass insulation. Radiation, the transfer of heat through electromagnetic waves, is also mitigated when spray foam is installed under a radiant surface, such as a hot roof.

Vapor Resistance

Open cell foam has a perm rating of around 15 for a 2-inch depth, allowing water vapor to pass through it. In contrast, closed cell foam acts as a vapor barrier when applied to a thickness of over 1.5 inches, with a perm rating of less than 1 for a 2-inch depth. The need for vapor retarders or barriers varies depending on the climate zone.

Water Resistance

Open cell foam is moisture permeable, allowing water to move through it, although its performance may vary by brand. Closed cell foam is 100% moisture impermeable, not permitting water movement.

Fire Resistance

Both open and closed cell spray foams meet Class I fire standards, with a flame spread index of less than 25 and a smoke developed index of less than 450. While they char and flake when burned, they do not melt and drip like foam coffee cups.

Cost

The cost of both open and closed cell foam has fluctuated significantly in recent times due to raw material shortages and supply chain issues. Roughly, open cell spray foam costs between 50 cents to 1 dollar per board foot, while closed cell foam costs $1.5 to $3 per board foot. Though more expensive than traditional insulation options like fiberglass and rockwool, spray foam offers considerable energy savings, potentially reducing mechanical ventilation system size and cost.

Uses

Open cell is good for soundproofing. It has about twice the sound resistance in normal frequency ranges as closed-cell foam. Open cell foam is future-proof. If you need to renovate a house or run new wiring, open cell spray foam is going to be much easier to work with. Closed cell foam’s rigid structure and durability make it ideal for exposed walls because it won’t be damaged if it is bumped by machinery or tools.

Conclusion

Flash and batt insulation is a hybrid system where closed cell spray foam and fiberglass are both used to insulate a space. It is more effective than just fiberglass and less expensive than all closed cell spray foam. It is not recommended to use open cell foam for this application. I wouldn’t say one is better than the other. Your project, budget, location and other variables will determine which spray foam is best for you. They offer many benefits to traditional insulation, but they also have many drawbacks like risks to the environment and our health.