Spray polyurethane foam (SPF) is the undisputed apex predator of home insulation. It completely stops airflow and massive heat transfer. But when contractors quote your home, they force you to choose between "Open-Cell" and "Closed-Cell." Making the wrong choice can either blow your budget needlessly or fail to protect a metal structure. Here is the strict engineering difference.
1. The Cellular Structure (Water & Density)
The terms "open" and "closed" refer to the microscopic bubbles inside the foam itself once it dries and cures on your roof deck.
Closed-Cell (High Density)
The microscopic bubbles are completely sealed and tightly packed. It dries extremely rigid, almost like concrete. Because it is totally impenetrable, it acts as a 100% moisture and vapor barrier. It's so strong it physically increases the structural "racking strength" of the building walls. Ideal for metal buildings, crawl spaces, and exposed exterior walls.
R-Value: ~R-7 per inchOpen-Cell (Low Density)
The microscopic bubbles are deliberately left broken (open). This allows the foam to expand up to 100x its original size. It dries spongy and soft. While it blocks air leakage perfectly, it does not block water vapor. However, because it expands so massively, it is incredible at sound dampening and fills awkward wall cavities perfectly. Ideal for standard residential roof decks and attics.
R-Value: ~R-3.8 per inchFor 95% of Dallas-Fort Worth residential attics, Open-Cell Foam is the correct choice. We apply it thickly (usually 6 to 8 inches) against the roof deck to achieve an R-22+ air seal. If your roof develops a shingle leak during a storm, Open-Cell allows the water to pass through, so you can clearly see the leak and fix your roof. Closed-cell would trap that water rotting the wood deck indefinitely.