Open-cell spray foam vs closed-cell spray foam: which statement is true?

• A. Open-cell is higher density (~2 lb/ft³) and better vapor barrier
• B. Closed-cell is low-density
• C. Open-cell is low-density (~0.5 lb/ft³) and permeable; Closed-cell is high-density (~2 lb/ft³) with strong air and vapor barrier
• D. Open-cell has higher R per inch than closed-cell

Answer: (C)

The main idea here is that open-cell and closed-cell spray foams differ in structure, density, and how they handle air and moisture. Open-cell foam has an open, interconnected cell structure, giving it a low density (around 0.5 lb/ft³) and making it permeable to air and moisture. It’s softer and less rigid, and its vapor barrier capability is limited. Closed-cell foam, on the other hand, has cells that are all closed, producing a high density (about 2 lb/ft³). This denser, rigid structure acts as a strong air barrier and offers much better moisture resistance, effectively serving as a vapor barrier in many installations. Because of the tighter cells and higher density, closed-cell also delivers more insulation per inch—its R-value per inch is higher than that of open-cell.

So the statement that accurately captures these facts is that open-cell is low-density and permeable, while closed-cell is high-density with a strong air and vapor barrier. The other claims don’t fit: open-cell isn’t higher density or a reliable vapor barrier, and closed-cell isn’t low-density; plus, open-cell does not have the higher R-value per inch.