What is the Difference between Open cell and Closed Cell Polyurethane Foams?
This may be one of the most important topics of concern in regards to spray polyurethane foam insulation.
“Open-cell” foam refers to the millions of tiny foam cells that are filled with air, each cell acting as an insulator, that are created when an open cell polyol resin mixes properly with an Isocyanate. Typical open cell foams are usually ½ pound per cubic foot and roughly R-3.8 per inch. The foam is meant to absorb some of the heat from a hot roof deck, but not so much as to let the heat through to the conditioned side. Open Cell foam is weaker or softer feeling than closed-cell foam, but a much better sound barrier than closed cell due to the open nature of the cells. Open Cell is not a water barrier, it will hold moisture if exposed to the outside environment. It is perfect for interior insulation and sound control and is the most popular type of spray foam insulation in residential applications in North America. Our experience has proven that Open Cell is the best insulating foam for the price in terms of thermal transfer inhibition and sound control. It’s what’s in my house, and there are hundreds to choose from.
Closed-cell foam differs in that all of its tiny foam cells are closed and packed together, hence its a solid. They are extremely small and filled with a gas that helps the foam rise and expand and become a greater insulator. Typical closed cell foams are usually 1.7 to 3 pounds per cubic foot and roughly R-7 per inch. The foam is designed to not allow any heat penetration within itself. Therefore it is technically a greater insulator in terms of thickness when compared to Open Cell. Closed Cell is primarily used in commercial roofing, cold storage, crawlspaces, below grade applications or as a structural adhesive. It is superior in strength and R-value per inch with a greater resistance to the leakage of air or water vapor. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch possible. It is considered a vapor barrier.
The disadvantages of closed-cell foam: it is denser, requires more material, and therefore, is more expensive. Though it has a better R-value, typically the cost per R is still higher than open cell foam. It is important to note that Closed Cell foam may void some asphalt shingle warranty’s due to “shingle burn”. In Florida, when Closed cell foam is applied to the underside of a roof deck, it has been known to cause typical asphalt shingles to retain the heat from the sun while not letting any heat pass through to the attic. Therefore the shingles rise in temperature much more than the manufacturer’s specifications allow for and become weaker resulting in a shorter life span. Closed cell still remains the best choice under a tile or metal roof since these types of roofing applications don’t absorb as much heat as asphalt shingles. The “shingle burn” theory is not agreed upon by all spray foam contractors, therefore Central Florida Spray Foam LLC advises the consultation of a State Certified Construction Engineer to dictate what type of foam is suitable for an individual application. From our experience, its best saved for home’s on the beach!
Polyurethane Foam Strengthens Your Home
The walls in your home are the main structural component of the building. In wood frame construction, the weight of the roof, shingles, standing rain water and any snow add weight and exert downward forces on the walls resulting in a compressive force.
Strong winds and gusts from storms also impose lateral forces onto your home’s walls. These forces can distort the walls with what is called a “shearing force.”
Building codes require that your home’s walls be designed to withstand these various forces and loads. However, when walls are built to just the minimum standards, while still safe, symptoms of movement such as creaking and shaking during high winds or occupant usage often occurs.
Higher density, closed cell spray foam insulation inside your stud walls fully adheres to both the exterior sheathing and the studs, reinforcing both. With this added rigidity, there will be less wall movement due to wind, vibration, and occupant activity. Additionally your walls have greater than code required resistance to “racking events” such as hurricanes or other strong wind situations.
SPF also can add structural strength to buildings. NAHB Research demonstrated SPF filled walls could add from 75% to 200% racking strength to walls of OSB, plywood, light gauge metal, vinyl siding or gypsum board.
Racking Test
Shearing forces on a wall tend to distort the wall from its original shape as a rectangle into a parallelogram. To test a wall’s resistance to the shear forces imposed by wind loading, engineers use a “racking test.” An 8 ft. x 8 ft. model wall is built and placed in a large frame. The base of the wall is secured to the frame and a horizontal (lateral) force is applied at one upper corner. The force is increased in 400 lb. increments until the wall structure fails.
Spray Foam’s affect on wall strength
In a series of racking tests1, walls with and without spray-applied polyurethane foam insulation were compared. Two exterior facing materials were tested:
- Vinyl siding over 15-lb. building paper
- Textured plywood siding.
All wall panels were faced with ½-inch sheetrock on the interior side and used 16 inch stud spacing. For the stud wall panels that were insulated with spray-applied polyurethane foam, the stud cavities were essentially completely filled with foam of 1.5 lb/ft³ density.
As the graph indicates, stud walls filled with spray-applied polyurethane foam add significant strength to home walls. Furthermore, for each load applied, the foam filled walls deformed less and offered greater resilience.
In a second series of racking tests2, spray-applied polyurethane foam insulation was compared with conventional R-19 glass fiber batts. In one comparison, the wall panels were faced both sides with dry wall. In the other comparison, the wall panels were faced one side with OSB (oriented strand board) with dry wall on the opposite side. In both cases, the wall panels used steel studs spaced 24 inches on center and the average foam density was 2.26 lb/ft3.
Once again, the graph indicates the greater strength of the spray foam insulated wall system.
In a second series of racking tests2, spray-applied polyurethane foam insulation was compared with conventional R-19 glass fiber batts. In one comparison, the wall panels were faced both sides with dry wall. In the other comparison, the wall panels were faced one side with OSB (oriented strand board) with dry wall on the opposite side. In both cases, the wall panels used steel studs spaced 24 inches on center and the average foam density was 2.26 lb/ft3.
Once again, the graph indicates the greater strength of the spray foam insulated wall system
- Test results are reported in “Testing and Adoption of Spray Polyurethane Foam for Wood Frame Building Construction” (May 25, 1992) prepared by NAHB Research Center for The Society of the Plastics Industry/Polyurethane Foam Contractors Division.
- Test results are reported in a letter from Bob Dewey, Mechanical Engineer, NAHB Research Center to Mason Knowles, The Society of the Plastics Industry/Spray Polyurethane Foam Division (November 18, 1996).