Strengthens Walls with Spray Foam Insulation

The main component that provides structure to your home is the walls. In wood frame construction, the weight of the roof, shingles, standing rainwater, and snow load all contribute to compressive forces exerted downward on the walls. Additionally, lateral forces from gusts and high winds during storms can impose further stress on your home’s walls. 

These lateral forces can create a “shearing force” that distorts the walls. To effectively withstand these pressures and loads, building codes dictate that walls must be constructed with specific designs. However, many walls are built to meet only minimum standards, leading to sounds of creaking and shaking during movement or high winds. 

To enhance both the studs and the exterior sheathing within the walls, it is advisable to use higher density closed-cell spray foam (SPF), as it adheres fully to both materials. This added rigidity helps reduce wall movement caused by vibration, wind, and occupant activity. Moreover, closed-cell SPF can increase the racking strength of your walls significantly, improving resistance against racking events such as hurricanes or severe wind situations beyond what is typically required by building codes. NAHB Research has shown that SPF-filled walls can provide an increase in structural strength, offering an enhancement of racking strength ranging from 75 to 200 percent for walls made of materials like vinyl siding, gypsum board, light gauge metal, or OSB plywood.

A wall can change from its original rectangular shape into a parallelogram due to shearing forces. To evaluate the wall's racking strength, engineers conduct a racking strength test to measure its resistance to shear forces imposed by wind loading. In this test, a model wall measuring 8 ft. x 8 ft. is placed within a large frame following its construction. A horizontal (lateral) force is then applied at one upper corner of the wall while the base remains secured to the frame. The force is gradually increased in constant 400 lb. increments until the wall structure fails, thus assessing its overall structural strength.

Walls insulated with spray-applied polyurethane foam were compared against those without insulation in a series of racking strength tests. These tests evaluated two types of exterior facing materials: plywood textured siding and 15-lb. building paper topped with vinyl siding. The interior side of all wall panels utilized 16-inch stud spacing and was finished with half-inch sheetrock. The stud cavities in the walls filled with spray-applied polyurethane foam contained foam with a density of 1.5 lb/ft^3. 

The stud walls filled with spray-applied polyurethane foam significantly enhanced the structural strength of the home, as indicated by the test results. Additionally, these foam-filled walls exhibited less deformation under each applied load, providing greater resilience.

In a second series of racking strength tests, spray-applied polyurethane foam insulation was compared with traditional R-19 fiberglass batts. One comparison involved wall panels faced with drywall on both sides, while another used drywall on one side and oriented strand board (OSB) on the opposite side. The average foam density in this test was 2.26 lb/ft3, and the wall panels employed steel studs spaced 24 inches on center in both configurations.

Once again, the test results demonstrate that the spray foam insulated wall system offers higher structural strength.

These findings are documented in the report titled “Testing and Adoption of Spray Polyurethane Foam for Wood Frame Building Construction” (May 25, 1992) prepared by the NAHB Research Center for The Society of the Plastics Industry/Polyurethane Foam Contractors Division. Additionally, results were presented 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).