Building science goes ballistic

Does a house of foam stand up to flying projectiles better than a house of wood? Yes, but not as well as a house of laminated glass, according to recent research from Building Science Corporation in Westford, Mass. Foam sheathing can boost a house’s energy performance, but many builders worry about foam’s ability to resist projectiles during storms. A team of engineers tested the impact resistance of walls built with foam sheathing in place of plywood or oriented strand board (OSB). To test a foam-sheathed wall’s impact resistance, the team from Building Science shot 2x4s out of an air cannon at different wall assemblies using the standard impact test for hurricane glass: the ability to withstand an 8-ft. 2x4 shot with a muzzle velocity of 34 mph, or 50 ft. per second (fps).

The engineers used four wall systems, all framed with 2x6s at 24 in. on center and clad with fiber-cement siding over housewrap. The standard wall had 7/16-in. OSB sheathing; the others used OSB or rigid-foam sheathing and spray foam in different combinations.

The best-scoring wall (1-in. foam sheathing outside and 2 in. of closed-cell spray foam between the studs) withstood a 2x4 at 43 fps—that is, the 2x4 broke through the fibercement siding, but bounced off without penetrating the foam sheathing. The standard wall with 7/16-in. OSB couldn’t withstand the force of the flying 2x4.

But neither wall met the hurricane-resistant window standard. To meet the 50-fps window standard, the team sheathed the wall with both OSB and foam, adding 2 in. of closed-cell spray foam between the studs.

Does this mean that in a hurricane, you’re safer standing in front of a window rather than a wall? “No,” says Peter Baker, an engineer with Building Science, “because walls do not shatter.” For more on this test, as well as videos, go to www.buildingscience.com/impact.