Cold spots and streaking

Tests show that foam sheathing reduces the potential for dust streaking on the inside surface of walls in northern climates. On the test wall with only plywood sheathing, the gypsum wallboard temperature remained constant over the insulated cavity. However, it began dropping rapidly about 2" from the edge of the stud. Directly over the stud the gypsum wallboard temperature was 10°F lower than the temperature over the cavity. The wall with 2" of XPS sheathing showed only a 2.5°F temperature drop from cav­ity to stud. These cold spots sometimes promote condensation on the inside wall surface. Dust attracted to the water causes streaks, sometimes called ghosting.8

The temperature of the interior wall surface plays a major role in this process. The report cited previous research indicating that slight ghosting can be expected when the wall temperature over the stud is 3.3°F lower than the temperature over the cavity, and that severe streaking occurs when the temperature is 8°F lower. In this test, walls with plywood sheathing showed 8 to 10° F lower temperatures over the studs, making them likely to develop streaks. The report states that no ghosting problems would be expected in any of the walls with insulated sheathing. However, walls with Iм of XPS showed a temper­ature drop of about 4 to 5°F. If a 3.3°F temperature difference is the magic number, then these walls may face some risk for streaking.8