For flash-and-batt insulation to perform properly and to reduce the chances of mold growth and structural damage, it's important to consider a few critical details. If you aren't willing to address each of these areas, then flash and batt isn't the system for you. FOAM THICKNESS There's more to the flash coat than just air-sealing; thickness is important, too. The flash layer must be thick enough so that the inside surface of the foam stays above the dew point (the temperature at which water vapor condenses) except for short periods of time, such as during an extremely cold spell, which the wall assembly can accommodate. According to building scientist Joseph Lstiburek, flash-and-batt installations require at least 1 in. of spray foam in climate zone 5, 1 1/2 in. in zone 6, and 2 in. in zone 7.
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