Arkansas HVACR NewsMagazine March 2026

HVACR NewsMagazine March 2026

Tech News

Vapor Pressure and Permeability

Open-Cell Spray Foam and the Ping Pong Effect In many cases with unvented attics, we’ll see open-cell spray foam applied directly to the roof sheathing. This spray foam is meant to serve as insulation, but its low density also means it is vapor-open — about 30 perms. Water vapor travels into and out of it easily, and that water makes its way to the wood based roof decking. During the day, the sun beats on the roof and raises the vapor pressure in the building materials. Raising the vapor pressure forces moisture OUT of the roof decking, through the spray foam, and into the attic. Since warm, wet air is less dense than dry air, the water vapor rises. At night, it’s a different story. The temperatures drop at night, and the water vapor returns to the building materials, where it is stored. The next day, the process repeats, and the water vapor travels a little higher with each “ping” and “pong” between the roof material and attic air. That moisture accumulates at the attic ridge over time. When moisture can’t exit the attic ridge and just passes back and forth between the building materials and the air, it can rot building materials that don’t dry very well. When Convection and Vapor Pressure Don’t Drive Air & Moisture Movement… We have yet to mention a significant pressure driver: wind. Wind helps move air and creates the pressure differentials needed to drive air out of the attic. Soffit vents take in air down low, and wind drives the motion that brings air to those.

On top of the density-driven movement, water vapor can pass through vapor-permeable building materials, even when air cannot. As a result, water vapor can get into the roof decking and other foundational materials based on their vapor permeability. From there, that water vapor responds to changes in vapor pressure. Vapor permeability is measured in units called perms . No, not the 80s hair — these perms indicate how well water vapor can pass through a material. Though if you want to get real nerdy, one perm is equal to one grain of water vapor per hour, per square foot, per inch of mercury (pressure). A building material with more perms allows more vapor to pass through (and is at least vapor-open, even if it doesn’t allow air to pass and is air -closed), and a building material or vapor retarder with fewer perms allows less to pass through. Plywood sheathing has a permeability of 10 perms under “wet - cup” conditions (which is considered semi-permeable, bordering on permeable), whereas foil-faced polyiso has a permeability of <0.1 perms (practically vapor impermeable). Oriented strand board (OSB), another common sheathing material, Plywood and OSB sheathing are at least semi permeable and will allow some water vapor to pass and stay in the material. OSB becomes more permeable when wet, too. The movement of water vapor is driven by pressure, following the high-to-low rule. When the roof decking absorbs solar radiation and heats up, the vapor pressure increases. Since vapor moves from areas of high pressure to low pressure, some of that water vapor in the high-pressure roof decking will be released to the lower-pressure surrounding air and into the upper areas of the attic. Ideally, it would exit via a ridge vent at the top, but that’s not always the case.