The short answer
A liquid applied weather barrier, also called a fluid applied weather resistive barrier, is a coating sprayed or rolled onto exterior wall sheathing. It cures into a continuous membrane that keeps bulk water out of the wall and, depending on the product, helps control air leakage.
How it differs from housewrap
Housewrap is a sheet product stapled or nailed to the sheathing, with taped seams and laps. Every seam and fastener is a potential leak. A fluid applied barrier has no seams and no fasteners, so it forms one continuous surface across the wall.
Vapor permeability
Walls need to dry. Many fluid applied barriers are vapor permeable, letting moisture vapor escape while blocking liquid water. Others are designed to slow vapor more. The right choice depends on the climate zone and the wall assembly, which is why product selection should match the design.
Detailing openings
Windows, doors, and penetrations are where walls fail. Fluid applied systems include compatible flashing products for rough openings and transitions, so the whole envelope works as one system.
Where it is used
Fluid applied barriers are used on custom homes, multifamily, and light commercial buildings, especially where builders are chasing tighter air leakage numbers or want a more durable barrier during construction.
Related service
Liquid Applied Weather Barrier