The building codes refer to it as a water-resistive barrier, and building scientists call it the drainage plane. In reality, it’s a layer of asphalt felt paper or housewrap, stapled to the wall sheathing shingle-style so that the upper pieces lap over the lower pieces, as each course of shingles overlaps the lower course.

Housewrap has two purposes: it blocks air infiltration and drains water that gets behind the exterior cladding, making the building durable.

No cladding sheds all the rain that hits it, and some claddings let a lot of water through. Stucco and brick are porous. Vinyl siding is loose fitting. Wood siding and trim shrink, opening up joints. None of these claddings will keep a house dry without a secondary water-resistive drainage plane material to back them up.

And yet, workers still get housewrap installation completely wrong: They reverse the lap so that water running down the paper face of the upper sheet flows behind the lower sheet and gets trapped between the paper and the sheathing. Over time, the result can be a catastrophe of rot, where sheathing and even studs are composted into crumbly brown fluff that’s good for growing plants. The time to catch this mistake is before it’s covered up because if left in place, it can be a time bomb.

Tradesmen working from the top down, or haphazardly, sometimes install housewrap so that the upper course tucks behind a lower course, directing rainwater “down and in” rather than “out and down.” Over time, trapped moisture can destroy a wall system.

Supervision and training during construction are the keys to getting housewrap installed correctly. Once the stucco, brick, vinyl, or wood siding is on, the problem is hidden from view—until it announces itself as a crop of fresh mushrooms. The time to catch this problem is before it’s covered up. Remove the housewrap, install it right, and welcome the opportunity to teach your crews the proper installation technique.

Spray-applied or roller-applied liquid air-barrier and drainage-plane membranes are the new alternatives for water-resistive barriers. They take skill to apply—you have to prep and tape joints and detail the flashing according to manufacturer’s instructions—but research shows these membranes do a great job of water protection and air sealing.