Overview
The weather-resistive barrier, the WRB, is the layer behind your cladding that does the unglamorous work of keeping water out of the wall. Siding, brick, and stucco all leak. Wind drives rain past them, and the WRB is what catches that water and runs it back out. Think of it as a drainage plane: a continuous surface, lapped so water always flows down and out, never in. Code requires a WRB behind most claddings for exactly this reason, and inspectors check for it.
Here is where people get confused. The WRB sheds bulk liquid water while still letting water vapor pass through, so the wall can dry. That dual job is the whole point. It is not the air barrier, and it is not the vapor barrier, though some products are detailed to do double or triple duty. A taped housewrap can serve as an air barrier if every seam is sealed. A polyethylene sheet is a vapor barrier, but a lousy WRB. Know which job each layer is doing in your assembly, because mixing them up is how walls rot from the inside.
The Types: Housewrap, Felt, and Integrated Sheathing
Four families cover most jobs. Mechanically-attached housewrap is the spun-bonded polyolefin most crews reach for, Tyvek being the name everyone uses generically. It rolls out wide, goes up fast, and resists tearing. Asphalt felt and building paper, the #15 and #30 black rolls, are the old standby. Felt is cheap, forgiving, and self-seals around fasteners better than plastic wrap does, but it is heavier to hang and can wrinkle when it gets wet.
Liquid-applied WRB is the newer player: you roll or spray it on like paint, and it bonds to the sheathing as a single continuous membrane with no laps to reverse. It shines on complex framing, but it costs more in product and labor and demands dry weather to cure. Integrated, taped sheathing systems, ZIP System being the common one, build the WRB right into the panel face; you just tape the seams. Fast and tidy, but the whole system lives or dies on the tape job and on hitting the right nailing. Each trades speed against detailing and cost. Match the product to the crew that will install it.
Permeability and the Drainage Plane
Perm rating is the number that matters most and the one most crews ignore. It measures how much water vapor passes through the material. A WRB needs to stop liquid water while letting vapor through, so a wet wall can dry to the outside. Most housewraps run somewhere in the high-perm range, which is what you want on a standard wood-framed wall. Go too low on perm and you trap moisture against the sheathing; that is a vapor barrier in the wrong spot, and it rots the wall it was supposed to protect.
Drainage is the other half. Reservoir claddings, stucco, brick, and manufactured stone, soak up water and hold it like a sponge, then release it inward against the WRB. A flat sheet of wrap pressed tight to that wet mass cannot drain. You need a gap: either a drainable wrap with a built-in textured or crinkled surface, or a separate rainscreen furring strip that holds the cladding off the wall. That air space lets water fall straight down and out the weep holes at the bottom. No gap behind a reservoir cladding is a rot clock you have already started. The building-science world has documented this for decades; Building Science Corporation has the research if you want the deep version.
Installation: Laps, Tape, and Flashing
Water runs downhill, so you install from the bottom up and lap every upper course over the lower one, weather-board fashion. Vertical seams overlap six inches or so, horizontal laps a couple of inches, upper sheet always on the outside. Get that backward anywhere and you have built a funnel that pours water into the wall. Tape the seams if the product calls for it, and use the manufacturer's tape; generic tape lets go in the heat and your air barrier opens up.
The window and door openings are where the real work is. The WRB and the flashing have to shingle-lap together in a specific order: sill flashing first, lapped over the WRB at the bottom; then the window; then jamb flashing; then head flashing tucked up under the WRB above so water sheds down and over, never behind. That sequence is non-negotiable and it is where most leaks are born. Fasten the field of the wrap with cap nails or cap staples, plenty of them, so wind does not balloon it off the wall before the siding goes on. Plan the order of operations before you unroll the first sheet.
Common Mistakes
Reverse-lapping leads the list. A crew runs out of wrap, starts a new course at the bottom, and laps the lower sheet over the upper one. Now every horizontal seam catches water and steers it into the sheathing. It looks fine from the curb and it is invisible once the siding goes on, which is why it keeps happening.
Not integrating the window flashing with the WRB is the expensive one. If the flashing tucks behind the wrap at the head but the wrap is not lapped over it at the sides, water finds the gap and runs into the rough opening. Then there is skipping the drainage gap behind stucco or stone, which I covered above and which guarantees trapped moisture. Leaving wrap exposed to UV too long is another: most housewraps have a stated exposure limit, and crews blow past it when siding gets delayed. Sun degrades the material until it tears and loses its water resistance. Last, using the wrong perm rating for the climate and assembly, a low-perm wrap on a wall that needs to dry outward, sets up rot you will not see until it is structural.
WRB vs Air Barrier vs Vapor Barrier
A wall has three separate control layers, and they do three different jobs. The water control layer is the WRB, the drainage plane I keep talking about. The air control layer stops air from leaking through the assembly, dragging heat and moisture with it. The vapor control layer slows how fast water vapor diffuses through the wall. People treat these as one thing because some products handle two or three at once. A taped housewrap controls water and air. A taped ZIP panel can do all three depending on the panel. That does not mean the jobs are the same.
Here is where crews get hurt. The vapor retarder belongs on the warm-in-winter side of the wall, which in a cold climate is the interior. Staple poly to the inside in Minnesota and you slow vapor before it hits cold sheathing. Do that same thing in a hot, humid climate and you trap moisture against the drywall every summer. And never put poly on the exterior where it doubles as a low-perm WRB; now the wall cannot dry outward and the sheathing rots behind siding that looks perfect. Know which layer each product is, and put it where the climate says it goes.
Flashing, Penetrations, and the Tricky Spots
Windows get all the attention, but the WRB leaks at a dozen other spots crews wave past. Every pipe, every electrical penetration, every dryer vent and hose bib punches a hole through the drainage plane. Each one needs a gasket or a bead of compatible sealant tied back into the wrap so water sheds around it, not through it. Skip that and you have drilled a funnel straight to the sheathing.
Then there is kickout flashing, the single most-missed piece of metal on the whole house. Where a roof eave dies into a sidewall, water sheets down the roof, hits the wall, and needs a kicked-out diverter to throw it into the gutter. No kickout means that water dumps behind the siding, and that exact spot rots more walls than anything else I see. Deck ledgers need their own flashing lapped under the WRB above and over it below, or the bolts wick water into the band joist. Same shingle logic governs every transition: WRB to roof underlayment, WRB to foundation, WRB to a roof-wall step. Lap upper over lower at each one, in order, so water always rides down and out.
FAQ
Is housewrap the same as a vapor barrier? No. A WRB sheds liquid water but lets vapor pass so the wall can dry. A vapor barrier blocks vapor. Confuse the two and you trap moisture in the wall and rot the sheathing.
Do I need a WRB if my sheathing is taped ZIP System? No separate wrap needed; the WRB is built into the panel face. But the whole system depends on taping every seam correctly and flashing the openings, so the detailing still matters as much as it would with wrap.
Why do I need a drainage gap behind stucco or stone? Those reservoir claddings soak up water and release it inward against the WRB. A flat sheet pressed tight cannot drain. A drainable wrap or rainscreen furring strip gives water a path down and out the weeps.
How long can housewrap stay exposed before siding goes on? Check the product's stated UV exposure limit, usually a matter of months, not open-ended. Sunlight degrades the material until it tears and stops resisting water. If siding is delayed past the limit, the wrap may need replacing.
Which side of the wall does the vapor barrier go on? The warm-in-winter side. In cold climates that is the interior, behind the drywall. In hot, humid climates, an interior poly sheet traps summer moisture, so you want a vapor-open assembly instead. Match the vapor retarder to your climate zone, and never confuse it with the exterior WRB.
What is kickout flashing and why does it matter? It is the diverter where a roof eave meets a sidewall, kicking roof runoff out into the gutter instead of behind the siding. It is the most commonly skipped flashing on a house, and missing it rots that wall section faster than almost any other defect. Add one at every roof-to-wall intersection.