Quick answer
A weather-resistant barrier (WRB) is a continuous sheet or coating installed over the studs or sheathing of an exterior wall, behind the siding, that catches water getting past the cladding and drains it back outside before it can soak the wall.
The building code calls it a water-resistive barrier. Common forms are house wrap, asphalt felt, and fluid-applied or panel-integrated coatings. It works with flashing at every window, door and penetration.
Quick facts
The key facts about Weather-Resistant Barrier (WRB), with sources:
- Code name
- Water-resistive barrier, 2021 IRC Section R703.2 1
- Where it goes
- Over studs or sheathing, behind the exterior wall covering 1
- Accepted materials
- No. 15 felt (ASTM D226 Type 1), ASTM E2556 Type 1 or 2, ASTM E331 tested systems, other approved materials 1
- Lap rule (felt and E2556 wraps)
- Upper over lower at least 2 inches; joints at least 6 inches 1
- Main job
- Drainage plane: sheds water that gets past the siding 2
- Usual vapor behavior
- Vapor permeable, so the wall can dry outward 5
Key takeaways
- Siding is a rain screen, not a seal; the WRB behind it is the real line that keeps water out of the wall.
- The 2021 IRC requires at least one layer of water-resistive barrier on all exterior walls, continuous to the top of the wall.
- Laps run like shingles: upper over lower by at least 2 inches, with joints lapped at least 6 inches for felt and E2556 wraps.
- Most leaks happen at windows, doors and penetrations, so WRB work is only as good as its flashing.
- A WRB is not a vapor retarder; most wraps let vapor out so the wall can dry.
What a WRB is, without the jargon
Every type of siding lets some water through. Wind drives rain into laps, joints and nail holes. The WRB is the layer behind the siding that catches that water and lets gravity carry it down and out. Building Science Corporation calls this layer the drainage plane, a water-repellent surface in a drained wall 2.
You have seen it on houses under construction: the white printed wrap, the black felt paper, or the green or blue coating on some sheathing panels. Once the siding goes on, it is hidden for decades, which is why getting it right during a remodel matters.
The code term is "water-resistive barrier." "Weather-resistant barrier" and "house wrap" are trade names for the same idea. It is one layer of the building envelope, the full shell that separates inside from outside.
How a drained wall handles rain
Building Science Corporation describes rain control in three steps: deflect it, drain or store what gets in, and let the rest dry 2. The WRB is the drainage step.
- Deflect. Roof overhangs, sills and siding throw most rain off the wall. BSC calls overhangs and sills the first and most important step 2.
- Drain. Water that gets past the siding runs down the face of the WRB under gravity 2.
- Direct at openings. Flashing at window heads and sills turns that water outward; BSC says head flashing and sills should extend at least 2 inches past the jambs 2.
- Exit at the base. Flashing and weep paths at the bottom of the wall let water out 2.
- Dry. A vapor-open WRB and a small air gap let the wall dry outward after a wetting.
- Siding or claddingDeflects most rain
- Drainage gapSmall space where water runs down; can be a rainscreen
- Weather-resistant barrierThe drainage plane, lapped like shingles
- SheathingStructural panel the WRB is fastened over
- Studs and cavity insulationThe wall framing
- DrywallInterior finish and often the air barrier
Water that reaches the WRB drains down and out at flashings; the wall behind stays dry. Source: [2] [1]
The 2021 IRC writes this into law: the wall must prevent water building up inside it by providing a water-resistant barrier behind the cladding and a means of draining water that gets past the cladding to the exterior 1.
Types of weather-resistant barriers
| Type | What it is | Code basis |
|---|---|---|
| Asphalt felt | No. 15 felt paper, the traditional option | ASTM D226, Type 1 1 |
| House wrap (mechanically attached) | Vapor-permeable flexible sheet stapled or nailed on 5 | ASTM E2556, Type 1 or 2 1 |
| Tested wall system | A WRB system tested for water penetration | ASTM E331 per Section R703.1.1 1 |
| Fluid-applied WRB | A coating rolled or sprayed on the sheathing | Approved material per maker’s instructions 1 |
| Panel-integrated WRB | Sheathing with a factory-applied facing; seams taped | Approved material per maker’s instructions 1 |
WRB vs air barrier vs vapor retarder
A WRB sheds liquid water. An air barrier stops air flow. A vapor retarder slows vapor diffusion. Taped house wrap can help with air, and BSC notes wraps can cut airflow through an imperfect air barrier, but the main air barrier is often the drywall or sealed sheathing.
Example: lapping house wrap on a two-story wall
This example applies the 2021 IRC lap rule for No. 15 felt and ASTM E2556 wraps to a sample wall. The wall size and roll width are example numbers; the lap minimums are from Section R703.2 1. Always check the product’s own instructions, which may ask for more.
| Step | Figure | How it was worked out |
|---|---|---|
| Wall height to cover | 18 ft (216 in) | Example two-story wall |
| Roll width (example) | 9 ft (108 in) | Example product |
| Horizontal lap | At least 2 in, upper over lower | 2021 IRC R703.2 1 |
| Coverage per course after lap | 108 − 2 = 106 in | Each upper course loses 2 in to the lap |
| Courses needed | 216 ÷ 106 = 2.04, so 3 courses | Round up; a third short course is needed |
| Vertical end joints | Lapped at least 6 in | 2021 IRC R703.2 1 |
| Top of wall | WRB continuous to the top of the wall | 2021 IRC R703.2 1 |
- Bottom course of WRBStarts at the base of the wall
- Sill flashing at the windowSloped to drain outward
- Window set and side flashingPer the window maker’s instructions
- Upper WRB coursesEach lapped over the one below
- Head flashingTucked under the WRB above
- Tape seams and repair tearsBefore siding goes on
Shingle order means every layer above sits outside the layer below, so water always runs out, never in. Source: [2] [1]
Even a simple wall needs planning. The bottom course goes on first and every course above laps over it, so water always runs onto the face of the layer below and never behind it. Where a course meets a window, flashing must be woven into the same shingle order.
Where the WRB shows up on a remodel
On a siding or window quote. A good bid names the WRB product, says whether old wrap will be removed or patched, and lists flashing at every window, door and penetration. "Install new siding" alone is not enough.
On plans. For an addition, the wall section drawing should show the WRB, the flashing and how the new wrap ties into the old wall.
At inspection. Many jurisdictions inspect the WRB and flashing before siding covers them. The order of inspections is explained in our permits and approvals guide.
On the product. House wraps are printed with the brand and often the standard they meet. Photograph the wall before siding goes on so you have a record.
Photograph the wrap
Ask the crew to pause before siding starts so you can photograph each wall, especially around windows. Those photos settle many later disputes about leaks.
Benefits and limitations
What a WRB does
What it cannot do
- Cannot make up for missing or reversed flashing.
- Tears easily and degrades if left exposed too long; follow the maker’s exposure limit.
- Not a structural layer or an insulation layer.
- Not a vapor retarder for cold-climate interiors.
Limits worth knowing
A WRB only works as a system. BSC warns that windows, mulled joints and window corners often leak into the wall, and recommends sub-sill flashing because leaks there are close to certain 2. A perfect wrap with a poorly flashed window still lets water in.
Water also needs a way out. BSC notes water can drain through gaps as small as the thickness of a dime, but a capillary break works best around 1/4 inch, with about 3/8 inch often quoted as a practical minimum air space 2. Some claddings and climates call for a dedicated rainscreen gap.
Finally, a WRB is not meant to be a perfect barrier wall. BSC describes face-sealed "perfect barrier" joints made of sealant as having a very high chance of failure 2. Drainage is the safer strategy.
What drives the cost of WRB work
We do not publish prices for wraps or labor. The main cost drivers are the amount of wall, the number of openings to flash, the product type (felt, wrap, fluid-applied or integrated panel), access height and whether old siding and wrap must come off.
Taping seams, adding a rainscreen gap and replacing damaged sheathing found under old siding all add cost.
The biggest cost risk is hidden damage. When siding comes off an older house, rot behind it can turn a siding job into a sheathing job. Hold a contingency for it, as our remodeling cost guide explains.
A WRB is meant to last as long as the cladding over it. Its life depends on staying covered and dry. Most products set a maximum time they can be left exposed to sun before siding goes on; ask for that limit and keep the schedule to it.
How WRB work fits into a re-siding or addition project
This is the general sequence for a contractor-led job, usually under a permit.
- Remove old siding and inspect the sheathing for rot or mold; replace damaged panels.
- Install the WRB from the bottom up, lapping upper over lower at least 2 inches and joints at least 6 inches where the IRC rule applies 1.
- Flash windows and doors in shingle order: sill first, then sides, then head under the wrap above 2.
- Seal or flash every penetration: hose bibs, vents, wires and light boxes.
- Carry the WRB continuously to the top of the wall and tie it into roof and foundation flashing 1.
- Repair tears and tape seams per the maker.
- Pass any required inspection, photograph the walls, then install siding.
Protecting the WRB once it is covered
You cannot see the WRB after siding is on, but you can protect it.
- Keep siding, trim and caulk joints in repair so less water reaches the WRB.
- Keep gutters clean and downspouts directed away from walls.
- Patch the WRB whenever siding is removed for repairs or new fixtures.
- Flash new penetrations for vents, lights and hose bibs rather than relying on caulk alone.
- Keep soil and mulch below the siding so the base of the wall can drain.
Warning signs of a WRB or flashing failure
- Stains, peeling paint or soft drywall under windows or at corners inside.
- Swollen or rotting trim and window sills outside.
- Buckling siding or siding that stays damp long after rain.
- Musty smells in rooms along an exterior wall.
- Mold on the back of baseboards or inside closets on outside walls.
Ladders, lead paint and rot
Siding work means ladders, scaffolds and work at height. In pre-1978 homes, old exterior paint may contain lead, and paid work that disturbs it must be done by an EPA RRP-certified firm 7. Rot found behind siding can reach structural framing; have it assessed before closing the wall.
EPA advises fixing the water source and drying water-damaged areas within 24 to 48 hours to prevent mold 6. Call a siding or building envelope contractor to open the wall and trace the leak; for soft or damaged framing, involve a licensed engineer or contractor before repairs.
What the 2021 IRC requires
Weather-resistant envelope. 2021 IRC Section R703.1.1 says the exterior wall envelope shall prevent water from accumulating within the wall assembly by providing a water-resistant barrier behind the cladding, as required by R703.2, and a means of draining water that penetrates the cladding to the exterior.
Exceptions cover concrete or masonry walls flashed per the code and wall systems proven by testing 1.
The barrier itself. Section R703.2 requires not fewer than one layer of water-resistive barrier over the studs or sheathing of all exterior walls, with flashing per Section R703.4, continuous behind the veneer and to the top of walls, and terminated at penetrations and appendages to meet R703.1.
Materials must be No. 15 felt (ASTM D226 Type 1), ASTM E2556 Type 1 or 2, ASTM E331 tested per R703.1.1, or other approved materials per the maker 1.
Laps. No. 15 felt and ASTM E2556 barriers are applied horizontally, upper over lower at least 2 inches, with joints lapped at least 6 inches 1.
Local adoption. States and cities adopt and amend the IRC on their own schedule, and some add rules for stucco or high-wind areas. Check yours with the building department or our permit checker.
WRB vs. related layers and terms
| Term | What it controls | Main difference |
|---|---|---|
| Weather-resistant barrier | Liquid water behind cladding 1 | The drainage plane |
| Window flashing | Water at window openings 2 | Directs water at openings; must lap into the WRB |
| Siding rainscreen | A drainage and drying gap behind siding 2 | Adds a clear space in front of the WRB |
| Vapor retarder | Vapor diffusion, usually on the inside 4 | Slows vapor, not liquid water |
| Air barrier | Air flow through the enclosure 3 | Can be drywall, sheathing or taped wrap |
Myths about house wrap
- Myth Good siding does not need a WRB behind it.
- Reality The 2021 IRC requires at least one layer behind the veneer on all exterior walls 1, because claddings let some water through 2.
- Myth House wrap is a vapor barrier.
- Reality ASTM E2556 covers vapor-permeable sheets 5; most wraps let the wall dry outward.
- Myth Caulk can replace flashing.
- Reality BSC says face-sealed sealant joints have a very high chance of failure 2.
- Myth Tape every seam and the wrap becomes the air barrier.
- Reality It may help, but BSC notes the air barrier system includes every joint and penetration 3; design it on purpose.
- Myth Any direction of lap is fine.
- Reality Upper layers must lap over lower ones so water runs outward 1.
How climate and rain exposure change the details
Rain exposure, not just climate zone, drives WRB detailing. BSC notes that most parts of the world see significant wind-driven rain and that exposed sites demand better rain control 2, which is why wet, windy coasts often add a rainscreen gap.
Climate also shapes vapor choices: BSC says an exterior vapor barrier is usually not acceptable in cold climates 3, so cold-climate walls rely on vapor-open wraps.
Some jurisdictions amend IRC Chapter 7, for example adding stucco or high-wind rules, so confirm local requirements.
When the WRB should be on your checklist
Put it on your list any time siding comes off, windows or doors are replaced, or a new exterior wall is built. That is the one chance to inspect, repair and properly lap the layer, and to tie it into new flashing.
It matters less for interior-only remodels, though a kitchen remodel that moves a window or adds a range hood vent still cuts through the wall and needs flashing.
Your next step: ask each siding or window bidder which WRB they use, how they flash openings, and whether they will leave the wall open for photos or inspection before cladding.
- If Re-siding the house
- Then Yes; inspect, repair or replace the WRB
- If Replacing or adding windows or doors
- Then Yes; new flashing must lap into the WRB
- If New vent, light or hose bib through an outside wall
- Then Yes; flash the penetration
- If Interior-only remodel
- Then Usually no
Questions about Weather-Resistant Barrier (WRB)
Is a weather-resistant barrier the same as house wrap?
House wrap is the most common type of weather-resistant barrier. The code term is water-resistive barrier, and it also covers No. 15 asphalt felt, fluid-applied coatings and sheathing panels with a built-in facing. All of them do the same job: shed water that gets past the siding.
Is house wrap required by code?
Yes, in places that adopted the 2021 IRC, some water-resistive barrier is. Section R703.2 requires at least one layer over the studs or sheathing of all exterior walls, continuous behind the veneer and to the top of the wall. It does not have to be house wrap; felt or other approved materials qualify.
How much should house wrap overlap?
For No. 15 felt and ASTM E2556 wraps, the 2021 IRC requires horizontal laps of at least 2 inches, upper over lower, and at least 6 inches where joints occur. Many product instructions ask for more. Follow the larger of the two.
Is house wrap a vapor barrier?
No. House wraps covered by ASTM E2556 are vapor permeable, so water vapor can pass through and the wall can dry outward. A vapor retarder is a different layer, usually on the inside of cold-climate walls, that slows vapor coming from indoor air.
Which is better, felt paper or house wrap?
Both are accepted by the 2021 IRC when they meet the named standards: No. 15 felt to ASTM D226 Type 1, or house wrap to ASTM E2556. The choice depends on the cladding, climate and the installer’s system. What matters most is continuous coverage, correct laps and proper flashing.
Do I need to replace the WRB when I replace siding?
Often yes, and it is the only easy chance to do it. Once old siding is off, inspect the wrap and sheathing. Torn, brittle or missing wrap should be replaced, and any rot found behind it should be repaired first. Ask your siding bidder to include this in the scope.
What is a drainage plane?
A drainage plane is a water-repellent layer behind the cladding that lets water run down and out of the wall. Building Science Corporation names it as one of the parts every drained wall needs, along with a drainage gap, flashing at the base and a way for water to exit.
How long can house wrap be left exposed?
It depends on the product. Most makers set a limit on how long their wrap may be exposed to sunlight before cladding covers it. Ask for that number and plan the siding schedule around it, since sun can weaken wraps left uncovered too long.
Can taped house wrap be the air barrier?
It can contribute. Building Science Corporation notes house wraps can significantly reduce airflow through an imperfect primary air barrier. A true air barrier system includes every joint, seam and penetration, so if the wrap is meant to be the air barrier, the design should say so and the details must be sealed.
Sources
- ICC, 2021 International Residential Code, Chapter 7 Wall Covering (R703.1.1, R703.2), retrieved .
- Building Science Corporation, BSD-013: Rain Control in Buildings, retrieved .
- Building Science Corporation, BSD-014: Air Flow Control in Buildings, retrieved .
- Building Science Corporation, BSD-106: Understanding Vapor Barriers, retrieved .
- ASTM International, E2556/E2556M Standard Specification for Vapor Permeable Flexible Sheet Water-Resistive Barriers Intended for Mechanical Attachment, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- U.S. EPA, Lead Renovation, Repair and Painting Program, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Code requirements, accepted materials and laps were read from 2021 IRC Sections R703.1.1 and R703.2 on ICC Digital Codes.
Rain-control principles, drainage-gap widths and flashing guidance come from Building Science Corporation digests BSD-013, BSD-014 and BSD-106. The ASTM E2556 title was checked on astm.org.
The lap example is our own arithmetic on example wall and roll sizes, labelled as such. No prices are given.
Suggest a correction. We fix errors and say what changed.