Siding alone does not keep a wall dry. Rain will get past panels at joints and openings, so every sound wall needs a second line: a barrier that drains, plus flashing that directs water out at openings and roof junctions. When those layers are lapped correctly, small leaks drain harmlessly. When reversed or torn, water reaches framing.
Request a free quote using the quote button on this page. For a full renewal, read siding replacement. For a localised leak, read siding repair and partial replacement.
The outer skin sheds most rain, not all of it
Think of siding as a rain screen. It sheds most water by gravity, but it is not a sealed tank. The U.S. Department of Energy explains that most exterior wall cladding systems leak at some point because wind pressure and capillary action can drive rain through cracks, joints, and small gaps, and that sealing every hole is almost impossible.
That changes what good work means. A provider should show how water that gets in will get back out: where the drainage plane is, where it ends, and how it passes each opening.
What the barrier layer is meant to do
House wrap is one common form of water resistive barrier. Other forms include building felt, rigid foam with taped joints, and coated sheathing. Whatever the material, the job is the same. The U.S. Department of Energy guide to the weather resistive barrier describes a drainage approach in which water that leaks through cladding runs onto a water resistant surface, such as house wrap or rigid foam, and drains down and out.
A barrier also helps with air movement when seams are sealed, but its first task is liquid water. In a replacement, the whole barrier can be renewed as one system. In a repair, the new section must tie in with correct laps.
Laps and tape decide whether water drains or enters
Order is everything. Upper layers must lap over lower layers, in shingle style, so water steps outward at each overlap instead of stepping in. A reversed lap feeds water behind the barrier.
Department of Energy guidance on water managed walls notes that house wrap should sit behind the cladding, lap over flashing around openings, and be taped at seams without open gaps it calls fish mouths. Tears and untaped seams break the plane.
Check the base too. The barrier should carry water out over the foundation edge or base flashing, not end behind soil where water can pool.
Why openings need their own flashing plan
Windows and doors interrupt the barrier, so they need a sequence, not a single strip. Sill, sides, then head, each layer lapping to shed water outward. The sill is the most vulnerable point.
Guidance from the U.S. Department of Energy on fully flashed window and door openings describes flashing the sill with a back dam and self adhesive flashing, and using formable flashing that bends at corners so one piece can cover the bottom and sides. A back dam is a raised edge at the inside of the sill that stops water running into the room.
Head flashing catches water running down the barrier above and throws it out over the trim. If the barrier tucks behind it, water is guided in. Ask for photos during fitting, since this is hidden once trim is on.
Junctions where roofs, decks, and pipes meet walls
Every penetration is a small opening problem. The barrier guidance above also notes that holes for spigots, vents, HVAC line sets, and wiring should be thoroughly sealed or gasketed.
Roof to wall junctions carry more water than a plain wall. Step flashing and a kickout piece direct flow into a gutter instead of behind the siding. A missing kickout is a common hidden cause of rot beside a roof. Deck ledgers must also be flashed.
Clearance, grade, and room to dry
Moisture control starts away from the wall. Virginia Tech Extension guidance on preventing moisture problems in construction includes keeping clearance between cladding and finished grade, sloping grade away from the building, priming siding and trim on all sides, and sending downspout discharge away from the house.
The same guidance stresses preventing liquid water moving through components and vapor being trapped in wall assemblies. A wall should be able to dry to at least one side. Keep soil below the siding, and hold plants away from the wall.
Clues that the hidden system is not working
Some clues are direct: torn wrap at a base, or flashing that ends short of a corner. Others are indirect. Paint that blisters low on a wall, trim that softens at window corners, or siding that stays dark after other walls dry can point to water held behind the face.
None of these clues proves a fault alone. Gutters and roofs can produce similar marks. A site check traces the water path first.
Cost factors are driven by openings, existing flashing condition, roof access, and sheathing repair found once walls open. Ask which barrier will be used and how each sill will be flashed.
You can see the areas we cover on the areas hub. Good moisture work is order and patience, and it decides how long siding lasts.
Frequently asked questions
What does house wrap do behind siding?
House wrap, also called a water resistive barrier, is the drainage layer behind siding. It sheds rain that gets past the outer panels and guides it down and out of the wall. It also helps limit air movement through cracks when seams are taped. It is not a vapor seal, and it works only when laps face the right way.
Can caulk replace flashing around a window?
No. Caulk is a surface seal that ages, cracks, and needs renewal. Flashing is a shaped layer that directs water out by gravity, even when a seal fails. Windows, doors, and roof junctions need flashing first, with caulk as a small finishing detail. A wall that relies on caulk alone will need frequent checks and repairs.
How can I tell if flashing or wrap has failed?
Look for stains under windows, soft trim, peeling paint near junctions, and damp or musty areas inside after rain. Outside, check for reversed laps, torn wrap, unsealed seams, and flashing that tucks into the wall instead of out over the layer below. Some faults stay hidden until siding is opened, so a site check matters.
Should a weather barrier seal the wall completely tight?
No. It should drain and dry. A correct barrier lets liquid water run down its face and exit at the base, while still allowing the wall assembly to dry over time. Trapping water between tight, impermeable layers raises decay risk. The goal is a continuous drainage plane with clear laps, not a sealed bag around the frame.
Sources
- U.S. Department of Energy, Water Managed Walls
- U.S. Department of Energy, Water Managed Walls: Weather Resistive Barrier
- U.S. Department of Energy, Fully Flashed Window and Door Openings: Flashing Details
- Virginia Tech Extension, Preventing Moisture Problems in New Construction
Common questions
What does house wrap do behind siding?
House wrap, also called a water resistive barrier, is the drainage layer behind siding. It sheds rain that gets past the outer panels and guides it down and out of the wall. It also helps limit air movement through cracks when seams are taped. It is not a vapor seal, and it works only when laps face the right way.
Can caulk replace flashing around a window?
No. Caulk is a surface seal that ages, cracks, and needs renewal. Flashing is a shaped layer that directs water out by gravity, even when a seal fails. Windows, doors, and roof junctions need flashing first, with caulk as a small finishing detail. A wall that relies on caulk alone will need frequent checks and repairs.
How can I tell if flashing or wrap has failed?
Look for stains under windows, soft trim, peeling paint near junctions, and damp or musty areas inside after rain. Outside, check for reversed laps, torn wrap, unsealed seams, and flashing that tucks into the wall instead of out over the layer below. Some faults stay hidden until siding is opened, so a site check matters.
Should a weather barrier seal the wall completely tight?
No. It should drain and dry. A correct barrier lets liquid water run down its face and exit at the base, while still allowing the wall assembly to dry over time. Trapping water between tight, impermeable layers raises decay risk. The goal is a continuous drainage plane with clear laps, not a sealed bag around the frame.