WINDOWS + FLASHING

How Proper Window Flashing Protects Your Home From Hidden Moisture Damage

What Homeowners Usually Think

“If the window isn’t leaking inside, it must be fine.”

What Actually Happens

Most window leaks never show up on the interior.
They leak into the wall first, soaking the sheathing, insulation, and framing long before water appears inside the house.
In the Pacific Northwest—one of the wettest climates in the country—incorrect flashing is a major cause of structural rot, mold growth, and siding failures.

A window can look perfect and still be leaking behind the scenes.

Why Windows Fail in the PNW

1. Poor or Missing Flashing

Many older homes have:

  • no head flashing

  • no sill pan

  • paper or felt that stops at the window instead of lapping over it

  • siding run tight to the window with no drainage path

This traps water behind the trim and forces it into the wall assembly.

2. Incorrect “Contractor’s Caulk Everything” Approach

Caulk is not flashing.
When caulk is used as water management, it eventually cracks or separates and water is driven behind it.

3. Solar Vapor Drive from Wet Claddings

If siding (especially stucco, brick, or fiber cement) gets wet and is heated by the sun, vapor is pushed inward — often right through the window opening if the flashing isn’t perfect.

4. Nailing Flanges Not Integrated With WRB

If the window's nailing flange sits on top of the WRB (weather-resistant barrier) instead of integrated behind it, water flows directly into the framing.

5. No Rainscreen Gap

Without a drainage gap, water driven behind siding has nowhere to go except inward.

6. Poorly Installed Replacement Windows

Insert windows (“retrofit” windows) often bypass proper flashing entirely.
They rely on caulk and trim — and fail quickly.

Signs of Window-Related Moisture Damage

Most signs appear long after the damage has started:

  • Staining or bubbling around the interior trim

  • Stiff or swollen window sills

  • Drafts or cold areas around the window

  • Musty smell near a wall

  • Exterior paint peeling around windows

  • Soft siding or trim

  • Water droplets inside during rain

  • Mold around baseboards nearby

Even subtle symptoms can indicate major hidden rot.

Best Practices for Window Flashing in the PNW

1. Sill Pan First (Always)

A sill pan catches water and drains it outward.
Best practice includes:

  • a positive slope

  • back dam

  • end dams

  • fully sealed corners

  • continuous drainage to exterior

This is the foundation of a long-lasting installation.

2. WRB Integration

The weather barrier must be installed in a proper shingle-lap pattern:

  • WRB over the flange at the top

  • WRB behind the flange at the sides

  • WRB lapped over the head flashing

If water hits the WRB, it should drain to the outside of the window—not behind it.

3. Flexible Flashing Membranes

High-quality flashing tape or liquid-applied membranes should:

  • wrap the sill

  • seal the flanges

  • integrate with the WRB

  • remain flexible over time

Cheap tapes often fail in cold or damp conditions.

4. Head Flashing (Drip Cap)

A metal head flashing diverts water away from the top of the window.
Without it, water will run behind the siding and directly into the opening.

5. Rainscreen Assembly

A rainscreen gap behind siding:

  • provides drainage

  • reduces solar vapor drive

  • dramatically improves durability

This is especially important with fiber cement, cedar, brick, and stucco.

6. Correct Siding Termination

Siding should never be hard up against the window frame.
A small, uniform gap allows the flashing system to function.

How Emerald State Contracting Fixes Window Flashing

1. Diagnose the Existing Assembly

We identify whether failures are due to the WRB, head flashing, sill pan, nailing flange, or siding details.

2. Remove Trim and Siding Strategically

We expose only what’s needed to correct the flashing and identify hidden damage.

3. Rebuild the Flashing Layer

We install:

  • fully wrapped sill pans

  • sealed corners

  • continuous flexible flashing

  • proper WRB integration

  • metal head flashing with correct slope

4. Reinstall or Replace the Window (If Needed)

If the window or flange is damaged, we replace it with a modern unit and integrate it correctly.

5. Rebuild the Rainscreen System

We reinstall trim and siding with proper clearances, drainage gaps, and weather management.

6. Repair Rot and Structural Damage

If framing, sheathing, or trim has been compromised, we replace or reinforce it using durable, code-compliant materials.

Why This Matters

Proper window flashing protects your:

  • wall assemblies

  • insulation

  • indoor air quality

  • structural framing

  • siding lifespan

  • energy efficiency

A well-flashed window lasts decades.
A poorly flashed window may fail in just a few years—often completely unnoticed.

When to Call a Professional

You should have your windows evaluated if:

  • You plan to replace siding

  • Your windows are older than 20 years

  • You see staining or peeling paint near windows

  • You’ve dealt with interior mold

  • You feel drafts around openings

  • The home has fiber cement, brick, or stucco siding

  • You experienced leaks during wind-driven rain