MOISTURE 101

How Water Moves Through a Home — and Why It Matters in the Pacific Northwest

What Homeowners Usually Think

“Moisture problems come from leaks.”

What Actually Happens

Leaks are only one part of the story.
In the Pacific Northwest, moisture sneaks into homes in four major ways, and most of them are silent, invisible, and ongoing.

Understanding those pathways is the foundation of keeping a home dry, healthy, and durable.

The Four Ways Moisture Damages a Home

1. Bulk Water (Liquid Water)

This is the obvious one — rain, roof leaks, plumbing leaks, bad siding, overflowing gutters, poor grading, etc.

When bulk water gets into a wall, crawlspace, basement, or roof assembly, it causes:

  • rot

  • mold

  • swelling of materials

  • structural damage

Bulk water = the fastest path to failure.

But it’s not the most common cause of hidden damage.

2. Vapor Diffusion (Invisible Moisture Through Materials)

Moisture can move straight through porous materials — concrete, drywall, wood, paint, and even some insulation.

Examples:

  • Moist soil drying upward through a slab

  • Damp concrete walls “breathing” moisture inside

  • Solar vapor drive pushing moisture inward from wet cladding

It’s slow, but persistent.
Left unmanaged, it loads wall and ceiling assemblies with moisture.

3. Capillary Action (“Wicking”)

This is water being pulled upward or sideways through tiny pores in building materials.

Think of:

  • water rising up a paper towel

  • soil moisture wicking up concrete

  • wet siding pulling water behind the WRB

  • deck or siding boards soaking water at joints

Capillary action moves water against gravity.

This is why:

  • foundation walls get wet without leaks

  • sill plates rot

  • siding edges fail

  • crawlspaces smell musty

Stopping capillary movement is essential in wet soils.

4. Air Leakage (Moisture Hitchhiking on Air)

This is the most overlooked moisture pathway — and often the most destructive.

Warm indoor air always moves upward (stack effect).
When that air carries moisture into a cold attic, wall cavity, or roof assembly, it condenses.

Examples:

  • Mold on attic sheathing even with “good ventilation”

  • Condensation inside walls

  • Wet insulation

  • Flat roof moisture buildup

  • Window condensation

Air moves far more moisture than vapor diffusion.
This is why air sealing is more important than adding vents.

Why Moisture Problems Are Worse in the Pacific Northwest

1. Long Wet Seasons

Homes stay damp for months at a time — far longer than in dry states.

2. High Humidity + Cool Temperatures

Condensation risk is extremely high, especially in attics and small ADUs.

3. Shade + Vegetation

Slows exterior drying.
Shaded walls and ADUs stay wet longer.

4. Wood-Framed Construction

Wood stores moisture easily.
Slow drying = rot.

5. Modern Tight Homes Without Proper Ventilation

Air-tight construction without proper ventilation traps humidity indoors.

Where Moisture Usually Shows Up First

These are the early-warning zones:

  • crawlspaces

  • attic sheathing

  • rim joists

  • basements

  • bottom of exterior walls

  • around windows

  • flat or low-slope roofs

  • behind fiber cement or stucco

These areas show the real story of a home's moisture health.

How Moisture Damages Homes Over Time

Stage 1 — Silent Moisture Loading

No visible signs.
Moisture accumulates in assemblies.

Stage 2 — Mild Symptoms

  • musty smells

  • cold floors

  • slight staining

  • window condensation

Most homeowners ignore this stage.

Stage 3 — Material Breakdowns

  • mold blooms

  • peeling paint

  • soft drywall

  • swollen trim

  • sagging insulation

Stage 4 — Structural Damage

  • rotted framing

  • compromised sheathing

  • damaged rim joists

  • failing deck ledgers

  • roof sheathing deterioration

Stage 5 — Indoor Air Quality Decline

Mold spores and damp building materials begin affecting health.

The Building Science Strategy for Moisture Control

To build a home that lasts in the PNW, you need four layers of defense:

1. Drain Water Away From the Building

  • gutters + downspouts

  • grading

  • drainage planes

  • rainscreens

  • flashing

2. Stop Water From Getting In

  • high-quality WRBs

  • correct window flashing

  • sealed roof penetrations

  • sound siding assemblies

3. Control Air Leakage

Air sealing is more important than adding insulation.
Warm, moist indoor air should never enter attics, walls, or roofs.

4. Control Indoor Humidity

  • bath fans

  • kitchen exhaust

  • HRV/ERV systems

  • dehumidification if needed

  • avoiding over-tight construction without ventilation

Moisture management = water control + air control + vapor control.

How Emerald State Contracting Uses Building Science

Every project — from a crawlspace to an ADU — starts with moisture logic:

1. Diagnose the Source

We identify whether the problem is:

  • bulk water

  • vapor diffusion

  • capillary action

  • air leakage

  • or all four

2. Design the Correct Assembly

We build roof, wall, and floor systems that manage moisture correctly, not just to code.

3. Combine Air Sealing + Ventilation

This stops condensation inside assemblies.

4. Build With Durable Moisture Pathways

We make sure your home can drain and dry — not just stay sealed.

Why This Matters

Moisture is the #1 reason homes fail early in the Pacific Northwest.
Getting this right protects:

  • your home’s structure

  • your insulation

  • your roof

  • your walls

  • your indoor air quality

  • your remodeling investment

  • your long-term comfort

A dry home is a healthy, durable home.

When to Call a Professional

You should have a moisture assessment if:

  • you smell mustiness

  • you see staining or swelling

  • your attic or crawlspace shows condensation

  • your windows sweat

  • you’re planning a remodel or ADU

  • you have a flat/low-slope roof

  • your home is older than 1980

  • you’ve had previous water issues