Photo of The Crawlspace Crew owner Dillon fixing a sump pump in a crawlspace.

Why Your Crawlspace Gets Damp in Winter (And How to Fix It Before Mold Grows)

If you live in the Pacific Northwest, you already know: winter never means “cold and dry.” It means rain—relentless rain—plus humidity, groundwater, and freezing temperatures that can leave the air damp and push moisture into the places you never see… like your crawlspace.

And while a damp crawlspace might not sound like an emergency, it can quickly become one. Moisture leads to:

  • Mold and mildew growth

    Photo of mold in a crawlspace.

  • Wood rot

    Wood rot photo

  • Pest activity

    Photo of rodent access point in the ground.

  • Musty odors rising into the home

  • Higher energy bills

  • Reduced indoor air quality

So why does this happen every winter, and what can you actually do about it before mold shows up?

Let’s break it down.

Why Crawlspaces Get Damp in PNW Winters

1. Constant Rain Saturates the Soil

The Pacific Northwest is one of the wettest regions in the U.S., which means the soil around and under your home stays soaked for months.

Once the ground beneath your home becomes saturated, ground moisture evaporates upward—right into the crawlspace.

➡️ Learn more about soil saturation and moisture movement from Energy.gov.

2. Humid Outside Air Enters Through Vents

Many older homes have open crawlspace vents. The idea was to “circulate air,” but in our climate it does the opposite.

During cold months, outside humid air enters and condenses on cold surfaces—similar to how a cold soda sweats on a summer day.

This leads to:

  • Wet insulation

  • Condensation on ductwork

  • Mold forming on joists

➡️ See how condensation forms: EPA – Moisture Control Basics.

3. Lack of or Damaged Vapor Barrier

A vapor barrier is like a raincoat for the ground. Without it, there’s nothing stopping moisture from the soil from evaporating into the air.

Common issues:

  • Torn plastic

  • Crumpled or missing sections

  • Gaps around posts

  • Old 4-mil plastic that no longer does its job

➡️ More on vapor barriers: U.S. Department of Energy Crawlspace Guidelines.

4. Poor Drainage or Standing Water

If water pools around the foundation—or worse, inside the crawlspace—it creates a moisture problem that never goes away.

Causes include:

  • Misaligned gutters

  • Downspouts draining near the home

  • French drains clogged or missing

  • High water table

Standing water is one of the biggest red flags for future mold.

➡️ See CDC recommendations on mold prevention:
CDC Mold Prevention.

What Happens If You Ignore Winter Moisture?

Mold doesn’t wait.

According to the EPA, mold can start growing in 24–48 hours under the right conditions. Once it takes hold, it spreads quickly across wood, insulation, and HVAC systems.

Other consequences include:

  • Sagging floors

  • Structural damage from rot

  • Air quality issues (crawlspace air becomes indoor air)

  • Costly repairs in spring

A damp crawlspace is not a “later” problem—it’s a winter priority.

How to Fix a Damp Crawlspace Before Mold Grows

1. Install or Replace the Vapor Barrier

A continuous 6–12 mil vapor barrier reduces ground moisture by up to 85%.
Make sure it is:

  • Fully taped off

  • Laid flat

  • Covering 100% of exposed soil

2. Fix Wet or Fallen Insulation

Wet insulation loses effectiveness and becomes a mold magnet.

Signs you need new insulation:

  • It’s sagging or fallen

  • It’s heavy or damp

  • You smell mildew in the home

3. Seal or Condition the Crawlspace

Many PNW homeowners are shifting to conditioned crawlspaces for long-term moisture control:

  • Seal vents

  • Add insulation to the walls (instead of the floor)

  • Install a dehumidifier

This stabilizes humidity, reduces mold risk, and lowers energy bills.

4. Improve Exterior Drainage

Often the moisture problem starts outside:

  • Extend downspouts

  • Clear or install French drains

  • Regrade soil to slope away from the home

5. Add a Sump Pump (If Needed)

If your crawlspace has standing water in winter, a sump pump may be the only way to keep it out.

Want to Know What’s Happening Under Your Home?

Most homeowners don’t check their crawlspace until they smell something or see mold—which means the damage is already underway.

That’s why we offer a Free Moisture Inspection this winter.

We’ll check for:
✔ Condensation
✔ Wet insulation
✔ Mold or mildew
✔ Vapor barrier damage
✔ Standing water
✔ Foundation or ducting issues

Preventing mold is always cheaper than removing it.

👉 Schedule your Free Moisture Inspection today.