By AJ Howard, DIY Specialist at Crawl Space Ninja Supply

If you've been researching crawl space humidity, mold, musty odors, encapsulation, or dehumidifiers, you've probably run into one of the biggest debates in the crawl space world:

Should crawl space vents be open or closed?

Honestly, I get this question all the time.

Usually it sounds something like:

"AJ, if builders put crawl space vents there for a reason, why would I seal them?"

Or maybe:

"Won't sealing my crawl space vents trap moisture under the house?"

Or:

"Do I need a vent fan if I already have a dehumidifier?"

The truth is, there isn't a one-size-fits-all answer. Building codes still allow vented crawl spaces today, and millions of homes across the country still have open foundation vents.

But building codes also allow sealed and encapsulated crawl spaces.

At Crawl Space Ninja, we generally prefer controlling humidity ourselves instead of letting Mother Nature control it for us.

Why?

Because humidity is the number one enemy of a crawl space.

You usually can't see humidity until it starts causing problems.

Mold.

Wood rot.

Musty odors.

Condensation on ductwork.

Poor indoor air quality.

High moisture levels in floor joists.

All of those problems often start with excess moisture.

Think about it this way. Imagine it's 90 degrees outside with high humidity, but your home is a comfortable 72 degrees because the air conditioner is running. Now imagine opening every window in your house.

That hot, humid air immediately starts entering the home and fighting against the conditioned environment.

Your air conditioner has to work harder.

Humidity rises.

Comfort drops.

Open crawl space vents can create a similar situation underneath your home.

Every time outdoor humidity rises, humid air enters through those vents. When that humid air reaches cooler surfaces like HVAC ductwork, plumbing lines, or framing members, condensation can form.

Over time, that can contribute to mold growth, wood rot, and elevated humidity throughout the crawl space.

That's why we often recommend sealing crawl space vents.

But here's the mistake I see homeowners make all the time.

They hear someone say:

"Seal your crawl space vents."

So they block every vent opening and call it a day.

Unfortunately, that's not how humidity control works.

In fact, sealing crawl space vents without a dehumidifier can sometimes create a worse situation than leaving the vents open.

If you completely seal the crawl space and don't actively remove moisture, you're simply trapping whatever humidity is already there. The crawl space becomes isolated, but the moisture doesn't magically disappear.

That's why vent covers are not the solution.

They're one piece of the solution.

The real goal is creating a controlled crawl space environment.

That means controlling ground moisture, controlling humidity, reducing outdoor air intrusion, and providing proper air movement when needed.

Common Crawl Space Vent Questions

Should crawl space vents be open or closed?

At Crawl Space Ninja, we generally recommend sealing crawl space vents and controlling humidity with a dehumidifier.

Open vents allow outdoor humidity to enter the crawl space. During warm weather, that moisture can contribute to condensation, mold growth, musty odors, and elevated humidity levels.

Why did builders install crawl space vents if they're bad?

They're not necessarily bad.

The original idea was that airflow would help dry moisture underneath the home.

The problem is that in many parts of the country, especially throughout the Southeast, the air entering those vents is already humid.

Instead of drying the crawl space, the vents often introduce additional moisture.

Will sealing crawl space vents lower humidity?

Not by itself.

Sealing vents helps prevent humid outdoor air from entering the crawl space, but it doesn't actively remove moisture that's already there.

That's where a crawl space dehumidifier comes in.

Can sealing crawl space vents cause problems?

Yes, if done incorrectly.

Completely sealing a crawl space without addressing humidity can trap moisture.

This is why we recommend pairing sealed vents with a dehumidifier and, in many cases, active ventilation.

What does a crawl space vent fan do?

A crawl space vent fan helps move air out of the crawl space.

Many homeowners think the fan lowers humidity.

Actually, that's the dehumidifier's job.

The fan's purpose is to assist with air exchange and help address potential soil gases while moving air throughout the crawl space.

What are soil gases?

Soil gases can include naturally occurring gases such as radon, methane, and carbon dioxide.

Air naturally enters the lower portions of a home and exits through the upper portions through a process known as the stack effect.

If crawl space air has nowhere to go, some of that air can move upward into the living space.

That's one reason active ventilation is commonly used in encapsulated crawl spaces.

How We Seal Crawl Space Vents

One of the easiest DIY upgrades you can make is properly sealing foundation vents.

We use our pre-cut Foundation Vent Covers because they're simple, effective, and provide insulation value as well.

Products Needed:

• Foundation Vent Covers

• Closed Cell Spray Foam Kit

• Utility Knife (if trimming is needed)

• Safety Glasses

• Gloves

Foundation Vent Covers:
https://supply.crawlspaceninja.com/products/foundation-vent-covers

Closed Cell Spray Foam:
https://supply.crawlspaceninja.com/products/spray-foam-insulation-kit

Step 1: Measure the Vent Opening

Most crawl space vents are close to 8" x 16", but always verify before installation.

Step 2: Test Fit the Vent Cover

Our Foundation Vent Covers come pre-cut at approximately 8" x 16" x 2.25".

If your opening is slightly smaller, trim the foam board as needed.

Step 3: Insert the Vent Cover

Place the vent cover into the vent opening.

Many times it will fit snugly, but that's only part of the installation.

Step 4: Seal Around the Edges

Using closed-cell spray foam, seal the entire perimeter around the vent cover.

This is one of the most important steps.

The spray foam helps prevent air leaks while also helping secure the vent cover in place.

Step 5: Trim Excess Foam

Once the foam cures, trim away any excess if desired.

Pro Tip:

If the vent cover is visible from outside the home, paint the exterior-facing side black. It blends in much better and often looks like a naturally dark vent opening.

How to Install a Lomanco PCV-1 Foundation Vent Fan

One of the biggest misconceptions I hear is that vent fans are meant to lower humidity.

That's actually the dehumidifier's job.

The vent fan's role is to move air and assist with air exchange.

At Crawl Space Ninja, we commonly use the Lomanco PCV-1 Foundation Vent Fan.

Lomanco PCV-1 Foundation Vent Fan:
https://supply.crawlspaceninja.com/products/lomanco-pcv1-foundation-vent-fan

Step 1: Choose the Correct Vent Location

Install the vent fan in the foundation vent furthest away from the dehumidifier.

This is important.

We want the fan pulling conditioned, dry air across the entire crawl space before exhausting it.

Doing this helps ensure the dehumidifier is treating the entire volume of the crawl space rather than just the area around the machine.

Step 2: Remove Existing Vent Materials

Remove any vent screens, louvers, or materials that would interfere with installation.

Step 3: Position the Fan

Place the Lomanco PCV-1 into the vent opening according to manufacturer instructions.

Step 4: Secure the Fan

Using appropriate masonry screws, secure the fan to the block wall.

Make sure it is mounted firmly and evenly.

Step 5: Seal Around the Fan

Just like the vent covers, seal any gaps around the fan using closed-cell spray foam.

This helps prevent unwanted air leakage around the unit.

Step 6: Connect Power

Power the fan according to manufacturer requirements.

The PCV-1 is designed for continuous operation and includes a built-in low-temperature shutoff feature.

Products Used in This Project

Foundation Vent Covers:
https://supply.crawlspaceninja.com/products/foundation-vent-covers

Closed Cell Spray Foam Kit:
https://supply.crawlspaceninja.com/products/spray-foam-insulation-kit

Lomanco PCV-1 Foundation Vent Fan:
https://supply.crawlspaceninja.com/products/lomanco-pcv1-foundation-vent-fan

Watch the Installation Process

Prefer watching instead of reading?

We've got videos that walk through these exact concepts.

How to Install Foundation Vent Covers

[Insert Video Here]

How to Install a Lomanco PCV-1 Foundation Vent Fan

[Insert Video Here]

Open vs Sealed Crawl Spaces

[Insert Video Here]

Crawl Space Humidity Control Basics

[Insert Video Here]

Final Thoughts

If you're trying to improve your crawl space one step at a time, don't get overwhelmed.

I talk to homeowners every day who are tackling these projects in phases.

Start by understanding what you're dealing with.

Measure humidity.

Look for standing water.

Check for condensation on ductwork.

Identify moisture sources.

Then focus on controlling the environment.

Seal the vents.

Control humidity with a dehumidifier.

Install active ventilation where appropriate.

Add a vapor barrier as budget allows.

Remember, the goal isn't simply moving air around.

The goal is creating a dry, healthy crawl space that protects your home, your structure, and your indoor air quality for years to come.