How To Waterproof Your Crawl Space Interior

Tucked just beneath your home is the crawl space. This low-clearance area houses critical systems like plumbing, ductwork, and electrical lines. But without proper waterproofing, it can become a source of mold, structural damage, and poor indoor air quality.

The good news? You can take proactive steps to protect this area using the Flex Seal Family of Products. This blog will show you how to waterproof your crawl space interior while preserving airflow and access.

Important Information: This how-to focuses on waterproofing the interior of a crawl space, NOT fully encapsulating or permanently sealing off a crawl space entry point.

What Causes Crawl Space Moisture?

Foundation wall cracks
Pipe or cable penetrations
Unsealed vents and access doors
High humidity in the soil or air
Improper drainage around the home

When this moisture rises, even small leaks can lead to warped flooring, musty odors, rusted components, and higher utility bills.

Important: Before using Flex Seal products, read all directions carefully and wear protective gear while applying.

With Flex Seal products, you can protect your crawl space by:

Patching gaps and cracks
Sealing wall joints and pipe penetrations
Waterproofing porous surfaces

Here’s what you’ll need to get started.

Materials

Flex Paste – for sealing cracks and rough masonry
Flex Glue – for bonding foam board, covers, or patches
Flex Caulk Hybrid – for detailed edge and joint sealing
Flex Seal Liquid – for brush-on waterproofing
Putty knife or trowel
Paint brush or roller
Caulking gun (for Flex Glue cartridge)
Wire brush or sandpaper
Caulk finishing tool (optional)
Rags or drop cloth
Gloves and safety glasses

Step 1: Inspect and Prep the Surfaces

Start with a full visual inspection of your crawl space interior walls. Look for:

Cracks or gaps in the foundation
Signs of previous leaks or water staining
Exposed pipe or wire penetrations
Old, flaking sealant or paint

Once the condition of your crawl space is assessed, clean the surfaces with a dry brush or vacuum. Use a wire brush to remove loose concrete or dirt and smooth rough spots with sandpaper if needed.

Flexpert Tip: Although these Flex Seal products can be applied to damp surfaces, clean and dry surface areas are best for maximum performance.

Step 2: Seal Cracks and Gaps with Flex Paste

Use a putty knife to apply Flex Paste directly into wall cracks, mortar gaps, or around irregular surfaces. Push it in firmly and smooth it over for a tight, even finish.
Let it dry for 24–48 hours, depending on thickness.

Flexpert Tip: Flex Paste bonds securely to porous materials like concrete and brick and won’t shrink or peel over time, making it perfect for handling crawl space temperature fluctuations.

Step 3: Install Panels or Covers with Flex Glue

Use Flex Glue if you're applying foam board insulation, patch panels, or vent covers:
Apply Flex Glue in beads across the back of your material or directly to the wall.
Press into place and hold for 15–30 seconds.
Let cure for 24–48 hours.

Flexpert Tip: Flex Glue’s instant grab helps hold materials in place instantly—no need for nails, screws, or extra hardware.

Step 4: Seal Tight Spots with Flex Caulk

Next, use Flex Caulk to close off small seams, pipe penetrations, or the base of wall joints. Its Trigger Flow Nozzle makes precise application easy, even in hard-to-reach corners.
Smooth out the bead within 10-20 minutes of application using a caulk smoothing tool or a gloved finger. Flex Caulk will fully cure in about 12-24 hours.

Flexpert Tip: Flex Caulk stays flexible and won’t crack or dry out—ideal for foundations that shift seasonally.

Step 5: Brush on Flex Seal Liquid

Once everything else has dried to the touch, apply Flex Seal Liquid to vertical surfaces, such as crawl space walls or seams where walls meet utility access panels.
Use a brush or roller to apply thin, even coats. Wait 6–12 hours between coats and apply at least two for optimal sealing.

Important Information: Do not apply Flex Seal Liquid to crawl space floors or underfoot areas. It is not designed to support foot traffic and may become slippery.

Step 6: Let It Cure and Inspect

Allow all products to fully cure (24–48 hours), then do a final check for missed spots, thin areas, or hairline cracks and touch up as needed.
After curing, the treated surfaces will repel water, resist mildew, and flex with seasonal movement.

Now you know how to protect your crawl space from unwanted water intrusion without cutting off airflow or accessibility. With Flex Seal products, you have the DIY power to seal and protect your home from the ground up.