Crawl Space Insulation: Why Floor Joist Insulation Causes Problems

November 24, 2025

Introduction: The Right Way to Protect Your Crawl Space

Insulating your crawl space floor joists may look like a smart idea on paper, but it creates major moisture and structural problems in the real world. Insulation stuffed between joists traps humidity, grows mold, sags, and weakens the wood over time.

At Dr. Lee’s Crawlspace Repair, we never insulate between floor joists.

Instead, we insulate the crawl space correctly: foam board along the foundation walls, sealed vents, and full moisture control. This method keeps the entire crawl space dry, stable, and energy efficient.

Why You Shouldn’t Insulate Floor Joists

Floor joists are meant to support your home’s flooring system, not hold insulation. When insulation is packed between joists, it creates issues such as:

• Cold floors in winter
• Condensation and mold growth
• Damp, sagging, or fallen fiberglass
• Lower indoor air quality
• Higher heating and cooling costs
• Moisture trapped against the wood
• Increased pest activity inside the insulation

This approach doesn’t solve moisture problems. It creates them.

The right solution is insulating the crawl space walls, not the joist cavities.

Correct Crawl Space Insulation Methods

1. Foam Board Along Foundation Walls (Recommended)

Rigid foam board provides a stable moisture-resistant thermal barrier. Installed along the foundation walls, it insulates the entire crawl space rather than small, confined cavities that trap moisture.

2. Sealing Crawl Space Vents

Open vents bring in humid outside air, pests, and cold air. Sealing all vents is essential for preventing condensation, mold, and energy loss.

3. Encapsulation Compatibility

This method pairs perfectly with vapor barriers, drainage matting, sump pumps, and dehumidifiers. Together, they create a dry, conditioned crawl space that lasts.

Benefits of Proper Crawl Space Insulation

When insulation is installed on the crawl space walls instead of the floor joists, homeowners see major improvements:

• Lower energy bills
• Warmer floors
• Better indoor air quality
• Stronger protection against mold
• Reduced moisture and wood rot
• Increased home comfort and value
• Longer-lasting structural stability

This is the method supported by building science and modern crawl space standards.

When Should You Remove or Replace Old Crawl Space Insulation?

You may need to replace or remove insulation if:

• Fiberglass is sagging, damp, or moldy
• Your home still has insulation between floor joists
• Crawl space vents are open
• You notice cold floors or unusually high energy bills
• Pests have nested in old insulation
• You're planning encapsulation or adding a vapor barrier

Fixing these issues early prevents major structural damage later.

Why Trust Dr. Lee’s Crawlspace Repair?

We use the correct building-science approach to crawl space insulation. Our process includes:

• Foam board insulation on all foundation walls
• Sealing every crawl space vent
• Full compatibility with encapsulation systems
• Moisture-resistant materials that work in Arkansas climates
• Long-term protection for wood framing
• Custom solutions for every home

We never install materials that trap moisture or hide problems behind your flooring system.

Ready to Insulate Your Crawl Space the Right Way?

Proper crawl space insulation keeps your home warmer, prevents mold, and protects your structure from the ground up. If your crawl space has joist insulation or moisture problems, it’s time for an upgrade.

Schedule your free crawl space evaluation with Dr. Lee’s Crawlspace Repair today and learn how proper insulation can make your home safer, stronger, and more efficient