Hi, I’m AJ with Crawl Space Ninja Supply. If you’ve ever been under your home and seen fiberglass sagging in the crawl space, you’ve probably wondered: Is this normal? Should I replace it?

Here’s the reality: fiberglass insulation was never meant to live in a crawl space.

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Why We Insulate Crawl Space Walls (Not the Subfloor)

A lot of older homes were built with fiberglass insulation stuffed between the joists overhead. The idea was to block cold air from coming up through the floor. Sounds logical, right?

But here’s the truth: the biggest source of heat transfer in your crawl space isn’t your floor it’s your foundation walls. That’s where outside temperatures meet your home. If you want effective insulation, you need to treat the walls, not just the subfloor.

That’s why modern crawl space systems use products like Insul-Barrier directly on the walls and foam board or spray foam on rim joists. This method actually controls heat transfer where it matters most.

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Why Fiberglass Insulation Fails in Crawl Spaces

So what’s the problem with fiberglass down there? A few big ones:

It traps moisture – Fiberglass works like a sponge. Once it gets damp, it soaks in water and holds it.

It presses moisture against your wood – That damp insulation pushes wetness directly against your subfloor, creating the perfect setup for wood rot.

It sags and falls – Ever seen insulation hanging in big clumps? That’s what happens once it’s heavy with water.

It becomes a mold factory – Damp fiberglass can harbor mold spores and contribute to poor indoor air quality.

It loses R-value – Insulation only works if it’s dry and fluffy. Once it’s wet, its ability to block heat is basically gone.

That’s why when you see fiberglass sagging in your crawl space, you’re not just looking at a mess you’re looking at a symptom of a humidity problem.

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Why Moisture is the Real Enemy

If fiberglass is failing, it’s because of one thing: humidity.

Humidity creeps into crawl spaces from groundwater seepage, leaky vents, or just the natural environment. Once relative humidity goes above 60%, your crawl space becomes a breeding ground for moisture problems.

And here’s the catch: until you lower the humidity, no insulation will hold up. The first fix isn’t replacing fiberglass with more fiberglass it’s controlling moisture with a crawl space dehumidifier.

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The Safer Way to Remove Fiberglass

If you’re tackling the job yourself, know this: fiberglass is irritating and potentially unsafe to handle. Tiny fibers cause skin rashes, lung irritation, and eye problems. If it’s moldy, that risk doubles.

The safest way to remove fiberglass:

• Full PPE (coveralls, gloves, long sleeves).

• Eye protection.

• A full-face respirator mask — not just a dust mask.

Once removed, you can finally start drying out your crawl space the right way.

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What to Do Instead of Fiberglass

Once fiberglass is gone, the best path forward is:

1. Run a Crawl Space Dehumidifier – This is the only way to regulate humidity and draw moisture out of wood.

2. Insulate the Walls, Not the Subfloor – Use Insul-Barrier on foundation walls and foam board/spray foam on rim joists.

3. Seal with a Vapor Barrier – Encapsulating your crawl space locks moisture out and keeps humidity under control long-term.

Fiberglass insulation in your crawl space isn’t protecting your home it’s putting it at risk. It soaks up water, sags down, presses moisture against your subfloor, and can lead to mold and wood rot.

The real solution isn’t more fiberglass. It’s moisture control, proper wall insulation, and a dehumidifier to keep humidity where it belongs.

If you’re staring at fiberglass sagging in your crawl space, don’t just push it back up. Remove it safely, fix the moisture problem, and insulate the right way.