What Happens to Fiberglass Insulation After It Gets Wet?

Fiberglass insulation is made of fine glass fibers that trap air to slow heat transfer. When water soaks in, it fills the air pockets and displaces the trapped air, which is what gives the insulation its thermal resistance. A wet batt can lose a large share of its R-value, and it often stays damp for a long time because the fibers hold moisture against the surrounding structure.

The weight of the water can also make the insulation sag or slump inside wall cavities and attic floors. That compression further reduces its ability to insulate, and it can pull away from the framing, leaving gaps where heat escapes. Even after the surface feels dry, the inner layers may still hold moisture, so judging dryness by touch is unreliable.

Small, minor damp spots might dry out with good airflow, a dehumidifier, and time. But in most cases, wet fiberglass insulation does not dry evenly, and the trapped moisture can lead to hidden problems later. The pros often recommend removing soaked batts because the cost of replacement is usually lower than the risk of leaving wet material in place.

If the water came from a clean source like a burst pipe and the insulation is only slightly damp, drying might be possible. But if the water was contaminated, such as from a flood or sewage backup, the insulation should be treated as unsalvageable and removed. When in doubt, treat the material as a loss.

• Remove wet batts and place them in plastic bags
• Dry the framing with fans and dehumidifiers
• Inspect for mold before reinstalling new insulation
• Replace with new, dry insulation of the same R-value
• Seal the source of water before closing up walls

Replacing wet fiberglass insulation typically costs between $1 and $3 per square foot for materials and labor, but the total depends on the size of the area, the accessibility of the insulation, and whether drywall also needs removal. The cost depends on how much insulation you need to replace and how complicated the job is.

If the water damage is widespread, you might also need drywall repair, which adds to the overall expense. Your insurance policy may help cover some of the costs, but policies vary, and your insurer decides what is covered. Check your policy and document the damage with photos before any work begins.