Effective fiberglass insulation is a primary defense against moisture problems in a building. When installed correctly, it controls humidity by slowing the transfer of heat, which in turn prevents the temperature drop that causes water vapor to condense inside walls, attics, and crawl spaces. The key is pairing the right type of fiberglass insulation with an appropriate vapor retarder for the climate and ensuring the installation is completely free of gaps or compression. Without these elements, the insulation cannot perform its function, and moisture issues can develop.
Fiberglass insulation is made from extremely fine glass fibers. Because glass is inorganic, the insulation itself does not support mold growth. A report from the U.S. Department of Energy confirms that fiberglass is naturally resistant to moisture. Its thermal performance comes from the billions of tiny air pockets trapped between these fibers, which slow the movement of heat. By keeping the interior surfaces of your walls warmer in the winter and cooler in the summer, properly installed insulation prevents condensation.
The two main types of fiberglass batt insulation are faced and unfaced.
The choice between them depends entirely on the climate and the specific application within the building.
While fiberglass insulation resists moisture, a vapor retarder is what truly stops it from getting into wall cavities. Water vapor is always present in the air and naturally moves from warmer, moister areas to cooler, drier ones. In the winter, this means vapor from inside your heated home tries to move through the walls to the cold, dry air outside. If it gets into the insulation and hits a cold surface (like the exterior sheathing), it will condense into liquid water, wetting the insulation and wooden studs.
Vapor retarders are classified by their permeability, which is a measure of how easily water vapor can pass through them. According to Building Science Corporation, a leading building science research group, materials are grouped into three classes.
| Class | Permeability (perms) | Examples | Common Application |
|---|---|---|---|
| Class I | 0.1 perms or less | Polyethylene sheeting, rubber membrane, foil facing | Used in very cold climates and high-humidity areas like pools or saunas. |
| Class II | >0.1 to 1.0 perms | Kraft-faced fiberglass batts, asphalt-coated paper | The standard for most mixed and cold climates in the United States. |
| Class III | >1.0 to 10 perms | Latex or enamel paint over drywall | Allows some drying to the interior; suitable for warm, humid climates. |
Bonus Tip: The performance of a vapor retarder is only as good as its installation. Use acoustical sealant to seal the plastic sheeting to the top and bottom plates and apply code-approved tape over every seam, staple, and penetration to create a continuous barrier.
The single most common reason for fiberglass insulation failing to control moisture is improper installation. Moisture damage in U.S. homes accounts for a significant portion of building-related problems. Many of these issues can be traced back to insulation that was not installed to manufacturer’s specifications.
Here are the most frequent mistakes that create moisture problems:
Before installing or upgrading insulation, several factors must be evaluated to ensure the project successfully manages moisture and improves energy efficiency.
Bonus Tip: When working on an older home, use a moisture meter to test the wood studs and sheathing inside the walls before installing new insulation. This helps find hidden moisture pockets that need to be addressed first.
Using fiberglass insulation to manage humidity is about more than just buying the product. Its success depends on selecting the right type for your climate, ensuring a perfect, detail-oriented installation, and making sure it’s part of a complete system that includes proper ventilation and air sealing. When all these pieces are in place, fiberglass is an excellent and cost-effective tool for creating a durable, dry, and comfortable building. Before beginning any project, take the time to evaluate your property’s specific needs and local code requirements to ensure a lasting solution.
For a thorough evaluation of your property’s specific insulation needs and potential moisture concerns, it is often best to consult with experienced professionals. A trained eye can identify subtle issues that might be missed and recommend the most effective long-term solution. For guidance on your project, contact H&R Insulation for an assessment by calling (979) 325-2419 or sending an email to [email protected].
Higher R-values are better at resisting heat flow, which helps prevent condensation. The best R-value is not a single number but is determined by your local building code and where in the house the insulation is being installed (walls, attic, floors).
In a high-humidity area like a bathroom, it’s best to use unfaced fiberglass batts and install a separate, continuous polyethylene vapor barrier on the warm side of the wall before the drywall goes up. This provides a much more effective seal against moisture than paper facing alone.
Proper attic ventilation is essential. It removes warm, moist air that escapes from the living space below before it can condense on the cool underside of the roof sheathing during the winter. Without good airflow from soffit-to-ridge vents, moisture can build up and drip onto the insulation, reducing its effectiveness.
Insulating basement walls can help, but it won’t solve a humidity problem caused by water leaking through the foundation or from high groundwater levels. The water intrusion must be stopped first with proper grading, sealants, or a drainage system. Once the basement is dry, insulating with rigid foam directly against the concrete and then adding fiberglass in a stud wall is a common and effective method.
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