Fiberglass insulation is a building material made from fine glass fibers that slows heat transfer through walls, ceilings, floors and attics. The fibers trap small pockets of air, which helps reduce conductive and convective heat flow. Fiberglass insulation is also called glass wool insulation.
Most fiberglass products provide about R-2.2 to R-4.3 per inch, depending on the product type and density. Fiberglass insulation does not usually stop air leaks by itself, so cracks, gaps and penetrations should be sealed before or during installation.
Fiberglass Insulation at a Glance
| Feature | Details |
|---|---|
| Main material | Spun glass fibers |
| Common forms | Batts, rolls and blown-in loose fill |
| Typical locations | Attics, walls, floors, ceilings and some duct systems |
| Main purpose | Reduce heat flow and improve indoor comfort |
| Key measurement | R-value |
| Main limitation | Fiberglass is air-permeable and does not replace air sealing |
| Installation concern | Fibers can irritate the skin, eyes and respiratory tract |
How Does Fiberglass Insulation Work?
Fiberglass insulation slows heat movement by holding air between layers of glass fibers. This reduces heat transfer between conditioned indoor areas and unconditioned spaces such as attics, garages, crawlspaces and exterior walls.
The material's performance is measured by its R-value. A higher R-value means greater resistance to heat flow. Actual performance also depends on thickness, density, installation quality, gaps and compression.
Fiberglass insulation performs best when it fills the intended cavity without being compressed, folded or cut short. Gaps around the edges can reduce the insulation's real-world performance.
What Is Fiberglass Insulation Made From?
Manufacturers melt glass and draw or spin it into very thin fibers. Those fibers are formed into flexible blankets or loose-fill material. Some products contain recycled glass. Binders and facings help hold the fibers together or protect the insulation.
Fiberglass insulation is different from fiberglass-reinforced plastic used in boats, tubs and panels. Insulation is a loose, fibrous material designed mainly to resist heat flow.
What Are the Different Types of Fiberglass Insulation?
Fiberglass Batts and Rolls
Batts are pre-cut sections of insulation. Rolls are continuous lengths that can be cut to fit a cavity. Both are commonly used in:
- Unfinished exterior walls
- Attic floors
- Basement and crawlspace walls
- Floors above garages or other unconditioned areas
- Ceilings and roof assemblies
ENERGY STAR lists fiberglass batts and rolls as options for unfinished walls, floors and ceilings.
Blown-In Fiberglass
Blown-in fiberglass consists of loose fibers installed with a blowing machine. It can work well on attic floors, inside enclosed wall cavities and in irregular areas that are difficult to fill with pre-cut batts.
The installer must use the manufacturer's specified depth and density. Settling, gaps and incomplete coverage can reduce the installed R-value.
Faced and Unfaced Fiberglass
Faced fiberglass insulation has a covering such as kraft paper, foil-kraft paper or vinyl. The facing can help hold the batt together. Depending on the product and building design, it may also serve as a vapor-retarder layer.
Unfaced fiberglass insulation has no attached covering. It is often used when an additional vapor retarder is not needed or when insulation is being added over existing insulation. The right choice depends on the climate, wall assembly and local building requirements.
What R-Value Does Fiberglass Insulation Provide?
Fiberglass insulation provides about R-2.2 to R-4.3 per inch, depending on its type and density. A U.S. Department of Energy guide lists these approximate ranges:
- Standard-density fiberglass batts: R-2.9 to R-3.8 per inch
- High-density fiberglass batts: R-3.7 to R-4.3 per inch
- Blown fiberglass: R-2.2 to R-2.7 per inch
Actual ratings vary by product. Use the label on the specific insulation when planning an installation.
Thickness alone does not determine performance. A thick batt compressed into a shallow cavity may perform worse than insulation that fits the cavity correctly. Fiberglass should be installed at the required depth, with no voids or compression.
What Are the Advantages of Fiberglass Insulation?
Fiberglass insulation is widely used because it is affordable, easy to find and available in several installation formats.
- Lower material cost: Fiberglass batts often cost less than many other insulation materials.
- Easy availability: Home improvement stores commonly stock standard batt and roll sizes.
- Flexible installation: Products are available for walls, attics, floors and difficult-to-reach areas.
- Fire and moisture behavior: Glass fibers themselves are noncombustible, although facings and other components may have different properties.
- Pest and mold resistance: Fiberglass does not provide a food source for mold or pests. Dust, paper facings, wood and moisture can still create building problems around the insulation.
- Sound reduction: Fiberglass can reduce some airborne sound transmission when it is installed correctly inside wall and floor cavities.
Fiberglass is most practical where the installer can access the full cavity, such as open wall framing and attic floors.
What Are the Limitations of Fiberglass Insulation?
Fiberglass insulation can reduce heat flow, but it does not solve every building-envelope problem. Air leakage, moisture and poor installation can limit its performance.
Fiberglass Does Not Replace Air Sealing
Air can move through fiberglass insulation. Seal gaps around wiring, plumbing, ducts, light fixtures, attic hatches and framing joints with an appropriate caulk, foam or other air-sealing material.
Installation Quality Matters
Common installation problems include:
- Compressed batts
- Gaps around outlets and pipes
- Insulation cut too short
- Voids at corners and framing intersections
- Blocked soffit vents
- Insulation placed too close to heat-producing fixtures
These defects can create cold spots, reduce the effective R-value and contribute to moisture problems. Heat-producing fixtures may also require specific clearances.
Wet or Mold-Contaminated Insulation May Need Replacement
Fiberglass does not correct a moisture problem. Roof leaks, plumbing leaks, condensation and poor ventilation should be fixed before insulation is installed.
The U.S. Environmental Protection Agency recommends removing and replacing wet or visibly moldy fiberglass duct material rather than simply cleaning it.
Is Fiberglass Insulation Safe?
Fiberglass insulation can irritate the skin, eyes and respiratory tract during cutting, handling and removal. OSHA recommends protective measures such as long sleeves, gloves and head coverings, along with eye and respiratory protection when appropriate.
OSHA states that insulation glass wool is not classifiable as to its carcinogenicity to humans. OSHA also notes that the evidence is insufficient to establish that synthetic mineral fibers cause respiratory disease in humans. Safe handling, dust control and appropriate protective equipment are still necessary.
When installing fiberglass insulation:
- Wear gloves, long sleeves and eye protection.
- Use respiratory protection when product instructions or work conditions call for it.
- Avoid creating unnecessary dust.
- Follow the product's installation and clearance instructions.
- Keep insulation away from heat-producing fixtures, chimneys and flues as required.
- Wash exposed skin after handling the material.
Is Fiberglass Insulation a Good Choice?
Fiberglass insulation is a good choice for accessible attics, unfinished walls, floors and ceilings when it fits tightly and is combined with proper air sealing. It is affordable, widely available and offered in several R-values.
Fiberglass may be less suitable when the main problem is air leakage, an irregular enclosed cavity, limited installation access or a need for continuous insulation across framing. In those cases, air sealing combined with blown-in insulation, rigid board or spray foam may better address the building-envelope problem.
Before choosing a product, identify the installation area, required R-value, moisture conditions and air leaks. Seal the assembly first. Then install fiberglass insulation to the required depth, without gaps or compression.