Rigid Foam vs. Fiberglass Insulation Comparison Guide

Home improvement stores carry two major classes of insulation: fiberglass and rigid foam. Both are designed for residential use. While there is some overlap of applications, overall, one is not better than the other; each tends to have its own best areas of use. For example, rigid foam is commonly used for basement wall insulation because it resists the moisture from basement masonry walls. Fiberglass batts are typically insulated between wall studs, floor and ceiling joists, and roof rafters.

If you need insulation for a home renovation project and need to choose between foam board vs. fiberglass insulation, read on for more information.

Rigid Foam vs. Fiberglass Insulation: Major Differences

Rigid Foam Insulation

Rigid foam insulation consists of panels sold in 4-by-8-foot sheets in thicknesses from 1/4 to 2 inches. It is also sold in large bundled panels, joined at the ends in accordion style, for use as exterior sheathing material.

Rigid foam insulation uses one of three materials:

• High-quality, green-friendly polyisocyanurate
• Extruded polystyrene
• Expanded polystyrene

Rigid foam performs well when subjected to moisture and does not change dimensions, split, or crack. While spray foam insulation injected into wall cavities does dry to a rigid state, this is not considered rigid foam insulation. However, you can use rigid foam board instead of spray or fiberglass insulation where it makes sense, such as in wall cavities that may risk becoming wet or moist.

Fiberglass Insulation

Fiberglass insulation consists of tightly packed long rolls or individual batts (single unrolled pieces) of spun fiberglass fibers.

In contrast to rigid foam, fiberglass is soft and flexible. The millions of air pockets in fiberglass insulation provide a superior thermal barrier, but fiberglass can also trap moisture, leading to mold and mildew growth. Fiberglass should never be used when moisture is even remotely present.

Fiberglass insulation is typically sold in batts or rolls designed to fit standard wall and ceiling cavities with framing members spaced 16 or 24 inches apart, on center. The batts' thicknesses range from 3 1/2 to 12 inches. Fiberglass insulation can be sold in rolls ranging from 24 to 40 feet or in bundled batts, usually about 93 inches long.

Fiberglass insulation rolls or batts may be faced with paper to simplify attachment to framing members. Unfaced batts or rolls are better for laying as an insulating blanket.

Applications

Rigid Foam Insulation

• Exterior continuous insulation: Rigid foam can be used as sheathing, a continuous layer of insulation attached to wall exteriors before installing house wrap and siding.
• Basements: Indoors, rigid foam board is better than fiberglass insulation for basements. It's the material of choice on walls where the insulation will touch masonry. This mostly means the exterior walls in basement build-outs, except for daylight-facing walls found in walk-out basements. Rigid foam stands up to moisture better than fiberglass.
• Garages: Rigid foam is best for insuring garage doors and floors, but you can use either type of insulation for garage walls.
• Radiant heat floors: Rigid foam is used as a thermal barrier underneath radiant floor heating systems.
• Rim joists: Rim joists around the foundation's edge can be insulated with rigid foam blocks cut to fill the space and caulked in place. In some instances, you can use both types of insulation together; if concrete blocks have open cavities, they should first be stuffed with loosely packed fiberglass insulation.
• Other uses: Rigid foam can provide soundproofing to interior walls or sequester interior spaces that are not climate-controlled. It can also be used to build temporary privacy walls in shared apartments or dormitories.

Fiberglass Insulation

• Exterior walls: Rolled fiberglass insulation's main purpose is to fill the cavities of above-grade (non-basement) exterior walls when those walls are accessible from the inside. During new construction or major remodeling projects when wall surfaces are not yet installed, faced fiberglass batts are the insulation of choice. On walls with finished surfaces, blow-in cellulose insulation or spray foam is more commonly used to add insulation.
• Attics: Thick batts or rolls of fiberglass insulate the attic by preventing heat loss through the ceiling and roof. Faced batts are often installed between joists, or an attic blanket of unfaced batts can be laid across the tops of joists. Both joist cavity insulation and an attic blanket can be used for maximum insulation value. Blow-in cellulose insulation is an alternative to fiberglass and can also be used to add insulation over fiberglass insulation.
• Floors: Joist cavities above unheated crawl spaces or basements are often insulated with fiberglass batts. Fiberglass insulation in the floors will deaden sound transmission between two- or three-story homes.
• Rim joists: The joist spaces above foundations can be filled with loosely packed fiberglass insulation to prevent heat loss. This placement can be done alongside rigid foam, with fiberglass used to fill the void spaces at the top of concrete block walls and blocks of rigid foam used to block off the joist cavities above the foundation walls.
• Other uses: Loosely packed fiberglass is often used to fill in small gaps around window and door frames or the spaces where pipes and wires penetrate through exterior walls. Spray foam is also used for this purpose.

R-Values

The insulating properties of various building materials are measured by their R-value, which measures the material's thermal resistance. Higher R-values indicate better insulating values.

The thickness of the insulating material mostly determines the R-values of rigid foam and fiberglass. Rigid foam insulation packs more R-value per cubic inch than fiberglass insulation. If conserving energy is your main goal and there are no other influencing factors, such as moisture, rigid foam may be your first choice.

Rigid Foam Insulation

Rigid foam R-values range from R-1 for 1/4-inch thick sheathing panels to R-15 for 3-inch thick panels. The R-value is not entirely a function of the material's thickness, as the facing material and type of foam used will also affect it.

Fiberglass Insulation

Fiberglass R-values range from R-11 to R-38 for the most commonly available thicknesses:

• 3 1/2 inches: R-11
• 3 5/8 inches: R-13
• 3 1/2 inches (high density): R-15
• 6 to 6 1/4 inches: R-19
• 5 1/4 (high density): R-21
• 8 to 8 1/2 inches: R-25
• 9 1/2 inches: R-30
• 12 inches: R-38

Foam Board vs. Fiberglass Insulation Cost

When calculated by R-value per square foot, fiberglass insulation costs less than half the price of rigid foam board insulation.

For example, insulating a 10-square-foot wall area to R-15 costs about $3.40 to $4.00 with fiberglass insulation. A comparable R-value for rigid foam costs roughly $10.

Fiberglass insulation also produces less waste. When rigid foam is cut to size, it produces small pieces and thin sections that are virtually unusable. Fiberglass insulation can be torn off and tucked into small areas, so very little of it goes to waste.

Ease of Installation

Both rigid foam and fiberglass have pros and cons when it comes to installation. Rigid foam's main advantage is its clean, irritant-free product. Fiberglass's advantage is that it is flexible enough to mold around obstructions.

Rigid Foam Insulation

• Rigid foam can be cut with a fine saw or sliced and snapped like drywall with a utility knife.
• Foam does not require you to fully suit up and protect yourself against fibers as you do with fiberglass, though you should always wear a dust mask.
• The stiffness of rigid foam means that it will not accommodate obstructions in the wall, such as wires, outlets, junction boxes, and bracing. The foam must be carefully cut to fit around such obstructions.
• Because it is difficult to get a tight seal with rigid foam, expandable foam or caulking may be necessary to fill thin gaps between the foam and framing members.
• Foil-faced foam board insulation (polyisocyanurate) is a vapor barrier and does not require an additional vapor barrier, while rigid foam board insulation without foil may need one.
  

Fiberglass Insulation

• Fiberglass insulation is flexible and can fit around vents, wires, and wall bracing.
• Fiberglass insulation cuts with sharp scissors or a utility knife.
• Fiberglass insulation is an irritant to the skin, eyes, and lungs. To protect yourself before installation, wear a respirator rather than a dust mask, eye protection, gloves, long sleeves, and pants.
• Fiberglass should be stapled to the studs.
• Fiberglass insulation has a vapor-retarding facing that may eliminate the need for a separate vapor barrier.

Durability

Both rigid foam and fiberglass can last 80 to 100 years; however, fiberglass batting is more prone to water damage.

Rigid Foam Insulation

Rigid foam insulation is more resistant to damage from water or pests and will usually last longer than fiberglass insulation. However, although it is more resistant than fiberglass, rigid foam is not fully waterproof. It's best to use higher-density rigid foam for homes in wet areas with more moisture.

Fiberglass Insulation

Fiberglass insulation is not water resistant and is prone to getting wet, mold damage, or rot. Insects and rodent pests may nest in the material, leaving behind fecal contamination. Fiberglass insulation that gets water damage or pest contamination must be replaced.

Impacts on Air Quality

Most open-cell insulation products, like fiberglass, can absorb moisture, which could impact indoor air quality if it becomes contaminated by mold and mildew.

Rigid Foam Insulation

Depending on the rigid foam, some types can off-gas volatile organic compounds (VOCs). VOCs are most intense immediately after installation. Certain foams may also have additives that negatively affect the ozone layer, contributing to global warming and air pollution.

Fiberglass Insulation

Fiberglass has tiny fibers that tend to aerosolize during installation and over time. These small fibers affect air quality. If pests begin nesting in the materials, their urine and feces will also affect air quality.

Lower-quality fiberglass materials contain additives like formaldehyde, which are considered VOCs that can affect human health. Higher-quality fiberglass materials, like Greenguard-certified fiberglass insulation products, are safer for air quality and meet higher emission standards.

How to Choose Rigid Foam vs. Fiberglass Insulation

The size of your project, climate, and budget will ultimately determine the types of insulation you choose. Rigid foam has the highest R-value, is more durable, and is moisture-resistant. It is often selected as the insulator of choice for basements.

Fiberglass is less expensive and easier to install. It has fewer insulation properties than rigid foam, but it still works well and is often used as insulation between walls, in the wall framework, ceiling, joists, and roof rafters in a home.