Comparison of Fibreglass and Rigid Foam Insulation

21 October 2020

Home improvement stores have fibreglass and rigid foam for residential use. While there is some overlap of applications, each tends to have its own best areas of usage. For example, rigid foam is commonly used for basement wall insulation because it resists the moisture given off by basement masonry walls. Fibreglass batts, on the other hand, are typically used to insulate between wall studs, floor and ceiling joists, and roof rafters. Here is an in-depth comparison of the two.

Definitions of Fibreglass and Rigid Foam Insulation

These are panels of rigid foam, sold in 4-by-8-foot sheets in a variety of thicknesses from 1/4 to 2 inches. It is also sold in large bundled panels, joined at the ends accordion-style for use as an exterior sheathing material. Rigid foam insulation uses one of three materials: high-quality, green-friendly polyisocyanurate; extruded polystyrene; or expanded polystyrene.

Fibreglass insulation consists of tightly packed long rolls or individual batts (single unrolled pieces) of spun fibreglass fibres. In contrast to rigid foam, fibreglass is soft and flexible. The millions of air pockets in fibreglass insulation provide a superior thermal barrier but fibreglass can also trap moisture, leading to mould and mildew growth.

Uses for Rigid Foam Insulation

Exterior Continuous Insulation: Rigid foam can be used as sheathing—a continuous layer of insulation on wall exteriors attached before the installation of house wrap and siding.

Basements: Indoors, rigid foam is the material of choice on walls where the insulation will be touching masonry. Mostly, this means the exterior walls in basement build-outs, except for daylight-facing walls, such as are found in walk-out basements. Rigid foam stands up to moisture better than fibreglass.

Radiant Heat Floors: Rigid foam is used as a thermal barrier underneath radiant floor heating systems.

Rim Joists: Rim joists, which are around the edge of a foundation, can be insulated with rigid foam blocks cut to fill the space and caulked in place. If concrete blocks have open cavities, they should be stuffed with loosely packed fibreglass insulation.

Uses for ​Fibreglass Insulation

Exterior Walls: Rolled fibreglass insulation’s main duty 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 remodelling projects when wall surfaces are not yet installed, faced fibreglass batts are the insulation of choice.

Attics: Thick batts or rolls of fibreglass do an outstanding job of insulating the attic by preventing heat loss through the ceiling and roof. Faced batts are often installed between joists, unfaced batts can be laid across the tops of joists.

Floors: Joist cavities above unheated crawlspaces or unheated basements are often insulated with fibreglass batts. Between two- or three-story homes, fibreglass insulation in the floors will deaden sound transmission.

Rim Joists: The joist spaces above foundations can be filled with loosely packed fibreglass insulation to prevent heat loss. Sometimes, it is in conjunction with rigid foam, with fibreglass used to fill the void spaces at the top of concrete block walls, and blocks of rigid foam then used to block off the joist cavities above the foundation walls.


Wallender, L. (2020, January 07). Comparing Fiberglass Insulation and Rigid Foam Insulation. Retrieved October 14, 2020, from The Spruce:


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