What is Foam Glass?

Foam glass aggregate, also known as foamed glass gravel, or cellular glass. Whatever you call it, it’s a lightweight, thermally insulating building material made entirely from post-consumer recycled glass. Typically used as a substrate aggregate, foam glass gravel has wide and versatile uses. Because cellular glass is derived from recycled glass, it’s petroleum and chemical-free. This makes it one of the greenest building materials on the market.

Foam glass aggregate can reduce the embodied carbon of a building, but it can also reduce job site complexity. Not to mention create solutions to some of the construction industry’s most pervasive issues.

Glavel Process
Photo: Oliver Parini

History of Foam Glass

The discovery of foam glass was in the early 20th century by a Soviet scientist, but the material did not generate demand until later in the 20th century. High performance buildings and carbon footprint awareness were a catalyst for the increased popularity of foam glass applications.

This coincided with the growth of the recycling industry, and the need for municipalities to create valuable use of recycled glass. With foam glass being a lightweight product that also was capable of thermal insulation, it quickly became the product of choice for Passive House design and other high performance applications.

Foam glass aggregate’s popularization in the late 20th century has led to uses in Germany, Norway, Austria, and other countries. Applications include green roof fill, slope stabilization, soccer stadiums, rooftop gardens, parking garage substrate, subslab insulation, and lightweight concrete fill. Because foam glass is so lightweight, its usefulness is found in all walks of the construction industry.

Physical Characteristics

Lightweight – 9.8 pounds per cubic foot

Load bearing – 116 psi compressive strength

Thermal insulation – R1.7 per compacted inch

Self interlocking – Distributes loads evenly

Produced from recycled glass – Categorized as ‘clean fill’

Closed cell – Acts as a drainage layer

Frost heave resistant – Will prevent cycles of freeze and thaw

Non combustible – Will not burn, nor propagate fires

Water resistant – Closed cell structure facilitates runoff

Moundable – Pieces allow for mounds and up to 45° slopes

Inert – Prevents rodents, termites, bacteria, and rot


Production of foam glass begins with post-consumer recycled glass. Glass of all colors is cleaned to <1% impurity to maximize product efficacy. The glass is then mechanically ground into a fine powder and mixed with an environmentally-friendly foaming agent before being kiln fired. As the glass powder and foaming agent reach 1650 degrees Fahrenheit in the kiln, a chemical reaction occurs, causing the glass to foam up and create closed-cell micropores. These micropores provide the thermal insulation and strength to the foam glass.

Upon exiting the kiln, the foam glass cools and hardens into a substance that resembles lava rocks. At the end of the kiln, the foamed sheet tumbles off of the kiln’s conveyor belt and breaks under its own weight into aggregate pieces that are suitable to build with. With an additional annealing process, foam glass also be made into a board product, used for creating edge insulation systems and preventing moisture entry into foundations.

Foam Glass Uses

Sub Slab Insulation

As a subslab insulator, foam glass is capable of replacing both rigid foam board and backfill gravel. With thermal insulation, closed cell structure, and compressive strength of 116 psi, foam glass is water resistant, fireproof, pest and rot proof, and frost heave resistant.

Green Roof Fill

Weighing only 9.8 pounds per cubic foot, foam glass is 90% lighter than traditional gravel aggregate fill. A foam glass roof installation reduces building strain and increases the ability to build creative landscapes. Foam glass is a frictional product, when compacted it can be angled in slopes up to 45 degrees.

Embankment Stabilization

Foam glass can act as a fill for mechanically stabilized walls. Once compacted, foam glass will not shift or erode, nor will it decompose and compact over time. This ensures that walls are held stable over time with minimal risk of failure.

Example Install

On the left is a traditional install method with gravel and foam board. The gravel fills the bulk, while the foam board provides insulation. This method achieves R20 insulation with 4” of EPS foam sitting atop the gravel layer.

In a typical 1,000 square foot installation, a 10” layer of gravel weighs 104,000 pounds.

On the right is an install with Glavel. Taking the place of both gravel fill and foam board, 12” of Glavel fills the bulk and provides  R20 insulation. Pipes can be embedded within Glavel layers, and are also given the benefit of the thermal insulation to prevent freezing.

In a typical 1,000 square foot install, a 10” layer of Glavel weighs 8,000 pounds.

For more information about Glavel foam glass, resources can be found at Glavel’s Learn page.