Envelope and protection · Insulation
Foamed Glass Aggregate
Describes a placed, compacted granular insulation — a fill rather than a board — and the ground, drainage and compaction questions that follow from being laid like a sub-base.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What foamed glass aggregate is
Foamed glass aggregate is made by grinding recycled glass, mixing it with a foaming agent and firing it so the melt expands into a closed-cell foam. The cooled material fractures into angular lumps, which is what is delivered: a light, hard, sharp-edged gravel with sealed cells inside each particle and open voids between the particles. It is supplied loose or in bulk bags and is graded by particle size.
It is used by being placed and compacted in layers, in the way a granular sub-base is placed, rather than laid to a joint pattern the way a board is. That single difference is the whole of the material's character. There is no board layout, no cutting and no fitting tolerance; instead there is a placing depth, a compaction regime, a settled thickness that differs from the loose delivered thickness, and a need to confine the layer so it stays where it is put.
The reason it is raised is that one placed layer can do the work of two: a granular layer beneath a slab or a paving build-up that is also the insulating layer, without a separate board on top of a separate sub-base. Whether it can do that in a particular construction depends on the ground, the loading, the drainage and the design of the whole build-up, and that is a judgement for the engineer and the designer rather than a property of the aggregate.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Foam glass gravel
- Foamed recycled glass gravel
- Glass foam gravel
- Insulating glass gravel
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- A combined fill and insulating layer placed and compacted beneath a ground-bearing floor slab.
- Lightweight fill over weak, compressible or made ground where the weight of a conventional fill is the problem.
- Insulating and free-draining backfill against below-ground walls and around perimeter details.
- Frost-protection and levelling layers in external hardstanding and paving build-ups.
- Lightweight fill to podiums, planters and landscape structures where load on the structure below governs.
- Fill to buried tanks, chambers and service runs where a light, free-draining granular material has been specified.
Consideration
Where it is commonly considered
- Where the ground or the structure beneath cannot take the weight of a conventional granular fill.
- Where a designer wants a single placed layer rather than a sub-base plus a separate board.
- Where the layer needs to drain freely and the water has a designed route away from it.
- Where an engineer has assessed the ground and set out placing depths and compaction for the actual build-up.
- Where site access and plant allow the material to be placed and compacted in layers rather than tipped and levelled.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- In any loaded position, since what a compacted granular layer can carry is an engineering judgement about the ground, the layer and the construction over it.
- Where the ground conditions have not been investigated, which is the question this material's use turns on.
- Where the layer cannot be confined at its edges and could migrate into adjoining ground, voids or drainage.
- Where the compaction plant and the placing regime the supplier describes cannot actually be used on this site.
- Where groundwater, drainage and the route water takes away from the layer have not been designed.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Thermal behaviour
- Thermal behaviour is the reason the material is raised, but it is a topic here rather than a figure. Any value belongs to the supplier's documentation for the exact grade and compaction state, and to a calculation for the actual build-up.
- Moisture behaviour
- Water drains through the voids between particles while the particles themselves are closed-cell. Whether that is useful depends on where water arrives from and where it is designed to go, which is a drainage question about the whole site.
- Installation dependency
- The material is placed in layers and compacted, so the placed depth, the number of layers, the plant used and the settled thickness are the installation subjects. Loose delivered volume and finished compacted thickness are not the same thing.
- Substrate dependency
- What the layer is placed on — the formation, the subgrade, a membrane or a separating fabric — governs how it performs and whether it stays where it is placed. That is established by ground investigation, not by the material.
- Weathering and exposure
- The material is inorganic and does not rot, but a placed layer sits open to the weather during construction and can be trafficked, flooded or contaminated before it is covered, all of which are site conditions rather than properties.
- Repairability
- A placed layer is not repaired locally in any ordinary sense; a disturbed area is re-placed and re-compacted, and anything built over it has to come off first.
- Documentation
- Ask for the grade and particle size, the supplier's placing and compaction guidance, the relationship between delivered and compacted volume, and a clear statement of what the material is intended to do in the build-up.
Exposure
Exposure and environmental conditions
- What the ground conditions are and whether they have been investigated before anything is placed.
- Where groundwater sits, how it moves and where water passing through the layer is designed to go.
- Whether the layer will be trafficked by site plant before it is covered, and what that does to it.
- How the layer is protected from contamination by mud, spoil and run-off during construction.
- Whether frost, flooding or standing water during the works has been allowed for in the programme.
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The formation the layer is placed on, prepared and checked by the engineer before placing begins.
- Whether a separating or filtering fabric is required beneath or around the layer to stop intermixing.
- The placing depth and compaction regime set out by the engineer and the supplier for this build-up.
- What is built directly over the layer, and how the load from it is spread into the layer.
- How the layer is confined at edges, at perimeter details and at penetrations so it does not migrate.
Appearance
Appearance and finish considerations
- The layer is buried in every normal use, so it contributes nothing to the appearance of the finished building or landscape.
- The level and regularity achieved in the compacted surface carries directly into whatever is built over it.
- Where the material is used near the surface in landscape work, its pale angular particles can appear at edges and in planting and are then a visible detail.
- Any change in build-up depth follows through to thresholds, external levels and the relationship between the building and the ground around it.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How the layer is expected to behave under the loading the finished construction will actually apply.
- Whether further settlement is expected after compaction, and who has assessed that.
- What would happen if fine material from the surrounding ground migrated into the layer.
- How water reaching the layer over time is expected to leave it.
- How the layer behaves at its edges, where confinement and loading conditions differ from the middle.
Upkeep
Maintenance and repair considerations
- What would have to be removed to reach the layer if a problem appeared in the construction above.
- How a disturbed area would be re-placed and re-compacted to the same condition.
- Whether the same grade would still be obtainable if part of the layer had to be replaced.
- What record exists of the placed depths, compaction and testing carried out during construction.
- Who would be asked to assess a floor or pavement that had settled over a placed granular layer.
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- A formation prepared and checked before any material is placed.
- Placing in layers to the depths set out rather than tipping and levelling in one go.
- The compaction plant and number of passes described by the supplier and the engineer for this material.
- Any separating or filtering fabric required between the layer and the ground.
- Confinement at edges and around penetrations so the layer holds its shape while work continues.
- Control of site traffic over the layer, and protection from contamination before it is covered.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The grade, particle size range and supply description for the material delivered.
- The supplier's placing, layer-depth and compaction guidance for the intended use.
- A clear statement of the relationship between delivered loose volume and compacted thickness.
- Any documentation describing what the material is intended to do within a build-up, and what it is not.
- Records of the depths placed, the compaction carried out and any testing done on site.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What does the ground investigation say, and how does it shape the use of a placed insulating fill here?
- Who has assessed the loading on this layer, and what is that assessment based on?
- What placing depth and compaction regime are intended, and can the plant on this site achieve them?
- Is a separating or filtering fabric needed between the layer and the ground, and who decided?
- Where does water reaching this layer go, and who has designed that route?
- How is the layer confined at edges and at perimeter details?
- How is site traffic over the layer being controlled before it is covered?
- Which parts of this build-up need confirming against local requirements, and who is confirming them?
Blind spots
Commonly overlooked points
- It is placed and compacted like a fill, not laid like a board, so the questions are ground questions rather than product questions.
- Delivered loose volume and finished compacted thickness are different numbers, and ordering on the wrong one is a common mistake.
- The layer has to be confined, or it migrates at its edges where it is least visible and most loaded.
- Contamination by mud and spoil during construction changes how the layer drains and is easy to allow to happen.
- Because the layer is buried, the site records of depth, compaction and testing are the only evidence that survives.
What this page does not do
- This entry states no thermal, structural or drainage value and makes no performance claim.
- What a compacted granular layer can carry, and whether it suits a given site, are engineering judgements about the ground and the whole build-up.
- Build Design Hub does not test, approve, certify, specify or recommend any foamed glass product, grade, placing method or ground build-up.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
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Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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