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Recycled foamed glass aggregate generally offers lower reference thermal conductivity and bulk density than expanded clay, while the cited reference table lists a higher compressive-strength range for expanded clay. Those figures come from different tests and are not a like-for-like product comparison. There is no universal cost winner: compare construction-grade products using delivered, installed costs and the thermal and structural design values for your project.
At a glance
| Name | Best for | Key spec | Indicative cost | Checked |
|---|---|---|---|---|
| Recycled foamed glass aggregate | Lightweight insulating fill; evaluate where reduced dead load and sub-slab insulation matter | Reference density 100–150 kg/m³; conductivity 0.060–0.080 W/(m·K) | Relative material-cost figure 5.3–5.9 in the 2022 reference; not a current delivered price | November 2026 |
| Expanded clay aggregate | Insulating fill where the specified aggregate grade meets the project’s thermal and structural requirements | Reference density 220–300 kg/m³; conductivity 0.065–0.120 W/(m·K) | Relative material-cost figure 5.9–12.8 in the 2022 reference; one 2025 local-market report gives about €49–62/m³ | November 2026 |
Prices checked November 2026.
Reference properties are category-level figures, not guaranteed product specifications. The relative cost figures have no universal market or delivered-price basis; the euro figure is specific to the market reported in the 2025 source. Sources: reference table for aggregate insulation-fill materials and 2025 local-market report.
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What are foamed glass and expanded clay aggregate?
Both are lightweight granular materials discussed for building insulation and fill, but their shared use does not make them interchangeable. Cellular or foamed glass aggregate is made from glass formed into a cellular material. Glavel describes its product as recycled-glass aggregate for lightweight fill and insulation, and GLAPOR describes cellular-glass gravel as load-bearing insulating bulk material. Expanded clay is a separate lightweight aggregate category; its properties vary by grade and intended use.
That distinction matters at the specification stage. “Expanded clay” can refer to products for different applications, including horticultural granules. A bag of garden LECA is not automatically suitable as construction fill beneath a slab. Specify a construction-grade product intended for the project and obtain its technical declaration, applicable test methods, and design values. Likewise, treat foamed-glass figures as product-specific: a named aggregate’s declared properties are more useful than a broad category range.
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For product examples, Glavel’s foamed-glass aggregate page identifies lightweight fill and insulation uses, while GLAPOR’s cellular-glass gravel document describes its material. These examples establish product descriptions, not a matched performance or price comparison.
In practice, compare products intended for the same job: a specified construction aggregate for a slab or foundation should be evaluated against another construction aggregate proposed for that assembly. Ask each supplier for current product documentation, the size or gradation offered, and any installation instructions relevant to the proposed use. Do not assume that a shared label such as “lightweight fill” means the products have the same load-bearing capacity, thermal performance, or installation requirements.
Which material insulates better?
The cited reference values generally favor cellular glass for insulation: it lists a lower thermal-conductivity range than expanded clay. Lower conductivity means less heat flows through a given thickness under comparable conditions. The reference ranges overlap, however, and should not be mistaken for guaranteed values for products you can buy.
| Reference property | Cellular glass | Expanded clay |
|---|---|---|
| Thermal conductivity, λ | 0.060–0.080 W/(m·K) | 0.065–0.120 W/(m·K) |
| Bulk density | 100–150 kg/m³ | 220–300 kg/m³ |
Values are from the 2022 reference table in Structural Details in Energy-Efficient Buildings; they are reference ranges, not a guaranteed specification for every product.
A useful way to think about thickness is the idealized relationship R = d ÷ λ, where R is thermal resistance, d is layer thickness in metres, and λ is conductivity in W/(m·K). Using only the cited category ranges, a 300 mm layer would yield roughly R = 3.75–5.0 m²·K/W for cellular glass and R = 2.5–4.6 m²·K/W for expanded clay. These are calculations from the reference ranges, not design ratings: actual results depend on the specific product’s declared conductivity, layer condition, moisture, and the assembly’s design method.
The chapter authors J. Leclerc and A.W. Zia state that at comparable density and compressive strength, expanded clay’s conductivity is notably higher than cellular glass’s; they also say expanded clay requires a thicker layer for equivalent slab-on-ground thermal performance in the comparison they discuss. That is a relevant design warning, not a rule that every expanded-clay product or floor assembly will need a particular extra thickness. Start with the required thermal resistance, then size the layer from the product value accepted by the project designer.
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To make the insulation comparison useful, first identify the assembly target rather than choosing an arbitrary layer thickness. Ask the designer which thermal resistance or whole-assembly performance is required, then use the declared conductivity for the exact grade and thickness under consideration. If the supplier offers more than one gradation or product grade, confirm that the stated thermal property belongs to the offered one. For a slab-on-ground design, the floor assembly—not just the loose aggregate layer—determines the relevant performance, so a simple R-value calculation is a screening tool rather than a complete assembly assessment.
So, which insulates better? On the cited category ranges, foamed or cellular glass is generally the better starting point for lower conductivity at a given thickness. Expanded clay may still be suitable where its specific product value and the available layer depth meet the design target. The overlap between the ranges is one more reason not to apply the category-level comparison as a verdict on every product.
Which aggregate is stronger?
The reference table gives expanded clay a higher granulate compressive-strength range: 700–2,500 kPa, compared with 400–1,600 kPa for cellular glass. That does not establish expanded clay as universally stronger in a real installation. The source cautions that there is no uniform standard for determining properties of these aggregate insulation fills, so results from different materials and methods may not be directly comparable.
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Most importantly, a granulate compression result is not automatically the allowable design pressure for a slab or foundation. The project designer needs the relevant design value under the applicable standard and must account for the fill’s installation and the loads in the assembly. If a supplier provides only a headline strength without method and conditions, request clarification before treating it as a structural basis.
When asking “which is stronger?”, distinguish a reported test result from the value the engineer can use in design. A useful comparison records the test method, particle-size range, strain or compaction at testing, moisture condition, and the reported result for each proposed product. If those conditions do not match, the numbers describe different tests rather than a controlled head-to-head result. Ask the supplier or designer to explain how the declared value relates to the design allowable for the actual application.
The same caution applies to installation: a product tested at a stated compaction or strain condition should not be assumed to perform identically when installed under different conditions. Confirm the required placement procedure and the design assumptions together. A broad category range can help flag questions for the project team, but it cannot replace the product-specific documentation and structural review.
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How much does each material cost?
The available evidence does not establish a universal cost winner. The 2022 technical chapter gives source-specific relative material-cost figures of 5.3–5.9 for cellular glass and 5.9–12.8 for expanded clay. Those numbers are not current delivered prices, and the excerpt does not provide a universal currency or project basis. A 2025 article reports expanded clay aggregate at about €49–62 per cubic metre based on local-market data; it should not be extrapolated to another region, year, grade, or delivery arrangement.
A bag price or material-only price can also obscure the project comparison. If one material needs a thicker layer to deliver the required thermal resistance, comparing equal volumes may not compare equivalent performance. Freight, order volume, installation labor, compaction, excavation, disposal, and any separate drainage or insulation layers can also affect the installed cost.
Request quotes for equivalent scopes and compare delivered and installed cost for the designed assembly, not just the price per cubic metre. State the intended use, product grade, volume, delivery location, required thickness or thermal target, installation assumptions, and whether the quote includes placement or compaction. Then confirm the structural design value and thermal declaration for each offered product. The relevant source figures are in the 2022 Springer chapter and the 2025 local-market article.
Which is cheaper? The available price figures cannot answer that for a particular job. The relative-cost intervals are source-specific figures, not a common unit-price schedule, and the €49–62/m³ figure is tied to local-market reporting in 2025. A contractor’s quote can also include different scopes: one may cover delivered material only, while another includes placement or compaction. Normalize those items before drawing a conclusion.
A practical quote comparison should use the quantity required to reach the same thermal target and satisfy the same structural design assumptions. Include the material volume implied by the layer thickness, delivery charges and order size, installation labor, compaction where required, excavation and disposal, and any drainage or separate insulation work included in the design. Ask suppliers to state exclusions as well as included work. This gives the project team a more meaningful installed-cost comparison than a price per bag or nominal cubic metre.
Even then, a lower installed quote is not necessarily an equivalent offer. Check that each option meets the required thermal resistance with its proposed thickness and provides documentation acceptable for the intended loads and assembly. The chapter’s slab-on-ground comparison indicates that expanded clay may need more thickness for equivalent thermal performance at comparable density and compressive strength; that may affect quantities, but does not establish a total-cost result for every project.
Why does density matter for a slab or foundation?
The reference table lists cellular glass at 100–150 kg/m³ and expanded clay at 220–300 kg/m³. On those category figures, cellular glass is the lighter fill, which can matter for transport, handling, and the dead load contributed by the fill. Do not assume a particular delivery weight from these values alone: density may be reported for loose or compacted material, and product grades vary.
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Glavel lists dry bulk density of 9–10 lb/ft³ for its product. That is a manufacturer figure for a named material, not a replacement for the broad reference range or a specification for every foamed-glass aggregate. When comparing quotations, verify whether the stated density is dry bulk, loose, or compacted and whether the two suppliers use comparable definitions.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteDensity is relevant to more than transport. It affects the weight of fill included in the assembly and can influence how a project team thinks about handling and quantities. But a lower density is not proof of higher strength, and density alone does not establish whether a layer is appropriate beneath a slab. The values in the table are category-level references; confirm the product’s declared density and the state in which it is reported before using it in estimates or design documents.
For a delivery estimate, make sure the supplier and contractor are discussing the same volume basis. A loose-volume figure, a dry bulk density, and a compacted installed condition are not necessarily interchangeable descriptions. If a quote is based on a bulk volume, ask how it relates to the amount needed after placement. The comparison should use the supplier’s stated basis rather than an assumed conversion between different material states.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should you compare the materials for a project?
Use the design requirement first, then compare products that can meet it. This avoids choosing by a single broad-range number or a material-only price that does not represent the finished assembly.
- Define the application and loads. Confirm whether the material is intended as sub-slab fill, insulation, or another use, and give the designer the relevant loads and assembly requirements. Do not treat a product described as lightweight fill as automatically approved for every structural condition.
- Set the thermal target. Establish the required thermal resistance or assembly performance. Ask for the proposed product’s conductivity value and its basis, then calculate thickness using the method accepted for the project. If relying on an R-value per inch, confirm that it applies to the actual grade and design conditions.
- Obtain comparable strength declarations. Request the test standard, gradation or particle-size range, compaction or strain condition, moisture conditioning, and the design value the project team can use. A bare kPa or psi figure is not enough for an apples-to-apples comparison.
- Clarify density and installation state. Ask whether the quoted density describes loose, dry bulk, or compacted aggregate. Confirm installation and compaction requirements with the supplier and designer; do not assume a density or strength value measured in one state applies in another.
- Normalize the quote. Compare the cost to supply and install the quantity needed for the same project outcome. Include freight, volume, thickness, labor, excavation and disposal, and any separate drainage or insulation work in the scope.
- Check the product and application. Specify construction-grade aggregate, not a garden product simply because it is also called LECA. Review the supplier’s current documents and have the project designer confirm suitability for the assembly and applicable requirements.
This comparison is especially important where insulation, structural performance, and cost pull in different directions. The reference data point toward lower conductivity and density for cellular glass, while the category strength range is higher for expanded clay; neither observation substitutes for a product-specific design review.
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For a fair request for quotation, send both suppliers the same project information: the application, delivery location, approximate volume, thermal target or required thickness, design loads, and the installation scope you want priced. Ask them to identify the exact grade offered and attach current technical documentation. If a proposal does not state whether placement, compaction, or freight is included, clarify those items before comparing totals.
Keep the performance and price comparison on the same basis. For example, do not compare one material at the thickness needed to meet the design target with another at a thinner, lower-cost thickness that does not meet it. Likewise, do not treat a category-wide strength range as a design value for a named product. The point of the process is to narrow options to products that can satisfy the same job, then compare their costs and practical requirements.
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GLAPOR cellular-glass gravel and Glavel foamed-glass aggregate are examples of named construction products in this category. Expanded clay aggregate is also sold in grades intended for different uses. Their product pages or documents are useful starting points for identifying materials and requesting specifications, but they do not establish which option will perform better in a specific project or what it will cost delivered to a given site.
What does the evidence not establish?
The cited sources do not provide a matched independent test of the named foamed-glass and expanded-clay products using identical gradation, compaction, moisture, and loading. They also do not establish current delivered pricing for a defined project location and volume. Therefore, the ranges are useful for screening and asking better supplier questions, but they cannot by themselves determine which material is right or cheaper for a particular slab.
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The evidence also does not support a general environmental ranking. Recycled-glass content is a relevant product attribute, but a fair embodied-impact comparison would require comparable product-specific environmental declarations and consistent system boundaries. Do not infer a whole-life environmental winner from the recycled input or aggregate category alone.
The figures should therefore be read as orientation, not as a substitute for current product data. The 2022 chapter provides reference ranges and a relative-cost table; the 2025 article’s expanded-clay price reflects a particular local market; and Glavel’s published strength, density, and R-value are product-specific manufacturer values. Each answers a different question, with different scopes and conditions. Keep those distinctions visible when making a specification or budget decision.
Frequently Asked Questions
Which material insulates better, foamed glass or expanded clay?
The cited reference ranges show lower thermal conductivity for cellular glass overall: 0.060–0.080 W/(m·K), compared with 0.065–0.120 W/(m·K) for expanded clay. These are category references, not guaranteed values for every product; compare the declared value and required thickness for the exact grade.
Which aggregate is stronger?
The reference table lists a higher granulate compressive-strength range for expanded clay (700–2,500 kPa) than cellular glass (400–1,600 kPa), but the methods are not standardized across these materials. The figures do not establish a universal strength winner. Compare product-specific test conditions and design values.
Which is cheaper for a sub-slab project?
There is no universal cost winner established by the cited figures. The relative costs in the 2022 chapter are source-specific, and the approximately €49–62/m³ expanded-clay figure reported in 2025 applies to a local market. Request comparable delivered-and-installed quotes for the quantity and thickness each design requires.
Is construction-grade expanded clay the same as garden LECA?
Not necessarily. The name covers products for different uses, so a horticultural bag should not be assumed to meet construction-fill or structural requirements. Check that the supplier’s declared grade is intended for the building application.
Can I use the R-1.7-per-inch figure to size any foamed-glass layer?
No. That is a product-specific manufacturer figure, not a universal value for foamed-glass aggregate. Confirm the applicable grade, basis, and design conditions with the supplier and project professional.
The Bottom Line
For insulation-focused sub-slab fill, the cited category data generally favor foamed glass on conductivity and density. Expanded clay’s listed granulate-strength range is higher, but the figures are not matched tests and do not settle the design choice. Neither the relative-cost table nor one local-market price identifies a universal cheaper option. Compare construction-grade products on declared thermal and structural values, the thickness needed for the same design outcome, and delivered-and-installed cost.
