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Marmox Thermoblock and an aircrete thermal block are not the same product. Marmox Thermoblock is a structural thermal break, normally installed as a carefully designed course at a wall-to-floor junction or another linear thermal bridge. A “thermal block” usually means an aircrete masonry unit that forms the wall or foundation itself. Marmox explicitly says, “Marmox Thermoblock is NOT an aircrete thermal block.”
That distinction determines what you specify, how you install it and which performance figures matter. Use Thermoblock to interrupt heat flow through a junction without abandoning the wall system above it; use an aircrete block when you need the masonry body of the wall or foundation. The sections below compare construction, strength, thermal data, moisture behaviour, approvals and installation limits so the two products are not selected as interchangeable alternatives.
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At a glance
| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Marmox Thermoblock | A structural thermal break at a wall/floor junction, threshold, parapet or similar detail | XPS core with load-carrying epoxy/polymer-concrete columns and reinforced faces; average conductivity 0.05 W/mK; mean compressive strength 9.0 N/mm² | November 2026 |
| Aircrete thermal block (representative H+H Celcon grades) | Constructing the wall or foundation, with grade selected for strength, moisture and thermal requirements | Autoclaved aerated concrete; above-ground conductivity 0.11–0.18 W/mK for listed grades; strength 2.9–8.7 N/mm²; foundation products publish 0.24–0.29 W/mK | November 2026 |
The table is a functional comparison, not a ranking of one universal “best” block. The right row depends on whether the missing performance is at a junction or across the wall area.
Are Marmox Thermoblocks and thermal blocks the same product?
No. They solve different parts of the building envelope. Marmox Thermoblock is a narrow, load-bearing thermal-break element. An aircrete thermal block is a masonry unit used repeatedly to build a wall or, in a suitable foundation grade, below-ground masonry.
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The terminology causes confusion because both products are marketed in relation to thermal performance. Marmox describes ordinary thermal blocks as lightweight construction blocks for building walls in its FAQ material, while its Thermoblock is intended to stop a linear bridge where a wall meets a slab, floor or other construction. Replacing one with the other without redesigning the junction can leave a structural, moisture or compliance problem.
What is Marmox Thermoblock made to do?
Thermoblock combines a low-conductivity XPS insulation core with load-carrying epoxy/polymer-concrete columns and reinforced polymer-concrete faces. The insulating core reduces the heat path through the junction; the columns and faces transfer the wall load. It is therefore a composite structural component, not a conventional aerated-concrete masonry unit.
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Marmox’s 2025 specification gives an average thermal conductivity of 0.05 W/mK and a mean compressive strength of 9.0 N/mm². The NBS listing records 9 N/mm² minimum strength. Those figures describe the Thermoblock product, not the U-value of a completed wall or the psi-value of a junction.
Standard UK dimensions are 600 mm long, with widths of 100, 140 or 215 mm and thicknesses of 65 or 100 mm. Marmox identifies the 65 mm version as its standard UK block and the 100 mm version as an option where more insulation depth is required. Width must line up with the wall system and the approved detail; a wider or thicker block is not automatically suitable simply because its nominal insulation is greater.
Marmox states that Thermoblock is impermeable to water and suitable for damp conditions. That can be valuable at a ground-level junction, but it does not remove the need for the project’s damp-proofing, waterproofing, drainage and ground-gas design. Those surrounding details remain the designer’s responsibility.
What does “thermal block” usually mean?
In ordinary construction language, a thermal block is an aircrete masonry unit: autoclaved aerated concrete containing a network of air voids that reduces density and heat flow. The units make up the wall or foundation in a selected blockwork system. “Thermal block” is not one fixed specification, so the grade, thickness, moisture condition and application must be identified before its figures can be used.
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H+H’s Celcon block data illustrates the range. Listed above-ground grades publish conductivities of 0.11 W/mK for Solar, 0.15 W/mK for Standard and 0.18 W/mK for High/Super Strength. Published compressive strengths across the grades run from 2.9 to 8.7 N/mm². A lower conductivity grade may offer better insulation, while a higher-strength grade may be needed for the loads; the choice is a design trade-off rather than a universal upgrade.
Celcon face dimensions are commonly 440 × 215 mm, with widths from 75 to 355 mm depending on the product. For below-ground work, H+H publishes a separate foundation-block range. Its listed conductivity values are higher, about 0.24–0.29 W/mK, because moisture content and service conditions alter thermal performance. A walling block selected from an above-ground table should not be assumed to be a foundation block.
How do the materials and performance figures compare?
| Characteristic | Marmox Thermoblock | Aircrete thermal block (H+H Celcon examples) |
|---|---|---|
| Primary job | Interrupt a linear thermal bridge at a designed junction while carrying wall load | Provide the masonry and insulation body of a wall or specified foundation |
| Construction | XPS core, epoxy/polymer-concrete columns and reinforced polymer-concrete faces | Autoclaved aerated concrete |
| Published thermal conductivity | 0.05 W/mK average (Marmox UK, 2025) | 0.11 W/mK Solar, 0.15 W/mK Standard and 0.18 W/mK High/Super Strength above ground; 0.24–0.29 W/mK for listed foundation products |
| Published compressive strength | 9.0 N/mm² mean; NBS listing gives 9 N/mm² minimum | 2.9–8.7 N/mm² depending on grade |
| Typical dimensions | 600 mm long; 100/140/215 mm wide; 65 or 100 mm thick | Common 440 × 215 mm face; widths 75–355 mm depending on product |
| Water behaviour | Marmox describes the product as impermeable and suitable for damp conditions | Moisture changes published below-ground conductivity; foundation grades are specified separately |
| Certification/design position | BBA 10/4778 and ETA 20/0744 are cited by Marmox; it is not designed as a conventional Eurocode 6 masonry unit | Celcon products are BBA-certified aircrete masonry products with grade-specific applications |
These numbers are not interchangeable “better” scores. Conductivity (λ) is a material property, while a wall U-value depends on thickness, mortar, cavities, insulation, surface resistances and moisture. A junction psi-value also depends on geometry and the adjacent construction. A low-lambda Thermoblock can improve a junction without changing the U-value of the wall above it; an excellent wall block cannot, by itself, guarantee a good wall-to-floor junction.
When should a project use Marmox Thermoblock?
Choose Thermoblock when the wall system is already defined and the design needs a load-bearing thermal break at its base or another narrow junction. Typical locations include the first course above a floor or slab, a threshold, a parapet or a similar connection where a continuous masonry section would create a cold bridge.
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- the block width matches the wall and insulation build-up;
- the supporting substrate is level, sound and prepared for the approved bedding detail;
- the applied loads are within the product and project design limits;
- fire, damp-proofing, waterproofing and cavity or insulation continuity are resolved;
- the chosen detail has the required structural and thermal assessment for the project.
Marmox references BRE-accredited junction details and use in SAP/SBEM or DEAP/NEAP assessments. Ask the designer or energy assessor to use the assessed junction rather than substituting a generic wall value. The NBS/BIM listing and the manufacturer’s current technical documents are useful records for the specification and building-control submission.
How should Marmox Thermoblock be installed?
Installation must follow the manufacturer-approved detail; the internal load-carrying columns have to remain in their designed orientation. The following sequence captures the critical controls, but it does not replace the project drawings or structural design.
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- Prepare and level the support. Build on the level, prepared substrate identified by the detail. Correct unevenness before laying the block rather than forcing the block to bridge a hollow.
- Set the width and alignment. Use the Thermoblock width specified for the wall. Keep the wall above aligned with the block so loads pass through the intended faces and columns.
- Lay the approved course. Marmox says to use one course in load-bearing applications. Do not stack multiple Thermoblock courses unless a project-specific, approved design expressly permits it.
- Keep the orientation correct. Do not lay the blocks on their side. The load-carrying columns must remain vertical as designed.
- Butt adjacent ends. Bring ends together and, where a continuous barrier is required, seal abutting ends with Marmox MSP360 in accordance with the manufacturer’s instructions.
- Build the wall and adjoining insulation to the detail. Check that the wall, cavity insulation, floor insulation and damp-proofing remain continuous at the junction.
- Control cutting. Only cut lengths in the ways allowed by Marmox’s documentation. Do not assume a cut block, an altered column or an unusual orientation retains the published structural performance.
Record the product, dimensions, batch information where required, installation orientation and any approved deviations. If the substrate, wall width or loads differ from the design, stop and obtain a revised detail rather than improvising on site.
When is an aircrete thermal block the better choice?
Use an aircrete block when the product is intended to form the wall or foundation itself. Start with the load path and exposure, then select a grade whose strength, conductivity, dimensions and application match the design.
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- Establish the design load. Choose a grade within the required compressive-strength range. Do not select solely by the lowest lambda if the wall carries greater loads.
- Set the thermal target. Use the declared conductivity for the selected grade and moisture condition in the wall calculation. Do not treat the block’s lambda as the whole-wall U-value.
- Check dimensions and interfaces. Confirm width against the cavity, insulation, openings, floor zones and details at corners and junctions.
- Verify fire, moisture and compliance requirements. Use the product’s certification and grade-specific application data, then have the complete wall and foundation detail reviewed by the competent designer.
An aircrete wall can still need additional insulation or a separate junction treatment to meet the project’s energy design. Conversely, adding Thermoblock at the base of a wall does not turn an unsuitable walling grade into a suitable one.
Why can’t the conductivity figures be compared as wall U-values?
Because λ describes heat flow through a material under stated conditions, whereas U-value describes heat flow through the complete layered construction. Thickness, mortar joints, cavities, insulation, surface resistances and moisture all affect the assembly result. The psi-value of a junction adds the effect of the geometric connection between elements.
For Thermoblock, the relevant question is usually how the approved junction changes the linear thermal bridge. For an aircrete block, the question is how the selected block contributes to the wall or foundation build-up. Use the manufacturer’s assessed details and the project’s SAP, SBEM, DEAP or NEAP calculation instead of multiplying a single conductivity number into a whole-building conclusion.
What are the most common specification mistakes?
- Treating “thermal block” as a product name. State the manufacturer, grade, dimensions, strength, conductivity basis and intended location.
- Using an aircrete wall block as a junction break. A wall block may continue the masonry but does not automatically provide the engineered structural and thermal-break function of Thermoblock.
- Using Thermoblock as ordinary walling. It is a composite junction element, not a general replacement for a Eurocode 6 masonry unit.
- Stacking Thermoblocks. Marmox’s load-bearing guidance calls for one course unless a separately approved design says otherwise.
- Turning a block on its side. This can change the position of the internal load-carrying columns and invalidates the intended load path.
- Ignoring moisture condition. Below-ground aircrete figures differ from above-ground values; foundation products must be selected for that exposure.
- Assuming every cut is structural. Follow the cutting limits and obtain approval for any altered geometry.
- Leaving the ends unsealed where continuity is required. Use the specified MSP360 sealing detail at abutting Thermoblock ends.
- Submitting only a lambda value to building control. Provide the complete wall or junction detail, structural information, certification and the relevant energy calculation.
How can a designer document the choice?
A robust specification names the product family, exact size, intended position and design function. For Thermoblock, include the manufacturer’s 2025 specification, the selected 65 or 100 mm thickness, width, one-course arrangement, orientation, end-sealing requirement and the approved junction reference. Cite the relevant BBA/ETA information and the assessed psi-value or junction detail used in the energy model.
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For aircrete, name the Celcon or other specified grade, width, declared conductivity, compressive strength, wall or foundation application and moisture condition. Include the blockwork, insulation, cavity, damp-proofing, fire and support details that make up the complete assembly. Building-control acceptance and structural adequacy remain project-specific and should be confirmed by a competent designer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which product answers the actual problem?
If the problem is heat loss concentrated at the wall-to-floor line or another narrow junction, start with Marmox Thermoblock and its approved detail. If the problem is selecting the masonry that carries and encloses the building, start with an aircrete thermal-block grade. In many projects the two functions coexist: aircrete forms the wall, while a dedicated Thermoblock addresses a junction.
Make the decision from the drawings, not from the word “thermal” in a catalogue. Confirm load path, width, moisture exposure, fire strategy, whole-assembly energy calculation and the manufacturer’s current installation instructions before ordering materials.
Frequently asked questions
Can an aircrete thermal block replace a dedicated thermal-break detail?
Usually not as a direct substitution. The aircrete unit may provide the wall’s thermal mass and insulation, but the junction still needs a designed solution for its geometry, loads and linear thermal bridge. Substitution should be accepted only through a revised structural and energy detail.
Are all thermal blocks suitable for below-ground construction?
No. Below-ground moisture changes thermal performance, and manufacturers publish separate foundation products and values for that environment. Select a foundation-rated block and coordinate it with waterproofing, drainage and structural requirements rather than reusing an above-ground grade by habit.
Does the 100 mm Thermoblock always give a better result than the 65 mm version?
It provides more insulation thickness, according to Marmox, but it may not match every wall width, threshold or load path. The wider option is appropriate only when the approved junction, alignment and surrounding build-up are designed for it.
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What should be included in a building-control submission for Thermoblock?
Include the exact product and dimensions, the structural support and loading detail, the installation orientation, the end-sealing requirement, relevant BBA/ETA documentation and the assessed thermal-junction information used in the energy calculation. Also show how damp-proofing, waterproofing, fire provisions and adjacent insulation continue through the junction.
Frequently Asked Questions
Can an aircrete thermal block replace a dedicated thermal-break detail?
Usually not as a direct substitution. The aircrete unit may provide the wall’s thermal mass and insulation, but the junction still needs a designed solution for its geometry, loads and linear thermal bridge. Substitution should be accepted only through a revised structural and energy detail.
Are all thermal blocks suitable for below-ground construction?
No. Below-ground moisture changes thermal performance, and manufacturers publish separate foundation products and values for that environment. Select a foundation-rated block and coordinate it with waterproofing, drainage and structural requirements rather than reusing an above-ground grade by habit.
Does the 100 mm Thermoblock always give a better result than the 65 mm version?
It provides more insulation thickness, according to Marmox, but it may not match every wall width, threshold or load path. The wider option is appropriate only when the approved junction, alignment and surrounding build-up are designed for it.
What should be included in a building-control submission for Thermoblock?
Include the exact product and dimensions, the structural support and loading detail, the installation orientation, the end-sealing requirement, relevant BBA/ETA documentation and the assessed thermal-junction information used in the energy calculation. Also show how damp-proofing, waterproofing, fire provisions and adjacent insulation continue through the junction.
