Marmox Thermoblock: The Cold-Bridge Block Most Builders Will Try to Skip

Your structural engineer's drawings don't mention thermal blocks. Your builder hasn't priced for them. The drawings show standard 100mm dense concrete blocks running straight off the foundation, course one of the cavity wall, just like every extension on the street. Build it that way and the base of your insulated cavity wall sits on a slab of concrete with a thermal conductivity 23 times worse than the insulation above it. Heat flows out at that junction continuously through every winter the building is occupied. Approved Document L 2021 calls this a "reasonably avoidable" thermal bridge. The fix, for a typical rear extension, costs roughly £256 – £400 in materials. The block that does the job is Marmox Thermoblock.

What it is and what it's for

Marmox Thermoblock is a load-bearing masonry block with a built-in thermal break. Its core is extruded polystyrene (XPS), a closed-cell rigid foam. Running through that core are reinforced polymer cement columns that carry the structural load. The faces top and bottom are polymer cement reinforced with fibreglass mesh. The result is a single block that lays into a wall like any other masonry unit but conducts almost no heat through the wall-floor junction beneath it.

The composite block has a thermal conductivity (lambda) of

. A standard dense concrete block sits at - about 23 times worse. An aerated block (Thermalite, Celcon) is somewhere between the two at 0.11 to 0.19 W/mK. Marmox is what you reach for when an aerated block isn't good enough.

The block carries BBA Agrément Certificate

, valid in England, Wales, Scotland, and Northern Ireland. Compressive strength is 9 N/mm². It is not classified under Eurocode 6 (the European masonry code) because it isn't a homogenous block - its performance is certified by independent test rather than by density formula. Marmox Standard supports loadings consistent with up to a three-storey domestic structure. Above that you'd specify Heavy Duty.

The single line that matters for your extension: this block sits at the base of your cavity wall, forms a single course above the damp proof course (DPC), and breaks the otherwise continuous concrete path from outside to inside at floor level.

Why this matters under Approved Document L 2021

The 2021 update to Approved Document L (the conservation of fuel and power regulations for England) tightened the rules on thermal bridging. The text requires building fabric to be "continuous over the whole building envelope and constructed so that there are no reasonably avoidable thermal bridges." That language is the regulatory hook. Whoever signs off your SAP calculation must demonstrate compliance.

In June 2022, Accredited Construction Details (ACDs) were withdrawn from compliance use. ACDs were a set of generic junction drawings the industry had relied on for years to claim default psi-values without doing bespoke thermal modelling. They no longer count. Your SAP submission must instead use either:

1. Bespoke psi-values calculated for your specific junctions by a competent thermal modeller, or
2. Manufacturer psi-values certified through the BRE Certified Thermal Details scheme

If you use neither, SAP applies a default y-value of

across the entire heat loss perimeter of the building. That's a heavy fabric performance penalty. To compensate you'll need to thicken insulation elsewhere or upgrade glazing - both more expensive than just installing the right block at the wall base.

Marmox Thermoblock is BRE-certified for cavity wall junctions. The certified psi-values for the standard wall-floor junction sit between 0.03 and 0.07 W/mK depending on the detail. Using these certified values directly in SAP gives a far better fabric performance result than the default penalty, and the cost premium of the block pays back through reduced heating bills in four to six years on most domestic extensions.

Where it goes and where it doesn't

The most common use is the wall-floor junction at the base of an external cavity wall, sitting on top of the DPC at floor slab level. This is where the cold bridge would otherwise be at its worst, because the slab edge and the wall base meet without any insulation between them.

Other genuine applications:

• Slab perimeter - between an insulated floor slab and the external wall above, where the floor insulation and wall insulation should ideally meet at the same plane
• Door thresholds - eliminating the cold bar of concrete that runs under door openings at floor level
• Steel beam bearings - as a thermal break under steel beams that would otherwise conduct heat from a warm interior to a cold support point
• Wall starters into existing solid walls - though this is a more bespoke detail and structural engineers can be reluctant to specify it without a tested junction drawing

Where it doesn't go: above 11m or 18m height for fire safety reasons (specified in the BBA certificate), and never stacked two courses high under a load-bearing wall. Marmox is tested as a single layer only. Two courses double the eccentricity of the load path through the polymer cement columns and the structural performance has not been certified for that condition.

It is also not a substitute for floor insulation. The block breaks a thermal bridge at one specific junction. Continuous floor insulation, cavity insulation, and a properly insulated slab perimeter all still need to be detailed. Where Thermoblock pays its way is at the junction those layers cross, not as a replacement for them.

Sizes and which one you need

Two heights, three widths. That's the whole product range for the standard block. Get the dimensions right or your bricklayer will be cutting blocks that shouldn't be cut.

The 65mm height is the standard choice because it matches one brick course (65mm + 10mm mortar bed = 75mm = one brick course). The 100mm "extra" height variant exists for buildings where greater insulation depth is needed at the junction; it's a special order through specialist insulation suppliers.

The width must match what's above and below. The block above the Thermoblock course must be no narrower than the Thermoblock itself. If your inner leaf is 100mm aerated block, you fit a 100mm wide Thermoblock. If your structural engineer has specified 140mm inner-leaf blockwork at the lowest course, you need 140mm Thermoblock.

How to work with it

A bricklayer who has never used Thermoblock before will pick it up in the first hour of laying. It's lighter than concrete (the XPS core does most of the volume) and the bedding technique is the same as for any masonry unit. The constraints come from the manufacturer's installation rules, and a builder who hasn't read the BBA certificate may not know all of them.

Mortar coverage. The block bonds to the mortar bed via the polymer cement faces, not the XPS core. Both faces of the block are polymer cement; the textured face must point upward. Lay the block on a full bed of mortar, not a furrowed bed. Avoid voids between the block and the mortar where the load path will be eccentric.

Joints. Vertical joints between blocks should be sealed with Marmox MSP360 sealant (a flexible polymer adhesive) before the next course is laid. This is what creates a continuous airtight and moisture-tight thermal break across the whole perimeter. Mortar alone in vertical joints leaves a small thermal weak point at every block end. MSP360 fills that gap. A builder unfamiliar with the product will often skip this detail.

Cutting. The block can be cut on site but only along its length, not across its width. Cut the XPS core with a hacksaw or fine-tooth saw. Cut the polymer cement columns with a brick saw or rotary blade cutter. The minimum cut length is the width of the block - so a 100mm wide block must be at least 100mm long after cutting. You cannot make a transverse cut that goes through the structural columns. If you need a short fill piece at a corner, order a fitting piece or design the layout to use full or half blocks.

Curves. Up to 45 degrees of bend is possible if the joints are angled. Beyond that the structural columns no longer line up correctly. Most domestic extensions don't need curves; if yours does, talk to Marmox technical before designing the detail.

Orientation. The block must be laid flat with its long axis horizontal. The structural columns are designed to carry vertical compression. Laying the block on its side rotates the columns horizontally, which they aren't tested for. The textured polymer cement face goes upward; the smoother face goes down onto the mortar bed.

Stacking. Single course only. Never two courses of Thermoblock in a wall, ever. The compressive performance is certified for a single layer and degrades in stacked configurations.

DPC and DPM. The Thermoblock course goes on top of the DPC, not in place of it. The legal damp proof course requirement is unchanged by the presence of the block. Both DPC and DPM (the membrane in the floor slab build-up) must still be present and properly lapped.

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Why builders skip it (and how to stop yours)

The single biggest practical issue with Thermoblock is not the product. It's whether it appears on the drawings, in the specification, and on the materials order. Forum threads on BuildHub and the Green Building Forum document the same pattern repeatedly:

• Structural engineers don't always include Thermoblock in their drawn details. They focus on load paths and may leave thermal junction detailing to the architect - who may not have been engaged for this stage.
• Builders quoting a job from drawings that show "100mm dense block" at the base course will price 100mm dense block. They aren't being dishonest. They're pricing what's drawn.
• Building control officers vary in how strictly they enforce the psi-value calculation requirement. Some check the SAP submission carefully and flag missing thermal break details. Others accept the drawings as submitted.
• Builders who haven't used the product before are sometimes resistant to it. The "no stacking" and "no transverse cut" rules feel awkward compared to standard blockwork. Some will suggest substituting a 215mm aerated block on the inner leaf as "the same thing." It isn't.

If you want Thermoblock in your build, you need to put it there yourself. Do this:

1. Before structural engineer's design is finalised, ask them in writing to detail a thermal break at the wall-floor junction. Specify Marmox Thermoblock or Foamglas Perinsul HL by name. Ask for the BRE-certified psi-value to be used in any SAP or u-value calculations.
2. Before getting builder quotes, confirm the specification document and drawings name the product, dimensions, and supplier. A line item on the materials list is what gets ordered. A vague footnote on the drawings is not.
3. Before groundwork starts, walk the drawings with the builder. Confirm they understand a single course of Thermoblock goes above DPC at the base of the external cavity wall. Confirm MSP360 sealant is on the materials list.
4. At groundwork inspection, before the wall blockwork rises above the first lift, check that the right product was used at the right course and that the joints have been sealed.

The block is not expensive enough to argue about as a line item. A typical 4m by 6m rear extension has roughly 16 linear metres of external wall perimeter. Total Thermoblock supply for that perimeter at standard 100mm cavity width is in the region of £256 – £400. On an extension build costing eighty to one hundred and twenty thousand pounds, that is well under half a percent of the total cost. The argument against fitting it is almost always unfamiliarity, not money.

Cost and where to buy

£10 – £12 per block ex VAT for the 100mm wide variant, sold in packs of 18. The 140mm wide variant runs £14 – £18 per block ex VAT in 12-packs. The 215mm wide variant comes in around £15 – £17 per block.

Travis Perkins and Jewson don't routinely stock Marmox Thermoblock at branch level. The product is a specialist insulation line and the volume per project is small. The reliable supply route is through specialist insulation merchants who carry it as a regular line:

Marmox UK also operates a direct sales channel and can advise on which variant suits a non-standard detail. For a single extension's worth of supply, ordering through one of the specialist merchants is faster.

Lead times are short for standard sizes (a few days from a UK insulation merchant) but a standard size delivered to the wrong width is useless. Order from a supplier who confirms the cavity width with you before dispatch. One BuildHub poster reported a UK supplier delivered the wrong width despite explicit ordering, forcing a re-supply from Belgium. It happens. Confirm in writing.

Alternatives

Marmox Thermoblock isn't the only option. Two alternatives are worth knowing about.

Foamglas Perinsul HL is cellular glass - black foamed glass blocks at 65mm height like Thermoblock. The thermal performance is slightly worse (lambda 0.058 vs 0.05) and the compressive strength much lower (2.75 vs 9 N/mm²), so it's only suitable for lighter loadings. Its big advantage is the fire rating - Class A1, the highest possible. For most domestic extensions the Thermoblock fire class (Euroclass E) is acceptable, but for buildings near or above the 11m fire height threshold, Perinsul becomes the better choice. Cost per linear metre is comparable. Builders sometimes prefer Perinsul because it behaves more like a conventional rigid masonry block - no XPS core, no special cutting rules. Order direct from specialist insulation merchants.

An aerated concrete block at increased thickness (215mm Thermalite Hi-Strength rather than 100mm dense block) is the cheapest alternative, and a builder who doesn't want to use Thermoblock will often suggest it. It addresses some of the cold bridge but not all of it. The lambda is roughly two to four times worse than Marmox, so even at three times the thickness, the thermal break is significantly less effective. If your SAP calculation can carry the loss without dropping out of compliance, this works. If your designer is targeting a tight fabric performance figure, this won't get you there.

Armatherm structural thermal break pads are a different category - engineered thermoset polymer blocks for steel-to-steel and steel-to-concrete connections. Useful for thermally broken balcony connections, parapet upstands, and structural steel bearings. Not a direct substitute for Marmox at a wall-floor junction.

Common mistakes

Specifying Marmox in the SAP calculation but not on the drawings. The SAP assessor enters the manufacturer's psi-value into the model and the result passes. The bricklayer turns up to a drawing showing standard dense block and lays standard dense block. The building gets built without the thermal break. The SAP submission is now incorrect. This is the most common failure mode and it's invisible until someone with a thermal camera looks at the finished building.

Stacking two courses to compensate for "extra" insulation. Don't. The certified compressive strength is for a single layer. Two layers may look more insulating but the load path through the polymer cement columns is no longer tested. Order the 100mm extra-height variant if you need more depth.

Using only a partial perimeter. Thermoblock at the rear wall but not at the side walls means the cold bridge has just been moved, not eliminated. The block has to run around the entire heated perimeter of the extension to give the BRE-certified psi-value. Half measures don't earn half the credit; they barely earn any.

Mortar voids under the block. Bricklayers who furrow the mortar bed (running a trowel groove down the centre) for speed will leave point-load voids under the polymer cement columns. The block is rated for a full bed. A furrowed bed concentrates load on the column edges. Insist on full mortar coverage at this course.

Skipping the MSP360 sealant in vertical joints. Standard sand-and-cement mortar in the vertical joints is permeable enough to create a thermal weak point at every block end. The continuous thermal performance the BRE certification covers assumes sealed joints. Cost of sealant is small. Skipping it negates much of the reason for buying the blocks in the first place.

Treating it as a substitute for floor insulation. It isn't. Thermoblock is one detail in a continuous insulated envelope. Floor insulation, cavity insulation, perimeter upstand insulation (minimum 25mm under Approved Doc L 2021), and Thermoblock all do different jobs at different parts of the junction. Skip any one and the others can't compensate.

Where you'll need this

Thermoblock comes into play wherever an external wall meets the heated floor of an extension or new build. Across a kitchen extension specifically:

• Foundations and footings - design stage decision; specify Thermoblock on structural engineer's drawings before the foundations are poured, so the wall-floor junction detail accommodates it
• Walls and blockwork - installation stage; the single course goes above DPC as the first course of the cavity wall
• Insulation - the Thermoblock course must align with the floor insulation plane to give continuous thermal envelope across the junction

This block applies to any extension or new-build project that needs to comply with current Approved Document L thermal bridging requirements. The product itself is the same regardless of project type. What changes is whether your structural engineer, architect, and builder have all noticed it needs to be there.