Aerated Blocks Buying Guide: Thermalite Sizes, Strength & Uses
Aerated Blocks Buying Guide: Thermalite Sizes, Strength and Uses
Aerated blocks, often called aircrete or aerated concrete blocks, are lightweight masonry units used in many UK wall constructions. Their cellular structure contains tiny pockets of air, helping to reduce weight and improve thermal performance compared with many denser concrete products. They are also relatively easy to cut and shape on site, which makes them popular with builders working around openings, services and detailed blockwork.
JJ Building Materials stocks Forterra Thermalite products in useful building sizes, including 100mm Thermalite Shield 3.6N blocks, 140mm Thermalite Shield 3.6N blocks and 215mm Thermalite Turbo 2.9N blocks. This guide explains how size, product range, compressive strength, thermal conductivity, weight, wall type and fixing requirements should influence your choice.
What Are Aerated Concrete Blocks?
Aerated concrete is manufactured to create a lightweight cellular structure. Forterra describes Thermalite as having a closed micro-cellular structure with trapped air, giving the material useful thermal and sound-insulation characteristics as well as lighter handling. Unlike hollow concrete blocks, the low density comes from the material structure itself rather than large formed voids.
That makes aircrete useful in many internal and external wall applications where the specified product meets structural, thermal, acoustic, fire and exposure requirements. The word 'lightweight' should not be confused with 'non-structural': some aerated blocks are designed for load-bearing applications, but the wall design must identify the required unit strength and thickness.
Thermalite Blocks
Thermalite blocks are Forterra's aircrete masonry range. Different Thermalite products are optimised for different combinations of strength, density and thermal performance. JJ Building Materials currently stocks Shield and Turbo products, so it is important not to treat 'Thermalite' as one universal specification.
Forterra publishes technical data for each range. Thermalite Shield 3.6 has a declared compressive strength of 3.6 N/mm², design thermal conductivity of 0.15 W/mK and dry density of 600 kg/m³. Thermalite Turbo 2.9 has a declared compressive strength of 2.9 N/mm², design thermal conductivity of 0.11 W/mK and dry density of 470 kg/m³. Those figures show why product choice involves more than strength alone.
Thermalite Shield 3.6N
Thermalite Shield is a versatile aircrete block available in multiple widths. Forterra lists applications including external walls, foundations, partitions and separating walls, subject to the complete design. JJ Building Materials currently stocks 100mm and 140mm Shield 3.6N products.
The 100mm option is commonly useful where the wall detail calls for a standard-width block leaf, while 140mm provides a thicker masonry unit where the design requires that build-up. Both use a 440 x 215mm face format in Forterra's data, which makes quantity planning straightforward.
Thermalite Turbo 2.9N
Thermalite Turbo is a lower-density aircrete block designed to provide strong thermal performance. Forterra states a 0.11 W/mK design thermal conductivity and 470 kg/m³ dry density for Turbo 2.9. JJ Building Materials' live range includes the 215mm-wide Turbo block in packs of 25.
Its lower 2.9 N/mm² strength designation does not make it an inferior product; it reflects a different balance of properties. The correct choice is whichever product meets the wall's structural and thermal specification. Read our Thermalite Shield vs Turbo guide for a direct comparison.
100mm Aerated Blocks
100mm aerated blocks are widely used in internal partitions and as one leaf of cavity-wall constructions where the design calls for that width. A 100mm Thermalite Shield is relatively easy to handle and cut compared with dense masonry, making it convenient for general blockwork.
Do not assume 100mm is suitable simply because it is a common width. Wall height, loading, restraint, acoustic requirements, thermal design and fire performance can all influence the specified thickness.
140mm Aerated Blocks
140mm Thermalite blocks increase wall thickness by 40mm compared with a 100mm unit. That can change structural slenderness, thermal resistance, reveal details, cavity dimensions and the overall wall footprint. A designer may select 140mm to achieve a particular wall build-up or performance target.
Check door linings, window reveals, wall ties and junction details whenever block thickness changes. Substituting 140mm for 100mm affects more than the quantity of concrete in the wall.
215mm Aerated Blocks
The 215mm aerated block in JJ Building Materials' live range is a Thermalite Turbo 2.9N unit measuring 215 x 215 x 440mm. At this width, a single block spans a substantial wall thickness and changes the construction strategy compared with a 100mm leaf.
Because it is a Turbo rather than Shield product, both thermal conductivity and compressive strength differ. Read our 100mm vs 140mm vs 215mm aerated block guide before making a size substitution.
2.9N vs 3.6N Aerated Blocks
The figures 2.9N and 3.6N refer to declared compressive strength in N/mm². A 3.6N aerated block has a higher declared compressive strength than a 2.9N unit, but that does not automatically make it the correct product. Turbo's lower density can support improved thermal conductivity, so the designer balances several requirements.
Never replace a specified 3.6N product with 2.9N without checking structural requirements. Equally, do not assume a higher-strength substitution is automatically acceptable if it changes thermal performance, density, thickness or other system details. Our 2.9N vs 3.6N guide explains this in more detail.
Thermal Performance of Aircrete Blocks
Thermal conductivity, often shown as lambda or λ, indicates how readily heat moves through a material. Lower values generally indicate lower heat conductivity. Forterra's published design values show Turbo 2.9 at 0.11 W/mK and Shield 3.6 at 0.15 W/mK.
The U-value of a complete wall cannot be determined from block lambda alone. Insulation type and thickness, cavity, mortar joints, finishes, thermal bridging and junction details all influence overall performance. Use a proper wall calculation for compliance and design.
Aerated Blocks vs Dense Concrete Blocks
Aerated and dense concrete blocks solve different problems. Aircrete is lighter, easier to cut and generally more thermally insulating. Dense concrete blocks offer higher mass and are available in higher strength grades, which can suit different structural and acoustic requirements.
If you are comparing options, browse our concrete blocks alongside aerated blocks. Do not choose by weight alone; the complete wall specification should decide.
Are Aerated Blocks Easy to Cut?
One of aircrete's practical advantages is workability. Blocks can be cut with suitable hand or power tools, allowing accurate pieces around openings and services. Mark cuts carefully and keep edges square so mortar joints remain consistent.
Cutting generates dust. Use appropriate PPE, dust control and safe cutting procedures. Avoid unnecessary notches that could weaken the wall, and follow the design rules for chases and openings.
Fixing into Thermalite Blocks
Fixing into aerated concrete requires anchors designed for the substrate and intended load. Lightweight aircrete does not behave like dense concrete, so a generic plug selected without load data may not be suitable. Specialist aerated-concrete fixings, larger-diameter plugs or resin systems may be required depending on the application.
For heavy cupboards, boilers, rails or structural items, use the fixing manufacturer's technical guidance and confirm the load path. Our guide to laying, cutting and fixing Thermalite covers this in detail.
Laying Aerated Blocks
Set out the wall carefully and establish a level first course. Use mortar compatible with the blocks and wall design, maintain the required bond and keep joints filled as specified. Protect fresh blockwork from heavy rain, frost and rapid drying conditions.
Because Thermalite is absorbent, site conditions can affect mortar behaviour. Follow manufacturer and project guidance rather than soaking or treating blocks based on generic advice.
How Many Aerated Blocks per Square Metre?
Forterra Shield and Turbo blocks use a 440 x 215mm face. With typical mortar joints, around 10 blocks per square metre is a familiar preliminary estimating figure. For final quantities, calculate from the exact coordinating dimensions and wall area.
Subtract large openings, add returns and waste, then check pack quantities. JJ Building Materials' 100mm Shield is sold in packs of 60, 140mm Shield in packs of 40 and 215mm Turbo in packs of 25 at the time of writing.
Storage and Handling
Store blocks on stable level ground and keep packs protected from prolonged saturation. Aircrete is lighter than dense concrete but large widths can still be substantial individual units. Use safe manual-handling techniques and mechanical aids where required.
Aerated Block Buying Checklist
- Confirm wall type and specified block width.
- Check required compressive strength.
- Compare Thermalite Shield and Turbo technical data.
- Check thermal conductivity within the full wall design.
- Confirm block face dimensions and pack quantity.
- Plan compatible mortar and wall ties.
- Select fixings specifically for aerated concrete.
- Allow for cuts and waste.
- Check delivery and storage access.
Aerated Blocks FAQs
What are aerated blocks used for?
They are used in many internal and external masonry walls where the selected aircrete product meets structural, thermal, acoustic and fire requirements.
Are Thermalite blocks the same as aerated blocks?
Thermalite is Forterra's brand of aircrete blocks. Different Thermalite ranges have different properties.
What is the difference between Shield and Turbo?
Shield 3.6 has higher declared compressive strength and density, while Turbo 2.9 has lower density and lower design thermal conductivity. Use the product specified for the wall.
Is 3.6N stronger than 2.9N?
Yes in declared compressive strength, but strength is only one selection criterion.
Can I use 100mm Thermalite for an external wall?
It can form part of suitable external wall constructions where the design calls for it. Check structural, insulation and cavity details.
Can you fix kitchen units to Thermalite?
Yes with a fixing system specifically rated for aerated concrete and the required load. Heavy items need careful fixing design.
How many Thermalite blocks do I need per m²?
Around 10 blocks per m² is a common preliminary estimate for 440 x 215mm-face blocks with normal joints. Calculate exactly for ordering.
Where can I buy Thermalite blocks?
Browse JJ Building Materials' aerated blocks and Thermalite blocks collection for current sizes and pack quantities.
Final Advice
Choose aerated blocks from the wall specification outward. Width, compressive strength and thermal conductivity all matter, and Shield and Turbo serve different performance priorities. Once the technical choice is correct, compare pack size, handling and price to plan an efficient order.