Aircrete Inner Leaf: Replacing Dense Blocks in the Inner Leaf of a Traditional Cavity Wall with Mannok Aircrete Thermal Blocks

8th August 2022

Mannok’s Technical and Product Specification Manager, Jason Martin, explains why Aircrete Inner Leaf provides a high performing, more comfortable and sustainable home.

Cavity wall construction is the most common method used in building homes in Ireland. Traditional cavity walls can be elevated to deliver far superior comfort levels in your home simply by replacing standard dense concrete blocks with Aircrete Thermal Blocks in the inner leaf of the cavity, which is called Aircrete Inner Leaf.

Better Walls for Better Performance

Of the total heat lost from a home, typically, you can lose up to 15% of heat through the floors, up to 25% through the roof, up to 25% through windows and doors, and up to 35% through walls.

So, most heat can be lost through the walls of a home, and if the thermal performance of a wall is improved it will have a significant impact in reducing heat lost from a home.

Building the entire inner leaf of a traditional cavity wall with Aircrete Thermal Blocks will improve the thermal performance of the walls.

Why Mannok Aircrete Blocks are Best for the Inner leaf

Mannok Aircrete Thermal blocks are Ireland’s best block for combined thermal, structural and fire performance. It is the air pockets in the block that provide it with its lightweight, high insulating and compressive strength properties.

The Aircrete Thermal Blocks are autoclaved aerated concrete blocks, they have a unique microcellular structure filled with millions of tiny air pockets. The air pockets are formed during their production when a chemical reaction takes place between the aluminium and lime in the mix, and this chemical reaction produces hydrogen gas which fills the mix with tiny pockets of air.

The benefits of Mannok Aircrete Thermal Blocks

  • They achieve exceptional thermal performance thanks to their superb insulation properties
  • Reduce heat loss through thermal bridging by up to 80%
  • Using these blocks in your build will help easily achieve nZEB and Passive House standards
  • The blocks are made from up to 80% recycled content
  • They are the best performing block in Ireland in terms of structural, fire and thermal performance
  • Light, easy to build with, convenient to cut and generally there is less waste when using them.

Mannok has three different Aircrete block strengths:

  • 5N/mm² Mannok Aircrete Seven
  • 2N/mm² Mannok Standard
  • 2.9 N/mm² Mannok Super

The different block strengths have varied thermal conductivities, ranging from 0.19W/mK – 0.12W/mK, with the 2.9N/mm² Mannok Super block being the best thermal performing block.

When the Mannok Super Aircrete block is compared to a standard dense concrete block with a thermal conductivity of 1.33 W/mK, there is a huge difference in thermal performance. In fact, the Mannok Aircrete Thermal Block is up to 10 ten times more thermally efficient than a dense concrete block.

Thermal Bridging

Think of all the junctions in a building – where floors and walls meet, roofs and walls, around all the windows and doors – they are cold bridges – the areas where there are breaks in the insulation fabric which are open pathways for heat to escape. This heat loss can account for up to 35% of the building’s total heat loss, so it’s important to address it correctly. Mannok Aircrete blocks are widely regarded as the best solution to combat this heat loss, achieving up to 80% reduction in heat loss at these key junctions.

Typical Thermal Bridges

The most common thermal bridge that our Technical Team comes across daily is, floor to wall junctions. As you can see in fig.1 the wall insulation and the floor insulation don’t meet, they are separated by the inner leaf of the cavity wall which is built with standard dense concrete block. This separation is where the thermal bridge occurs as there is a break or gap in the insulation envelop of the building.

Fig.1: Thermal bridge at floor to wall junction

As shown in Fig.2, when two dense concrete blocks are replaced with two Mannok Aircrete Thermal Blocks, the separation at the floor to wall junction is plugged with an insulation product and there is continuity of insulation across the junction. This continuity of insulation reduces heat loss through the junction by up to 10 times.

Fig.2: Addressed thermal bridge with two Mannok Aircrete Blocks 

Another common junction where thermal bridging occurs, is the ceiling to wall junction. In this junction, the attic insulation and wall insulation are separated by the inner leaf of the traditional cavity wall construction built with dense concrete blocks. As a result, there is a significant thermal bridge, but by substituting two dense concrete blocks with two Mannok Aircrete Thermal blocks, which is illustrated in Fig.3, it improves the performance of the junction.

Fig.3: Addressed thermal bridge at ceiling to wall junction with two Mannok Aircrete Blocks

A common junction in Ireland, where thermal bridging occurs, is roof abutment usually for a sunroom at the side of the main house structure. With the gap between the roof insulation and wall insulation including an object with poor thermal conductivity, there is a heat path between the roof insulation and the wall insulation. However, as illustrated in Fig.4, the gap in insulation is very simply resolved with the use of two Mannok Aircrete Thermal Blocks, which have excellent thermal conductivity.

Fig.4: Addressed thermal bridge at roof to wall junction with two Mannok Aircrete Blocks

In any house there is 5-7 junctions where there is a break in insulation and where Mannok Aircrete Thermal Blocks should be introduced, as illustrated with purple lines in Fig.5. To fully address thermal bridging, Mannok Aircrete Thermal Blocks should be placed at all the locations highlighted in purple. However, when the thermal blocks are used at all junctions there is a patchwork effect of both dense concrete blocks and Mannok Aircrete Blocks. To overcome the patchwork effect and any potential issues, the Mannok Technical Team recommend building with entire inner leaf of the cavity wall with Mannok Aircrete Blocks.

Fig.5: All the junctions in a typical house where thermal bridging needs to be addressed

Aircrete Inner Leaf Benefits

Addresses All Thermal Bridges 

All thermal bridges are addressed by default, when the inner leaf is built with Aircrete Thermal Blocks, which reduces the heat lost through thermal bridges by up to 80%.

Improves Wall U-value

By introducing a more thermally efficient Mannok Aircrete Thermal block, which insulates 10 times better than standard dense concrete blocks, the U-Value of a wall will be improved and more heat will be retained at the junctions of a home.

Cost Savings

Because the junctions are better insulated, other insulation products elsewhere can potentially be reduced which can result in cost savings.

No Mixing and Matching of Blocks

Using Aircrete Thermal Blocks to address the thermal bridges of a building structure creates a patchwork of blocks. When the entire inner leaf of the traditional cavity wall is built with Mannok Aircrete Thermal Blocks, there is no mixing and matching of blocks for builders and no risk of differential movement between the two types of blocks.

Faster Thermal Response

Aircrete Thermal Blocks have a faster thermal response when compared to dense concrete blocks, so rooms in a house built with Aircrete Thermal Blocks will heat up quicker.

Optimal Thermal Mass

A certain amount of thermal mass in walls is particularly important to capture and store energy from south facing glazing, which reduces the risk of overheating during the day and keeps the home warm for longer at night, as the stored heat is released back into the room as it cools. Aircrete Inner Leaf has an optimal thermal mass and this is one of the main reasons it should be considered over other lighter forms of construction such as timber frame or light steel frame.

Zero Risk of Condensation and Mould

As Aircrete Inner Leaf addresses all thermal bridges by default and reduces heat loss through thermal bridges, which increases the surface temperature at the junctions of a home, the risk of water droplets condensing on the cold surface of the wall and forming mildew is eliminated. Not only is mildew very unsightly but more importantly, it’s very unhealthy and a home built with an Aircrete Inner Leaf is more comfortable and healthier.

Thermal Mass in More Detail

Thermal mass in walls, absorbs heat energy from the sun which helps regulate the temperature of homes on hot sunny days. At night, when the temperature starts to drop and rooms become colder, the sun’s heat energy stored in the thermal mass of the house is released back into the rooms keeping them warmer for longer in the evenings. Therefore, thermal mass helps regulate and reduce the peaks and troths of temperature, which is demonstrated in Fig.6.


Mannok Aircrete Thermal Blocks have an optimum thermal mass, much better than other lightweight forms of construction such as timber frame or light steel frame.

Fig.7 shows two different rooms heating up from a temperature of zero, one room is built with lightweight construction (Mannok Aircrete Blocks) and the other with heavy concrete blocks. The research and results in the diagram show the room which is built with the lightweight construction (Mannok Aircrete Blocks) heats up significantly quicker than the room built with dense concrete blocks. Mannok Aircrete Blocks do not absorb the heat as much as the dense concrete block, keeping more heat in the atmosphere of the room.



Mannok Aircrete blocks are manufactured from up to 80% recycled materials, they have an A+ Green Guide rating and they are one of the first construction products to have an Environmental Product Declaration published by the Irish Green Building Council. Additionally, Aircrete’s superior performance means they will help to achieve a more energy efficient home, with passive house or nZEB standards much easier to achieve.

Contact Mannok’s Technical Team

If you’re interested in building a cavity wall construction with Aircrete Inner Leaf or have a project you’d like the technical team to look at, get in touch:

Call +44 28 67748866 or email