Do I need to coat Mannok’s Aircrete blocks in Bitumen?

10th June 2022

In the latest “Your Questions, Answered” video, we explore a common enquiry into our technical department relating to the use of our Mannok Aircrete Thermal Blocks  below DPC, and if there is a requirement for tanking the blocks in the bitumous type substance for use in this application

Full certification

Firstly, it’s important to note that Mannok Aircrete Thermal Blocks have full third party certification for use below DPC so all the relevant free-thaw tests etc. have been carried out and there are no issues with structural performance when using the blocks in this application.

We would also point out, however, that to get the best thermal performance, the blocks should be kept dry, but there should be no requirement in most instances to tank the blocks in bitumen to achieve this.

Floor to wall construction details

So, we’re going to look at two typical details to see how we can keep these blocks dry quite easily.

Floor construction with insulation below the slab

In Fig. A, we have a typical floor detail which has insulation below the slab. You will see we have a 150mm concrete slab and insulation below this of 175mm. It’s always good practice for insulation thickness to coincide with block work coursing.  There is a soap bar which is 100ml deep, and a standard block which is 225mm deep giving an overall depth of 325mm. So, if the sub-floor and insulation also equates to 325mm it makes life easier on site.

In some instances, 175mm may seem to be too thick given that it may achieve U-values well in excess of what we need. However, this allows us to look at alternative lower performing insulations. For example, in this detail we could use EPS Pearl, which is a grey EPS, or even ordinary white EPS insulation, which would give the U-values we require.

The detail uses partial-fill cavity wall insulation, we have a concrete block inner and outer leaf. The inner leaf could alternatively be a timber frame. The radon barrier is underneath the floor insulation and goes up over the block, out and down.

We have the DPC on the inner and outer leaf, which typically needs to be a minimum of 150mm above ground level. The DPC is sealed with the radon barrier, which is good practice.

At the point where the wall insulation and the floor insulation are broken, we have introduced a Mannok Aircrete Thermal Block. We have a free draining cavity below the cavity wall insulation and a perforated drainage channel outside, so any water running down the inside of the block freely drains out and away from the structure. It is good practice to have adequate drainage around your building.

So, looking at the Mannok Aircrete Thermal Block in this detail, it’s protected from moisture on both faces by the radon barrier, so the only way this block can get wet is from rising damp from beneath. So, to keep the block dry we can simply introduce a second DPC below the block, and let it on out underneath the radon barrier as necessary. This results in a Mannok Aircrete Thermal Block which is fully protected from moisture and will be dry in all instances, and so in this detail there would be no requirement for tanking or dipping the blocks in bitumen.

Floor construction with insulation above the slab

Fig. B shows a second common floor construction, this time with insulation above the slab; with a concrete subfloor, insulation and screed. Here we have 75mm of screed and 150mm of insulation, which adds up to 225mm, and again coincides with the block work coursing.

The wall construction is the same as with the previous example, with the option for a timber frame on the inner leaf and the free-draining cavity to allow any water running down the cavity to drain off. Insulation should not be sitting in water so it is important that the cavity below the insulation is fully drained.

Again, you can see the radon barrier running up over the block, and down and out. So, already in this detail, the Mannok aircrete block is protected from rising damp, meaning there is no requirement for any additional DPC in this construction.

We have the DPC on the inner leaf sealed to the radon barrier, which again is good practice. So, there is no requirement for any use of bitumous type products to coat or dip the blocks to keep them dry. The block is dry in this application and will achieve its best thermal performance.

Effects of moisture on Mannok Aircrete Thermal Blocks

We’ve looked at two situations where Mannok aircrete blocks can easily be kept dry at that critical floor-to-wall junction. We’re going to look now at what happens to the Mannok aircrete blocks when they are subject to moisture. It has been brought to our attention that, on some occasions, the detail has not been done correctly and the block may be subject to rising damp. So, to investigate, we’ve carried out some in-house testing to see how moisture will affect the thermal performance of the blocks.


Looking first at our typical Mannok Aircrete Seven block, the thermal performance in its dry state it 0.19w/mk. For the test, we then submerged the block in water for 48 hours and tested it straight away after removing from the water so the block was at full saturation. The test result revealed that the thermal conductivity of that block at full saturation was 0.33 w/mk. Looking again at our first detail, even if the DPC was not present underneath the block, the block would only be subject to rising damp so will never get close to full saturation. So, as our test figures shown are based on full saturation, it really is the worst-case scenario.

Next, looking at our Mannok Aircrete Super block, the dry thermal conductivity is 0.12 w/mk, the fully saturated conductivity tested at 0.29 w/mk. The interesting thing with these figures is, when we look at another common thermal block which is now being used throughout Ireland, the lightweight aggregate block, the thermal conductivity of that block in its dry state is 0.33 w/mk. So even if the detail is not done correctly, like we have shown in our earlier example, the thermal performance of the Mannok aircrete block in its fully saturated state is still on a par with a dry lightweight aggregate block. What we must then ask ourselves is “what is the thermal performance of a lightweight aggregate block in its wet state?”

Thermal bridging comparison study

So, why is it so important to use Mannok Aircrete Thermal Blovks at key junctions and to detail the junction correctly so that the Mannok Aircrete block is kept dry? A recent thermal bridging study carried out on 17 key junctions, compared the Psi values across those junctions between Mannok Aircrete, dense blocks and lightweight aggregate blocks.

Dense blocks are a standard concrete masonry unit, lightweight aggregate blocks are a thermal block and are manufactured using lightweight aggregates, and Mannok Aircrete is an autoclaved aerated concrete block.

It’s important to note that the Mannok Aircrete Thermal Block outperformed both of these other blocks across all of the 17 key junctions. Compared with the dense block, the Mannok Aircrete block performed up to 750% better, and compared to the lightweight aggregate block, the Mannok Aircrete block performed up to 200% better across the key junctions.

So, looking at these figures it’s quite clear that Mannok Aircrete Thermal Blocks should be used to get the best thermal performance across key junctions.

We’ve looked at two common details, and how the Mannok aircrete block can be kept dry to maximise thermal performance. A full range of the details are available in our Accredited Construction Details, including the examples shown. In addition, our technical team are on hand to work with you on any other details that might not currently be available. If you have further questions, don’t hesitate to contact the team on +44 28 67748866 or