Thermalite / aircrete

[ladwey]

The planning notes from the architect recommend Thermalite Turbo and Shield lightweight blocks for the cavity walls. I've not used them before; they look great from an insulation and handling point of view, but a builder friend has said that he's had problems with them cracking in the past. He recommends using the standard blocks instead. Has anyone used these 'aircrete' or similar blocks and had any problems with them? Was my mate just unlucky or are they bad in some applications?

 

[^woody^]

"Recommended" or "specified"? I suspect the later, and its not up to the builder to use the cheaper things on offer at the merchants or that they just be randomly swapped

The issue is whether alternative blocks have the same performance and meet the same u-value in the wall.

These "recommended" blocks, as well as and the ones the builder recommends are basically the same, and are each just as likely the crack. Each needs a weaker mortar to prevent cracking, and certain storage and handling precautions to minimise the risk of cracking.

 

[ladwey]

I should have been more specific, sorry -- the construction notes say 'Thermalite Shield ... or similar' at the relevant points. Don't worry about a builder swapping in cheaper materials!- I'm buying things directly myself and just wanted to check that this was a place where the architects weren't out of touch with what people find useful and durable to actually build with. The u-value of the walls is an important concern and I wouldn't do anything to compromise it.

 

[^woody^]

The block is normally chosen in conjunction with the wall insulation to get the overall wall u-value to meet building regulations.
Alternatively, a lazy designer just cut and pastes from his last plans.

Either way, you need to check that if you do swap the block, the wall still meets the required u-value, as sometimes you may need to swap insulation too if the block has lower performance

There are also 100mm and 110mm versions of the blocks, so again make sure its the specified/agreed type.

 

[jks7492]

Yes its quite common for thermalite blocks to crack

 

[noseall]

Yes its quite common for thermalite blocks to crack

Especially if the wall has been soaked while under construction.

 

[ladwey]

Thanks for the useful comments all! I was actually just speaking to a structural engineer this morning about an unrelated thing, and he mentioned that he'd just come from a house where they'd discovered that the thermalite blocks in their kitchen had cracked and were panicking -- he said he sees it all the time, and that one of the chief reasons is that while they're very structurally sound and excellent insulation, the thermalite blocks tend to crack with thermal expansion and contraction. He stressed that it's just a cosmetic issue and doesn't at all affect their structural integrity, so if you're (eg) drylining the walls it's not something to worry about at all. He also mentioned as Woody does above that the harmless cracking can be eliminated by using a weaker mortar mix.

 

[phatboy]

At the moment, you may find the biggest issue is finding the things!

We were going to do an extension with them, or other aircrete variants (Durox or Celcon), but in December they were all on allocation, and best case 3 months wait

Reverted to concrete.

 

[ketstone0000]

Ii used Thermolite it cracked top to bottom best to buy them early so they can dry out. If i were going to use them again I would consider creating some way the blocks can shrink like a movement strip bang in the middle of the wall or maybe every 2 to 3 meters apart depending on the width of the wall. but then a good look with the plastering as that will be another problem with the walls moving under it , I was in a position to let the wall dry out and settle for 6 months, then I used a rendering mesh and then rendered the walls no cracks to this,day 2 years later

 

[noseall]

The planning notes from the architect recommend Thermalite Turbo and Shield lightweight blocks for the cavity walls. I've not used them before; they look great from an insulation and handling point of view, but a builder friend has said that he's had problems with them cracking in the past. He recommends using the standard blocks instead. Has anyone used these 'aircrete' or similar blocks and had any problems with them? Was my mate just unlucky or are they bad in some applications?

Aerated lightweight (Celcon) type blocks are quite possibly the worst things known to man. Snap like digestives.
We had to tooth out some (floor to ceiling) on our last job, to allow us to slot in some 7n's.
When we removed tge plasterboard, the first couple of blocks had hairline cracks, top to tail. Fudging nightmare.
We need to do the same on our current job and I can see already that the existing blocks are cracked.

If you can find a way of achieving the correct heat loss numbers, use Fibolites or similar. We manage it with thd correct cav insulation.

 

[Deluks]

Turbo's are rated at 0.11w/mk. Be aware of this when substituting a different block. What insulation is specified?

 

[noseall]

Turbo's are rated at 0.11w/mk. Be aware of this when substituting a different block. What insulation is specified?

Plasmoor Fibolites will achieve June 2022 numbers with 100mm cavity and 90mm rebated PIR full fill system.
It will depend whether the OPs job requires more (SAP calc) though...?

 

[Deluks]

Plasmoor Fibolites will achieve June 2022 numbers with 100mm cavity and 90mm rebated PIR full fill system.
It will depend whether the OPs job requires more (SAP calc) though...?

That lovely expensive PIR that you so love to work with!

 

[noseall]

That lovely expensive PIR that you so love to work with!

Horrendous. And as for cutting up gables...!