Cavity vs 140mm & Dot+Dab vs 190mm & Dot+Dab?

[EViS]

What is the minimum aerated block size and dot+dabbed insulation thickness required to meet current Building Regulations and be equivalent to a filled cavity wall construction for an extension?

E.g. Would 140mm thermalite/celcon blocks with 50mm PIR be sufficient? Or would 190mm blocks be necessary?

 

[tony1851]

Don't quite understand what you mean, but 100 aac block is usually used, together with whatever type of insulation you propose (ie full-fill or partial fill).

140 + block is unnecessarily thick (unless you want better U-values, though thicker cavity insulation is far more cost-effective).

 

[EViS]

To re-phrase, if I want to build a single block wall (i.e. not cavity) extension, what size block do I require and what thickness insulation+plasterboard should I be dotting+dabbing?

 

[jeds]

Recently did one in 215 Standard aac block with 50mm PUR foam finished with thin coat render and dot/dab internally. From memory the U-value was about 0.26. This was a specific request - I'd probably stick with standard cavity if it were me.

 

[mfarrow]

There's minimum requirement in the building regs. Look for the prevention of moisture one (I forget the name and part unfortunately). It only recognises rendered walls as standard which I assume you want anyway with block work.

 

[theoldun]

There are several ways to achieve 28W/m2K with 215 blocks to comply with L1B, how ever all blocks must have 20mm of external render.
Celcon Solar.
Topblock Superbloc.
Thermalite Super
All have a lambda value of 0.11 W/mK and you can dot and dab with.50mm Gyproc Thermaline Super.

Celcon Standard
Thermalite Shied
Both have a lamda of 0.15W/mK and you can dot and dab with 65mm Gyproc Thermaline Super.
There are other ways where you can use a denser block with a higher K value, but then you have to use a thicker insulation such as Celotex PL4000
Regards oldun

 

[EViS]

That's great, thanks theoldun . Any chance you (or anyone else) know what block type and insulation thickness would be required using 190mm blocks?

And yes, the wall will be rendered.

 

[tony1851]

That's great, thanks theoldun . Any chance you (or anyone else) know what block type and insulation thickness would be required using 190mm blocks?

And yes, the wall will be rendered.

If you are using rendered aac blocks, I think you'll find that building control expect a minimum block thickness of 215mm. (see Approved Document C)

If you want to use 190 blocks,you would need some sort of rain screen arrangement, which will be expensive and take up more thickness.

 

[EViS]

Gotchya. 215 it is then.

Am I likely to experience cracking issues after applying a sand/cement render onto 215 aerated blocks in the same way that a 100mm aerated block is problematic?

Also, must I use a thin bed mortar with AAC blocks? Or can I use a sand/cement mortar?

 

[tony1851]

Gotchya. 215 it is then.

Am I likely to experience cracking issues after applying a sand/cement render onto 215 aerated blocks in the same way that a 100mm aerated block is problematic?

Also, must I use a thin bed mortar with AAC blocks? Or can I use a sand/cement mortar?

aac blockwork is prone to shrinkage cracking. The manufacturers recommend movement joints at around 6m intervals, but if you use too strong a mortar, it will still crack at less than this. It will of course also crack the rendering. If you have to have a joint, hide it behind a rwp. Don't use too-strong a mix for the rendering.

And don't be beguiled by 'thin-joint' work - it's expensive and a waste of time in domestic extensions.

 

[EViS]

So all this speak of thin joint mortar isn't really applicable to residential extensions. Interesting. I assumed that although more expensive to purchase, time saved in laying blocks would make the entire process more cost effective, no?

What's a good mortar mix for the joints between AAC blocks and an acceptable strength rendering mix?

 

[EViS]

Shameless bump, I don't suppose anyone could respond to the above question please ?

 

[limerockconstruction]

Celcon or Thermalite would tell you 6 and 1 for laying and if i'm not mistaken, they suggest you mesh the external blockwork and then render with something mad like a 8 and 1

 

[EViS]

What sort of mesh should I be using? Chicken wire or the large plastering mesh (similar to scrim tape)? Should it then be adhered to the scratch coat?

 

[limerockconstruction]

It should be fixed to the blocks and the scratch coat spread over the top of it.