Full fill cavity on block/render wall

[garyo]

I intend to have a 100mm full-fill cavity in my block/block rendered walls on my new extension. I thought this was fairly normal but have just read this...

"The combination of full fill insulation and rendering can also inhibit the drying out of any moisture that enters the outer leaf of masonry. This can potentially increase the risk of frost action and sulphate attack on the jointing and rendering mortars. Therefore it is not recommended for full fill insulation to be used on a rendered external masonry leaf."

Am I about to do something unusual/risky?

Thanks

 

[JohnD]

have just read this

where?

 

[garyo]

where?

Here... stumbled on it whilst Googling about weep holes...

https://www.labcwarranty.co.uk/media/118314/weep holes in rendered cavity walls.pdf

 

[JohnD]

looks like a very weak comment to me.

Unless you are going to plastic-spray the render, or use an impervious coating, I find it hard to believe it will prevent water vapour permeating.

Maybe it would make a difference if the house was part-built during winter and the blocks became saturated.

 

[garyo]

looks like a very weak comment to me.

It's confusing too. The next page goes on to say that they don't consider cavity tray/lintel weep vents to be required on render finished walls due to the lower risk of water permeating, and that any water that did enter would be able to dissipate through natural ventilation...

 

[JohnD]

IMO it is unlikely to be relevant unless the wall was already wet, or needed drying out after a leak.

 

[23vc]

I read exactly the same thing this week Gary, haha. I rang my bco who dismissed it instantly. Lucky really as I'm 4 courses up now with the blocks

 

[garyo]

I read exactly the same thing this week Gary, haha. I rang my bco who dismissed it instantly. Lucky really as I'm 4 courses up now with the blocks

Cheers 23. And are you bothering with weep vents then?

 

[23vc]

Not planning to mate unless he says I have to

 

[^woody^]

Longer term, render may crack and then there could be the problem of damp insulation.

So it is more of a case of increased risk, which would need to be mitigated with increased and regular inspection for cracks.

Most people won't bother inspecting regularly, in which case risk is higher, and so full fill insulation best avoided.