Internal insulation for block cavity walls

[Julestools]

Hi all, I want to add internal wall insulation to my 1970's block cavity walls. The cavities were filled with a mineral wool type fluff in the 90's, which appears to be dry where I've been able to see it and there is no evidence of damp or mould in or on the walls. The external render is also in good shape. 

We are leaning toward a stud frame fixed to the inside of the external cavity walls with mineral wool bats like knauf RWA45 or RS45, with plasterboard over.

Our reasons being, it's cheaper than using celotex or insulated plasterboard, it's a breathable option which, from what I gather, would carry less risk of issues with condensation and mould and we don't mind losing a bit of internal space. 

I'd love to hear your thoughts and knowledge on this approach to  internallly insulating cavity walls. Is it a good idea to do it at all? Is a vapour-open breathable approach better, or should I use a vapour barrier, as some manufacturers suggest? Any and all thoughts welcomed! Cheers, Jules

Edited yesterday at 14:23 by Julestools

[Redbeard]

Depends what you are putting it on top of. If it's gypsum painted in vinyl silk then your hopes of breathability are arguably less well-founded than with a lime plaster with breathable paint (opinions vary!!). I would make insulated studs and I'd use Intello membrane as VCL. 

[Julestools]

Thanks Redbeard. So would using Intello  increase the chances of a dry healthy wall and insulation space if using not-so-breathable plasterboard and plaster? 

 

 

 

 

[Oz07]

You'd use metal studs with the brackets and rockwool its easier than timber stud

[Redbeard]

I think , off hand, you'd want insulated stud, Intello, secondary (say 25 x 50) stud, plasterboard, but have a look at Pro Clima's site. Ecological Building Systems (UK importers of Pro Clima) may advise too. Note I was writing that when @Oz07's post came in.

 

I like Intello as it acts as a VCL but, if any moisture has got where it shouldn't, can become slightly vapour-open to let it out. 

[Oz07]

Ive seen big commercial renovations where they just use metal stud with rockwool behind and in-between. This is on 9" solid brickwork. Surely if good enough for that be fine for modern cavity?

[Redbeard]

I should probably advise a condensation risk analysis for IWI on top of CWI. WUFI is good but Glaser is at least indicative. My reason for insulated stud (in my mind a glued laminate of, say, 12mm OSB, xmm plastic insulation, 12mm OSB) is to reduce thermal bridging. Metal stud leaves a bridge all the way through the sandwich, but is a narrower highly-conductive thing against a fatter less-conductive thing. So the end result may not be so different. I don't know!

[Julestools]

45 minutes ago, Redbeard said:

I think , off hand, you'd want insulated stud, Intello, secondary (say 25 x 50) stud, plasterboard, but have a look at Pro Clima's site. Ecological Building Systems (UK importers of Pro Clima) may advise too. Note I was writing that when @Oz07's post came in.

 

I like Intello as it acts as a VCL but, if any moisture has got where it shouldn't, can become slightly vapour-open to let it out. 

Insulated stud sounds good, though someone suggested spacers at intervals to bring the studs away from the wall to minimise bridging. Then wool can go behind and between studs too. 

Intello is clever indeed! 

Would you regard pb with, say, the more porous 'one coat' plaster on it, breathable enough, or not?! 

[Julestools]

49 minutes ago, Oz07 said:

Ive seen big commercial renovations where they just use metal stud with rockwool behind and in-between. This is on 9" solid brickwork. Surely if good enough for that be fine for modern cavity?

Good point.. And yes I might be overyhinking it! We're they using vcl to keep moist air out, or somehow making it  breathable I wonder? 

[Mike]

4 hours ago, Julestools said:

Is it a good idea to do it at all?

If you have the cash and have already done / plan to do other obvious thermal improvements, then it's worth considering - unless you can apply external wall insulation, which would be preferable. If you're able to withstand the disruption of IWI then presumably it would be part of a larger improvement scheme?

 

With an insulated cavity wall the risk of condensation is greatly reduced / eliminated (depending on insulation thickness and provided rooms are adequately heated and ventilated). If it turns out that there is a significant risk then any gypsum plaster needs removing. Ensuring the airtightness of the existing wall (and all the junctions between the wall and other materials) is more important than a vapour control layer, as airflow through the structure is likely to cause much more condensation than the absence of / gaps in a VCL (as well as causing heat loss). If you add a VCL too, then only choose an 'intelligent' one such as Intello.

 

There's a long meandering thread here that's worth reading. It relates to solid walls, but still useful:

 

1 hour ago, Julestools said:

Would you regard pb with, say, the more porous 'one coat' plaster on it, breathable enough, or not?!

Plasterboard & regular plaster are vapour-permiable but, as mentioned above by @Redbeard, regular vinyl paints aren't - you'd need clay-based / lime-based paint to maintain permeability. With an insulated cavity wall permeability is normally less of a concern.

 

[Julestools]

Thanks Mike for a very helpful reply. Yes, this is part of an extensive refurb, including a roof upgrade later this year, and floor levelling and insulating currently underway. I am confident about the air tightness of the walls so currently not seeing a need to remove the  plaster, but perhaps a condensation risk assessment may be a good idea, as Redbeard suggests. 

Breathable paint, no probs! 

Thanks again👍