Ben1984

Are you saying that wet plaster won't make any difference to air tightness? I was under the impression that it was much easier to achieve a good air tight score with wet plaster than it is with dot and dab.

Yeah, I've run some calcuations (using some of the online calculators): based on a rendered block outter skin, 150mm cavity filled with mineral wool you would need to use a thermalite block on the inner skin to hit 0.18 u value. I suppose my question is really this: Can I hit 0.18 with a rendered 100mm block outter skin, 150mm mineral wool cavity and a wet plaster finish internally? Is there a block that I can use on the inner skin that is thermally efficient enough to hit 0.18 without causing me loads of issues with cracking? My current thinking is that it can't be done. I would either have to use a thermolite type block to hit the 0.18 which would then be a nightmare to plaster onto or I could use a concrete block on the the inner skin but increase the cavity to 200mm to hit < 0.18. I would prefer to keep a 150mm cavity but not sure this can be achived given my desire for a wet plaster finish internally.

Yeah, this is an option I am considering. But even if I go with a 200mm cavity it still makes sense to use a thermally efficient block for the inner skin if it can take the S&C render without cracking

Thanks for taking the time to reply - which white blocks are you reffering to? I would settle for a score of 2, but am aiming for something slightly better. Wet plaster would make this easier to achieve, and it also address some of the other things I dislike about dot and dab.

Hi nod, I hope you don't mind me dragging up your old post but it sounds like you might be a good person to seek advice from. I am trying to finalise the details for our self-build; we are doing a brick and block cavity wall construction and we want to avoid dot and dab which makes it a bit more difficult to achieve a decent u-value in the walls. Could we get away with a thermalite type block on the inner skin if we are planning to use sand and cement render with a skim top coat? Or is that asking for trouble trying to render onto that type of block? I hadn't considered insulated plasterboard, it alway seemed like a very expensive option. I'd be very interested to hear your rationale for choosing this if you would be kind enough to share it.