Having gone off-topic in http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17757&page=1#Comment_300578, am continuing a pargeing discussion here.

Seeking advice on using insulating lime plaster as airtight pargeing on old masonry before IWI.

What I'll be needing it for, is where well-built (flat/plumb but rough surface) rubble Edwardian stonework is lined with lime plaster on lathing on battens, with original skirtings and plaster cornice, to remain, which define the depth of IWI to replace the battened plaster. So prob 50mm IWI incl plasterboard. The Victorians/Edwardians imagined this would provide some insulation, but in fact does worse than the hard plaster that's used instead at 1stFl. This is because the hard plaster at least makes the wall airtight, while the unplastered rubble at GrdFl is highly wind-permeable (and equally permeable to chimney-effect-driven infiltration even when not windy). So at 1stFl the rubble's modest insulation value can have effect, as outside air is impeded from whistling right thro it, but at GrdFl it's unmitigated outside air right behind the lath-and-plaster - same as the plasterboard-tent effect of modern dot n dab in estate houses.

The IWI we will prob pay for Aerogel behind plasterboard, on the parge coat. The thinner the parge can be, the better, and if a little bit insulating itself, also better. My questions are:

1) can insulating lime plaster, with its insulating inclusions, in fact be airtight? At what thickness, for airtightness?

2) separate concern - what thickness must a) ordinary lime plaster, b) insulating lime plaster, be for ease of application, not becoming an expensive faff if too thin?

3) on flat, plumb but rough-surface rubble, obviously plenty of thickness where filling the dips, but a bare skim over the high spots? Will that work?

Thanks Dave, yes, and I've a hope that some (more) GBF regulars may have either upfifting or bitter experience on the matter.

I have no experience to offer but did in passing come across an offering from https://www.lincolnshirelime.co.uk/insulation-systems-46-w.asp which I though looked interesting & might be for your application: their LimeCO2te WarmCO2te product " is made with lightweight mineral as the aggregate in place of sand and my results so far put the Lambda value at around 0.125 W/mK, that is about 5 x more thermally efficient than a standard lime plaster and 8 x better than sand and cement. "

At the risk of hijacking this thread I understand that plaster shouldn't bridge the DPC. In some properties - we're a case in point - the DPC is well above floor level so either one ignores the potential issue as we did (through ignorance) or does something else. What should that be..? I know that under current regs the floor DPM should lap to the wall DPC - again something ours doesn't, due to a builder mess-up - but plastering over membrane doesn't seem practical?

None of this seems to be addressed in any of the IWI details one finds online?