Is the damp inside on all the affected walls or just in one place?
Is there damp inside both properties or just one?
Could it be a terrible coincidence? e.g. a leak inside that coincides unluckily with the efflorescence on the outside?

Having said that, the window sill is very green. It suggests very much that the gutters are overflowing down the walls. I have a pvc window sill on a north-facing flat-roofed bathroom that rarely gets cleaned (even though it is single storey so is at ground level!) and it's not as green as that. It did grow some moss at the ends though.

The cavity in the wall is (presumably) a continuous/connected space across each face of the building but (presumably) there are fire barriers dividing the cavity where the two semis meet

So the cavity can be wet on one property or elevation and dry on the adjoining one? Hence the efflorescence one side and not the other.

Question whether the cavities are still open and ventilated and acting as safe drainage planes as originally built, or whether they are filled/blocked, on each elevation?

If (some of) the cavities are filled and their weep holes blocked and they have been covered with impervious brick/cement, then there's nowhere for moisture to escape once it gets in. Eg if it gets in from dubious gutter detail, or from driving rain or humid internal air.

There are some parts of the country where cavities should not be filled because of driving rain.

The great thing about EWI is that it's been found, for over half a century in N Europe, and a bit less in UK, to be extremely reliable and problem-free, in fact elegantly solving typical other housing problems like mould, penetrating damp.

It takes a magnificent effort to mess it up, as in cheapskate schemes in UK, govt funded in a way custom designed to attract cowboy profiteers without any quality control, such as we see here, to create a body of typical problems and a bad reputation (just like with heat pumps) in the media/public mind.

Posted By: WillInAberdeenFull fill cavity with render finish is allowed anywhere - but not zone 4 with narrow cavities or injected foam

Here in Anglesey (Zone 4) a rendered cavity wall must have a 50 mm gap between the outer wall and the insulation. It is also permissible for there to be no weep holes over lintels. If it is a brick finish outer wall the gap should be 75mm.

Posted By: WillInAberdeenIsn't clear how this applies to brick slips on EWI on a filled cavity.

Probably a much greater risk of penetration of water into EWI than a full brick. (100mm v's a 10mm slip or thereabouts). Instinctively I do not think brick slips are a good idea over EWI even though the subject is covered in some depth in Pearson’s book External Wall Insulation as a suitable cladding. Recessed pointing is only recommended on the thicker slips and the pointing to be a specialised waterproof coloured mortar..

By the same token, if the EWI were impermeable then water wouldn't be getting out through it to cause the efflorescence. I guess there are joints between sheets of insulation, even if the material is waterproof..

Mikee said on the previous page that its unknown what material the EWI is or whether the cavity is filled so maybe those are the next things to find out, drill a hole.

Posted By: Peter_in_HungaryIf the cavity isn't (properly) filled then the chances are that much of the advantage of EWI would be negated.

So everyone says, but I'm not so sure. Is only true if cold outside air is able to enter the cavity, through the EWI and through the outer skin, or at some place bypass both those, and blow around in the cavity.

In a terrace or semi-detached, yes if neighbour's outer skin is un-EWI'd and stone cold, then their cold cavity will transmit its cold laterally into yours, inboard of the EWI, unless carefully blocked in some way. Is that the case here?

Yes - the inner skin, cavity space and inner face of outer skin all become part of the internal environment of the house, only slightly below internal temp. Then, if the cavity is in fact open to a cold loft, then there will be convective exchange between them. But not if the loft is also part of the internal environment i.e. roof insulation at rafter level. And not if the cavity, though unfilled, is blocked from the loft. So, that fear is in fact a worst-case, which can be thought through and remedied, in fact should be a strategic consideration in where to place roof insulation - at loft floor level is usually a unconsidered cheapo way of doing it. Therefore saying that EWI is compromised by unfilled cavity, is too rule-of-thumb.

Posted By: WillInAberdeenround outsides of window frames

One of the virtues of EWI - windows now seated/sealed in the insulation zone, not leaking direct into the cavity.

Posted By: WillInAberdeenground floor joist pockets

would be in the inner skin, not outer.

We should go round pointing any cracks in the exterior face, esp in the render, if it's rendered, which if in gd condition qualifies as a full airtight layer - and any other holes like electrician's, or underfloor air vents etc. These can be comprehensively seen, on the outside. Then add the rendered EWI over that incl good rain-sealing around windows incl ends of/under subcills - should then be pretty airtight - otherwise where is the airtight layer when EWIing a cavity wall house? Yes, the plaster layer too, but that a much harder task, as that indeed is where the multiple penetrations are.

Rafter level insulation can be done from the inside, if the extg tiles/slates aren't going to need re-laying in the near-foreseeable, and (big if) if the old non-permeable underslating felt is also undeteriorated (if there's been any roof-leaks history, then it isn't). Note, if insulated at rafter level, no need to maintain or improve eave-to-eave ventilation of the loft space (which has become part of the internal environment) - it can (should) be blocked, thus near-enough avoiding the traditional eaves-insulation-continuity problem.