Given your professional background I’m sure that you are probably right as far as building regs go. I can only relate personal practical experience which has had a rather different result from that which you suggest.
A thick stone exterior wall, had in decades past been fitted with wooden battens to which were screwed some ancient large panels of what I can only describe as a sort of asbestos and cement base with a final waterproof decorative coating somewhat akin to the old melamine veneers. This was no doubt done for easy cleaning reasons as the room used to be the village Butcher’s shop. The damp had rotted the battens and so the paneling was falling away from the wall.
Removing the paneling revealed that the original ancient horse hair type plaster had come off the wall at some time in the past. Must have been before the paneling was put up as there was hardly any plaster debris behind the panels.
Basic sand / cement render (with some added PVA) was used to fill the deep holes and roughly level the surface.
Following this, 30mm polystyrene panels were glued to the wall with MAP adhesive. Once dry, 10 + 40 plasterboard backed with polystyrene was glued in place with the MAP with all joints offset between the two layers by at least 30cm.
At the time of doing the work, a local French professional plasterer and dry-waller told me that what I was doing would not work because the adhesive would fail due to the rising damp in the wall. He was wrong.
After 17 years there is no sign of any damp at all, not even at the edges of the wall. The damp cannot penetrate as the polystyrene is of course completely waterproof.
I suppose that different methods will work well for different buildings.
I have done the interior surfaces of all our outside walls in the same way and have never had a problem with it. I find that this method has created a warm and dry interior, was fairly cheap to do, and was easy to work with.
Robert its obviously better to be at the site than just reporting this and that. Also carrying out prior study to determine what maybe causing damp isdues and remedying those first. Working on historic buildings under heritage England means cement could never come anywhere near the projects, current regulation on comercial retro fit is PAS 38 with PAS 35 for domestic homes. Cement holds on to moisture long after lime or clay bricks have dried so can lead to issues of errosion of stone or bricks on old buildings as many are softer than modern bricks.
I also follow Prof John Edwards, probably the leading expert on damp and mold in old buildings and how best to approach various types of buildings and materials.
Any mold would occur behind the insulated panels so unless you are removing them you are not likely to see anything wrong from the interior side. In a converted hospital we were called in because of a smell, stripped of the insulated plasterboard in this case a bathroom with two exterior wall and there was a thick blanket of mold from floor to ceiling. The residents had to evacuate while we carried out the work to clean and sterilise the walls to treat the long term mold build up, even we wore good masks.
As I said previously, I suppose that different methods will work well for different buildings.
Happily I don’t live in an historic building so I don’t need to return it to it’s original condition at considerable expense.
What I need is a system that works, is affordable, and will outlast me, so another 20 yrs will be just fine. I believe that the method I have used will meet my needs, and after that I expect the council will compulsory purchase and demolish the entire row of houses of which mine is the only habitable one.
A note of caution: apart from other considerations such as permeability, picture hangin…plasterboard finish changes the acoustics in the room dramatically.
The walls of old buildings require special attention: stone, brick or wooden structures have an intrinsic regulation of water vapour that requires the use of compatible insulation materials that are breathable. The most suitable insulation for old buildings are wood wool and wood fibre, cellulose, hemp-lime, hemp wool, etc., all plant-based materials that retain their insulating capacity even in the presence of water vapour.
I have seen hemp plaster used (hemp + lime). Here is just one link for information:
What I like about wood wool is you can skim it with lime plaster so maintaining the full breathability of the build fabric. Others need protection via vapour and non vapour barriers. Sheeps wool is AFAIK the only insultion that keeps working whilst damp/wet. It is a subject to get to grips with as is the ventilation aspect.
Placo can take the ring out of high frequency sounds but doesnt do much for the base frequencies but thats another rabbit hole.
So here’s my question to the crowd : in our Irish fixer-upper (in Kerry, so pretty humid), the construction is 1920s single-skin riverstone (aka rubble held together with whatever binder they could find at the time) coated inside and out with cement. I am still undecided about the choice of insulation to put between the wall (that same cement coating which has a very thin plaster skim coat and which has been painted over many times ). A fair amount of cement/plaster skim will need replacing as filamentous mould has infiltrated in between the layers, and we would obviously need to address that. At the moment, we seem to have managed to at least stop its growth by opening all of the window vents and through judicious use of some portable heaters.
I am at ease with the use of plasterboard and fibreglass/rockwool insulation, so I’m thinking maybe just go with that, because if any humidity does start appearing in the future, I can just rip it out and replace it. I’ve already put up most of the steel C & U section framing for the inside partitions upstairs. We ripped out the old studwork as it was full of woodworm, and have treated all of the floorboards and beams.
That said, I would welcome any other suggestions for consideration. One thing to bear in mind is that the cost of any building material in Ireland is higher even than in France (which is already quite high), and any high performance stuff is off the scale. As we’re not entitled to any of the SEAI grants, we’re just hoping to get the cottage to a state where it will retain a modicum of warmth and remain dry!
Suggestions ?
For an idea of what I’m talking about when I use the word “riverstone”, see picture below.
Two extracts from the above linked article: "Very Cold Climate
These include homes in locations that have icy winters despite experiencing hot summers. The barrier should be installed on the interior of the insulation.
Absorptive Building Cladding
Some building cladding materials are also absorptive and tend to retain and release moisture. These building materials include wood siding, brick, stucco, stone, and fiber cement siding.
When your home has this type of cladding, especially in a mixed climate, a vapor barrier should be installed on the exterior of the insulation."
I’m in the Charente - where temperatures can range from -11c to 40c.
My house has external stone walls which are 80cm thick.
The room which I’ll be insulating is an attic bathroom, which will have WEDI panels on the room side of the wall.
Bearing in mind the above, where should I place the vapour barrier?
On the interior or exterior of the insulation??
Vapour barriers are primarily there to stop the humid hot air inside a dwelling from passing into the insulation until it reaches it’s dew point, when it will then condense inside the insulation & make it wet & ineffective (not to mention the possibility of mould).
Any…
…needs to be ventilated to allow it to breathe i.e. to not get wetter & wetter& thus spread damp into the interior. This is why using modern cement mortar is a bad idea for stonework.
I’d look at the outside “render” first. One problem with render is when it starts to crack - water gets behind and then just soaks into the “building”. Even taking out cracked/loose render improves it - still gets wet but nothing gets “trapped”.
Then ventilate and heat inside before covering it. It should dry quickly once you solve root causes be it from inside or outside
This is why giving recommendations without being there is risky, it is a more complex issue irradicating damp and mold whilst maintaining a comfortable interior
