Reinstate lime plaster if external render cement&tyrolea

[NT]

sounds ideal, if you want condensation and black fungus

I'm a bit unclear why that would result. Is it a question of whether the ventilation provided is adquate for the level of anticipated water ingress?

The water level in a wall depends on the ratio of water ingress to evaporation. Cement renders crack, letting water in, but are not permeable enough to allow more than minimal evaporatoin to occur. If you then stop or greatly reduce evaporation on the other side too, you've got a recipe for a water content that goes ony one way, up and up, until saturation occurs. If well sealed this takes a long time, but the same process still happens. Unfortunately soaked walls can sustain considerable freeze thaw damage. And of course the interior insulation makes them much colder.

Now with interior insulation with vented cavity, your new insulation substantially drops the temperature of the existing wall material, causing condensation to occur at much lower RH than now or than original design. And you permit interior air access to this now cold wall.

Putting 2 impermeable layers

The insulating board planned is one integrated layer of insulation and plasterboard.

What type of material determines what porosity, but whatever it is youre still making the water balance much worse. Really youre heading into a likely problem situation imho. You might get away with it, but I wouldnt try.

I might begin by checking out the exterior render to see fi there are any weak or loose patches that could be removed and replaced with lime, to give evaporation areas.


NT

 

[paulbandler]

Thanks for the further response - I really appreciate you taking the time. I hope you don't mind me keeping up the quesions, its just that I would really like to be able to improve the insulation so am motivated to bottom these questions out so...

RH
? Relative humidity?

And you permit interior air access to this now cold wall.

Well not quite - the intension was to vent into the floor and ceiling spaces behind the board so the air would be relatively cool and not too laden with moisture from living. Will that help?

I might begin by checking out the exterior render to see fi there are any weak or loose patches that could be removed and replaced with lime, to give evaporation areas.


Thanks, patching with lime wherever possible seems very like a very good idea - but I think there is limited scope for this at this stage (at ground accessible areas anyway).

 

[BMcD]

In my opinion you'd be best off hacking off all the cement render and then re-render using insulated lime render. See link

http://www.telling.co.uk/unilitplasterandrender.htm#Unilit 20.

You can leave the interior lime based with no need for insulation boards.

 

[monkey writer]

Hello, how can you tell if the renter is lime or cement? I assumed that ours was the original lime render... but we just had large holes drilled through for the boiler flue and looking at the cross-section of the render seems very hard and solid so I'm now suspecting it is more likely to be cement. The property was built in the 1890s and is a Victorian terrace in Folkestone, Kent, in the workers end of town.