The walls of greater Dublin housing estates include mass concrete built in the Inter-War years (e.g. Crumlin estates), un-insulated solid and hollow concrete blocks in the 1950s, 60s and 70s (e.g. Raheny estates) and drylined hollow block from the late 70s till today (e.g. estates from Blackrock to Rochfort Bridge). It’s ironic that these walls are seen as being very different to the massive brick walls of Georgian townhouses and the rubble walls of Ireland’s vernacular buildings. While they are quite different in terms of the thickness of the walls and the materials that went into them, they’re very similar in one critical respect: they are all ‘single leaf’ masonry constructions.
This means the same masonry wall holds up the building, isolates the inside environment from the outside, and buffers wind-driven rain on the outside and moisture generated inside. As such it is subject to a complex range of forces as temperature, air pressure and moisture content change through the year across its width. In other forms of construction some of these forces are separated. For instance a well-built cavity wall, which has an inner leaf of timber or block interlinked but continuously separated from an outer leaf. The cavity isolates moisture gained from external conditions from moisture gained from the room. Both leafs have the potential to dry out towards the cavity or their other face.
The room face of an un-insulated single leaf wall can be within a few degrees of room temperature as it is warmed by the radiant heat of a fire or the moister convection currents of a wall-mounted convector heater. This means that the point where vapour condenses (the ‘dew point’) can be further into the masonry. If built with the right materials moisture tends to continue migrating slowly outwards. Switch off the heat in Winter, or increase the amount of moisture being produced in the room, and the ‘dew point’ can move back to the room face of the wall. How often have we all felt walls that were not only cold but damp to the touch?
It comes as a surprise to many people, even those within the Industry, that every drylined wall buildup, no matter how good, will result in vapour condensing inside it every Winter. This is because the insulation isolates the masonry wall so that its room-face cannot warm as before and consequently becomes more uniformly cool and also wetter . This is typically where the most extreme temperature change takes place and vapour condenses.
When a material or surface is warmer than about 15º C, relative humidity is greater than 75% and convection currents or radiant heat are unavailable the potential for mould growth is high. A well-designed system is one that thus limits the amount of moisture and vapour (i.e. water as liquid or as gas) reaching this critical point and then allows whatever does reach there to dry out as quickly as possible. Given the health issues associated with mould it may be argued that good internal insulation must primarily be about careful management of water and secondarily about retaining heat!
The Author believes that a conservation-based approach is the appropriate one to apply in upgrading all single skin walls, regardless of their age. A conservation architect and builder are trained to look at each building differently and to be very aware of the impact of orientation, exposure and the original building materials. Every step must allow the wall to dry out, generally in both directions. Ideally materials should be graded based on their vapour permeability: least permeable on the inside, most permeable facing the cavity or the Elements.
 Künzel, H.M., ‘Effect of interior and exterior insulation on the hygrothermal behaviour of exposed walls’,Materials and Structures, Vol. 31 (March 1998), pp 99-103