Does external wall insulation work?
The short answer is NO, external wall insulation doesn't work.
An external solid brick or stone wall: Room side heated to about 21°C air temperature. Outside it is cold, about 3°C.
Warm air in the room passes energy onto the plaster by continual bombarding until the plaster becomes the same energy level / temperature. Lots of energy is continually required.
Humidity in the room is in vapour / gas form. It will be under pressure and pushed into the wall fabric. As it cools down it becomes a liquid state (moisture / water). The change of state is termed the 'dew point'. The wall increases in moisture content.
Even winter Sunshine contains strong electro-magnetic radiation that energises the outer face of the masonry. Moisture is converted to vapour again and pressurised. It is easier for the vapour to push out into the air. The wall naturally dries out. Wind will assist by providing more energy to the vapour to disperse it into the air.
Insulation fixed to the outer wall surface and then rendered over provides little to no thermal insulation to the wall. The energy from the air atoms are still trying to 'heat' the inner wall surface to the same extent as having NO insulation.
Humidity (water vapour) in the room air will be pushed into the wall where it will condense to become moisture (water). It changes form from gas to liquid when the temperature falls. It is termed as the 'dew point'. The vapour condenses as it meets the insulation. The wall fabric increases in moisture content.
The external insulation prevents any solar radiation from energising the wall. The moisture (water) cannot re-vapourise, therefore cannot vent to the outside air. The wall remains damp as the moisture content slowly increases from the humidity.
Just like wearing rubber gloves makes your hand sweat. The glove prevents the vapour flowing into the air. It becomes liquid.
The same happens when external insulation prevents vapour release. Some firms advertise their products are vapour permiable. They will let the vapour out. However, if the molecules have insufficient energy to remain as vapour they form liquid that cannot pass through the micro-pores.
Walls become damp. Damp walls reuire more energy to raise the temperature. The result can be mould growth as the moisture content of the wall increases. During the colder nights more energy will be used to heat the air in the rooms. When the heating goes off the wall will cool down more quickly. Condensation forms from the humidity in the room coming into contact with the cooler surfaces. Mould heaven.
Putting the insulation on the room side of the wall significantly reduces the amount of energy to heat the room. The energy contained in the air atoms can easily cause the plasterboard to achieve thermal equilibrium
= efficient thermal insulation