About the walls
We saw precedently that as soon as the surface of the wall exceeds by few degrees the ambient temperature, the kilowatts of heat flees away, by convection and radiation. Only the wall of the house can, due to its bad conductivity, resists the thermal pressure, like a dam resists water pressure.
Losses by radiation are in parrallel, simultaneous, with the losses by convection, and are thus negligible with respect to the latter, especially that, during the day, the flux is reversed : a stone in the sunshine seems hot, while at night it feels cold, though in both cases it is at ambient temperature, or very close.
The conductance of the convection, with an average wind, is : 8740/220=40 W/m2.°C, that is to say a thermal resistance of R=1/G=0.025 °C.m2/W, which will thus be neglicted in future calculations.
If we want to limit the losses of a 120 m2 house to, for instance, 4 kW with -7°C outside and 20°C inside, we need a volumic inside coefficient of : 4000/27/300 = 0.49 W/m3/°C, which gives an average surfacic coefficient of 4000/27/350 = 0.42 W/m2/°C.
A long time ago, castles, and farms, because of the materials, had 60 to 80 cm thick walls, and even more, with very small openings. Without getting into to much details, this gave at best a surfacic coefficient of : 3/0.6 = 5 à 3/0.8 = 3.75, that is to say about 10 times what we are looking for. Fortunately, chimneys were big and forests too !!
With breeze blocks of 20 cm, we get a coefficient of about 2, which is still 5 times too much
The best off was the caveman, who with about ten metters of rocks above the haid did not fear cold neither rain, and because there always was a fire in his cavern, the vault radiated a sweet heat. What a comfort !!
Hence the necessity to increase the thermal resistance of the walls and go back to the technic of the cavemans.
The insulation of a house is the suppression of thermal losses in order to get a comfort at a good price.
In order to get a thermally resistant wall, it has to be made of light materials, but then it will not be mechanically resistant.
Thus two walls must be built : one which is mechanically resistant and one which is thermically resistant. Materials which have both properties at the same time do exist, but the thickness must be doubled. Furthermore they are often difficult to use : we did not got anything financially, and the problem of the thermal bridges and of the thermal comfort are still there.
A breeze block wall of 20cm thickness covered with 10cmm of polystyren has a coefficient of surfacic conductivity of k=0.33, for a cellular concrete wall of 40cm, k=0.50, for alveolar bricks of 40cm, k=0.70, for a sandwich wood-polystyrene-wood of 10cm, k=0.40.
Generally, nowadays, two walls are built or somehow.
Where the insulating wall should be place ? Inside or outside ? Nowadays, it is generally placed inside. But then, it is not a wall anymore, it is a juxtaposition of insulating panel, separated by load-bearing screeds and partition walls, that is to say separated by thermal bridges.