Ventilated facades solve at least two topical problems for civil engineering: it allows reducing the humidity (concentration of water vapor) on warmed wall surface and reducing heat losses from facade to the environment. The major factor in solving these problems is the air stream, washing the facade wall in the vertical direction. For the free-convective flow the width of the channel, providing the maximum consumption of the air is called optimum width of the ventilated channel. The free-convective flow in a vertical slot gap of facade ventilated space will be stylized by a barotropic flow with an indicator of a polytrope of n, n < k proportional to intensity of a heat transfer between hot wall and air. The motivation of a choice of the sizes of the vertical ventilated channel of ventilated facades in the conditions of the free-convective flow is given in the present article and it is based on using results of numerical experiments run on real ventilated designs.