Sand and cement rendering has three serious disadvantages:
structurally, sand and cement is inflexible so it is prone to cracking. During situations where the sand and cement rendering material experiences extreme weather such as during Summer or Winter, large temperature changes causes the render to expand (Summer) and contract (Winter) abruptly, resulting in cracks. Once this has happened, water gets into the cracks and soaks into any voids between the render and the substrate (e.g. brick or stone substrate). The older the render, the more chance there is of cracking and the bigger the voids due to the rendering material undergoing significant expansion and contraction. Once there, the water will not be able to leave the render and so instead of evaporating into the open air, it evaporates into the house itself. As a result, big patches of moisture form on the wall.
this render reduces the ability of a building to evaporate and circulate the moisture that’s produced inside the house itself e.g moisture coming from the occupants and areas of the house further away from condensation often occurs as a result. Note: on houses with cavity walls, neither of the first two problems stated above are particularly troublesome; if rainwater passes through the render and penetrates the wall’s outer skin, the cavity allows any water to trickle down to the base of the building and thence away. Similarly, any moisture in the internal atmosphere of the house e.g produced by the occupants etc can exit the building via the internal skin and evaporate into the cavity. Condensation can still occur on walls with these orientations but it’s less likely to be a serious problem if there’s a cavity.
Sand and cement render requires it to be repainted as a result of cracking occurring so this type of render has a fairly high upkeep.