Insulating primer in aqueous dispersion for gypsum-based substrates. Suitable for the insulating treatment of gypsum-based surfaces, prior to laying ceramic tiles with cementitious adhesives. Also suitable as anchoring coat prior to the application of gypsum-based plaster, levelling layers, cementitious levelling layers on absorbent substrates such as: expanded cellular cement, concrete, bricks. The film of the dried product gives the following characteristics: it reduces the substrate absorption. It increases the working time of the plaster and avoids the formation of shrinkage cracks. It consolidates the substrate surface. It increases the adhesion of various types of claddings (paints, plaster, wallpapers, tiles etc.).