Abstract: A formwork is made up of a series of plastic foam modules disposed in the manner of a brick wall and forming a mold into which concrete is poured, the formwork remaining permanently secured to the concrete to produce a concrete wall insulated both on the inside and on the outside. Each module is formed of two identical sections disposed in mirror position. Each module section is a panel having inner spaced ribs which terminate short of the top and bottom panel edges and inserts are embedded in the ribs, having apertures opening in the free spaces formed by the ends of the ribs and the panel edges. Once the two identical module sections are placed in mirror position, at the construction site, they are held together by horizontal tie-rods having hooked ends lockingly engaged in the insert apertures. Adjoining tie-rods, in modules, disposed one over the other, are further interlocked by vertical coupling rods to prevent separation of the modules during pouring of the concrete.