When masonry or wooden walls are insulated with hard-foam-slab insulation covered with plaster or mortar, reinforcement of the plaster or mortar can be omitted without incurring cracking if (a) the slabs are grooved, (b) the size and number of slab grooves have defined minimal values, depending on slab thickness, (c) the slabs have a residual shrinkage of at least 0.1 percent, (d) the plaster or mortar has a maximum plastic-resin content of 2.5 percent by weight and (e) the slab weight per cubic meter is, optionally, less than 20 kg per cubic meter.