A concrete slab underlayment product is used at an excavation area at which a concrete foundation slab is to be poured. The underlayment combines a vapour barrier layer with a set of foam insulation bodies. The vapour barrier layer spans fully over the entire set of foam insulation bodies, which are spaced apart from one another at least at lower ends thereof opposite the vapour barrier layer. This leaves drainage/air spaces open between the foam insulation bodies when laid in an installed position atop the floor of an excavated area. In use under a concrete slab, the vapour barrier layer forms a gas and moisture barrier, and the foam insulation bodies and the drainage/air spaces therebetween form a combination of void spaces, drainage channels and insulation blocks between the concrete slab and the floor of the excavation area.