FredRay, if you are referring to an INSIDE slab, the consequences may be little or great, depending on several geological factors. The use of a VB is prophylactic, so it is better to always use it. One reason is the fact that concrete is subject to capillary suction (water will "wick" up, as witness a concrete block set into a shallow puddle). Thus damp basements, leading to mould, rot, and health (IAQ) problems. A good drainage layer, such as 6 inches of crushed stone, is also an essential capillary break.. In addition, concrete is prone to vapour diffusion, whereby the small gaseous H2O molecules can easily work their way through the microscopic interstices in the concrete matrix. Some concretes, such as a high-volume fly ash (ASTM Class F) are more water and vapour-impermeable than others, but the additional protection of an extremely low permeance Vapour Diffusion Retarder (VDR) is essential. Thirdly, in areas prone to soil gases, such as radon gas (often coming through small fissures in a granitic formation), a low permeance shield is needed (along with sub-slab depressurization techniques to keep the radon out of the building entirely). The funny thing about radon gas is that one house may be radon-free, while its next-door neighbour could be riddled with it – all depending on the nature of the fissures in the rock underlay.
VDRs are rated according to their permeance ratings, in either (Imperial/US: grain/ft^2 * hr – inches Hg) or (SI: ng/Pa*m^2*s), and are usually categorized as Type I or Type II (Type 1s are better, having a metric permeance of up to 15 nanograms…, as opposed to a Type 2, with a permeance of up to a maximum of 60 ng/Pa*m^&2*s. As you can imagine, an aluminum foil has a very good permeance rating (0.03 mm = 0 perm), as does 6 mil polyethylene (3.4 perm SI). By comparison, standard 1-2-3 concrete has a perm SI of 184, and ½" gypsum wall board has a perm rating of 2026 SI!
To convert Perm SI to Perm Imp., multiply Perm SI by 0.0174
The big trouble I have with most applications of poly under a slab is the actual practice, which guarantees that it will be perforated in hundreds of places before the slab is placed. To avoid this, I generally spread a small layer of sand over the crushed stone before laying on the poly (which I lap and seal also). Then, heavy boots will not ruin the thin membrane as readily.
As to your second question, I have no experience in this area - with sealers, top membranes, etc. Perhaps another forum member can respond to this? Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
