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.->--,...--,:~ ~". FLOOR SLAB MOISTURENAPOR PROTECTION Floor slab design rel-ative to moi:::;turdvapor protection should consider the lype imd loca1jo~1 of two elements, a granular layer <Uld a vapor membrane (vapor retarder, wa(cr resistant harrier or vapocharricr). 11It11e lolJov..illg sections, the pros and cons of the possible options regarding these dcmcnts will be presenled, such that yoU and your specifier can make an engi.neering decision based on the benefits and costs of the choices. GRANULAR LAYER 111 American Concrete Institute (AU) :;02 r -'i6, a uhase mak~riar' is recoilllllenJed, f<:llh~'l' than the conventional deaner -'.-;and :.: "~shion" l~l<-lteriaI. TIle J!1'JHlwl m;ml1a~llS 0;;;1 '.1l';-tJ] .;.~~.nd (commOll "cushi0:J' "':.~; J I~; ~lii~lc,.lt 1" 1''..lmpact and mainttlil1 un,i] collcrde placement is complck. I\Cl recommend." a clean, fille graded Ill,Hcrial (\\ iUl ,1tlc.aSl ] 0% to 30% of particles p3ssillg :1 #l 00 si~\'e) which is not contaminated \,.-i1h c]a~'. silt or organic Ilw1criaJ. \Vc rc!cr you 10 ACI 302.1~96 for additional details n~galding tile requirements [or the base matcri::il In case...;; where potential stalic water levels or sigllificant perched water sources appear near or above the floor slab, an wlderfloor drainage system may be needed ".ilerein a draintik syslem is placed \\ithin a tilicker clean sand or gravella)"er. Such a system should be properly engincered depending on subgrade soil types an ratc/head or water inflow. VAPOR MEMBRANE The Ileed tor a vapor membrane depends on whether Ule floor slab will have a vapor sensitive covering: wilJ have vapor sensitive items stored on the slab, or i[1he space above the slab '\\111 be a humidity controlIed area. If the project does not have' this vapor sensitivity or moisture control need, placement of a vapor membrane may not be necessary. Your decision will tllen relate to whether to use the ACI base material or a conventional sand cusmon layer. However, if any of the above sensitivity issues apply, placement of a vapor membrane is reconmlended. Some floor covering systems (adhesives and flooring materials) require a vapor membrane to maintain a specified maximum slab moisture content as a condition of their warranty. VAPOR MEMBRANE/GRANULAR LA YER PLACEMENT A numberr of issues should be considered when deciding whether to place the vapor membrane above or belo\" the granular layer. The benefits of placing the slab on a granular layer, with the vapor membrane placed below the granular layer, include reduction of the following: a- Slab curling during the curing and drying process. " Time of bleeding, which allows for quicker finishing . Vapor membrane pWlcturing. . Surface blistering or delamination caused by an extended bleeding period. .. Cracking caused by plastic or drying shrinkage. The benefits of placing the vapor membrane over the granular layer include the following .. The moisture emission rate is achieved faster. ..Eliminatesapoten~_al water__!"e~r\'().ir_~_tbiIJ. th~ !IT<Hmlar lay~r above the membrane. " Provides-a '-'Slip slu-[ac~", thereby" reducing slab ;straint and the associated random cracking. if a membrane is to be used in conjunction v,'ith a granular layer, the approach recommended depends 011 slab usage and the construction schedule. The vapor membrane should be placed above the granular layer when: .. Vapor sensitive Ooor covering systems are used to vapor sensitive items wi11 be directly placed on the slab. . TIle area will be humidity controlled, but the slab will be placed before the building is enclosed and sealed from raill. a- Required by a Ooor covering manufacturer's system warranty. TIle vapor membr.ane should be placed below the granular layer when: . Used in humidity controlled area (without vapor sensitive coverings/stored items), with the roof membrane in place, and the building enclosed to the point where the precipitation will not intrude into the slab area, Considerations ~hould be given to ~light sloping of the membrane (0 edges where draintile or other disposal methods can alleviate potential water sources, such as pipe or roofleaks, foundation wall damp proofmg failure, fire sprinkler system activatioll, etc 'D1ere may be cases where membrane placement may have a detrimental effect on the subgradc support system (e.g., eXpilll$JVe soils). In these cases, y-ou decision will need to weigh the cost of subgrade option.s illId the performance risks.