Need advice on proper vapor barrier and capillary break placement
One of the requirements for certification under the NAHB Green Standard is to install a vapor barrier (in direct contact with the slab) and capillary break (min 4″ thick 1/2″ aggregate) somewhere beneath it. I don’t know about the rest of the country, but in my market (Oklahoma City) I would guess that 80%-90% of the slabs here do not have a vapor barrier and I’m not aware of any that have an aggregate capillary break so I don’t have a good local knowledge base of how to accomplish this.
In my research there seem to be many different schools of thought on how to layer everything and there seem to be good arguments for all theories. I would like to know from people with extensive practical experience rather than theories as to what works best. There have been scientific studies showing that concrete with higher water to concrete ratios (typical of the building industry) placed over sand is 30% stronger than concrete placed over plastic because the sand will effectively lower the water to concrete (w/c) ratio. Another study in the same article suggests that the sand decreases the strength of the concrete because it leeches water away necessary for proper curing of the slab so it should be poured directly on the vapor barrier. Here’s the link to that article by the way:
I realize proper finishing techniques and proper water to cement ratios are important, but difficult to make happen all the time. I’m looking for ideas for best practices to mitigate problems like cracking, curling, strength, etc.
My conclusion so far is to place 4″ of clean 1/2″ aggregate on the grade and then top it with 8″-12″ of fill sand (depending on my stem wall height). Next I would top it off with a 6 mil or better plastic and place the concrete on the plastic. I thought I might possibly spray a retarder on top to slow down the evaporation and reduce cracking. I would have to specify a water to concrete ratio as close to .5 as I could and try to get my contractors to place it. I have big slabs, some in excess of 5,000 s.f. so spreading low slump concrete could be an issue. I appreciate anyone’s comments and suggestions.
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