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Slab below grade vapor barrier

apkitz's picture

Yes this old question again. As I read through FHB and JLC forums... There is no one superior way.... and a ton of debate..

Parameters:  Building in Southern MN--mixed climate. Putting in a basement I want to finish. As of now I think I'm doing the footings with Fastfoot. I'd like to put down 2" of foam under the slab to help insulate.... I am going to have 4" PVC drain along the exeterior and interior of the footing and run to a sump pit. Will also have powered radon mitigation as radon is all over here. I don't plan to put radiant or anything in as I am putting in a Geothermal-forced air unit. I am putting up an SIP home with and SIP basment package--so I am going the extra mile for moisture control. Soil is a hard loam around here.

Bare with me.... these most common ways I have seen from hours of reading forums.... I have found about 4 main themes of prep people are using...

earth, poly, foam, gravel, slab

earth, foam, poly, gravel, slab

earth, gravel, poly, foam, slab

earth, gravel, foam, poly, slab

I realize foam/poly can be redundant if you tape the seams of the foam--but with our high radon around here..another $100 bucks doesn't bother me.

I have read that people used to think it was cool to put the sand mix over the vapor barrier to help even out drying, but then some say it will stay too wet if the sand was wet and you get curl and vice versa...

So what are the pros opinions on proper prep? What's working for people in the mixed climates--I ask this as I have a lot of shotty concrete guys here and have seen a lot of slabs fail--and some of these contractors change there routine--not based on science--but on how fast and cheap they want to be. I will be pouring this with an old family friend that I trust--but I want to see what others are using before I go after him with my ideas as he is old and set in his ways, but good. He just isn't as dorky or interested in the building science part as me.  

Thanks guys--Andy


Order (post #207345, reply #1 of 2)

Undisturbed or compacted earth, gravel also compacted, foam, poly, and then concrete with reinforceing meash, steel and fiber.


Concrete doesn't dry.  It cures through a chemical reaction called hydration.  Hydration is the process in which the water in the concrete reactes with the cement to form a matrix. The reaction starts immediately upon adding the water.  It will continue for the next 28 days until it is near complete at that time.

Adding to much water to a mix to make it easier to place reduces the final strenght of the concrete.  It is a common practice to soup up a mix to to make it more workable.  This is ussualy seen as excessive bleed water coming to the surface as a slab is finished..  Adding sand beneath a slab and over the vp is a way to let some of the bleed water migrate out the bottom of the slab.  IMO and many others this is wrong.  The purpose of the vp is two fold, (1) keep ground water away from the bottom of the slab where it can wick though it and creat a damp basement, and (2) the vp also helps control the cure rate of the concrete by keeping the water in the mix where it is supose to for the reaction.

Workability is always an issue with concrete crews. It is just plain old hard work to place concrete that is stiff and dry than it is souped up with water.  That means it takes longer and cost the contractor more in labor.  It also means that thier needs to be more and better finishers on the job because the intial set is going to be faster.  Again more cost to the contractor.  The solution is to have a super plastizer add mix addded to each load when it arrives on site.  The placing crew gets a nice workable flow so they can get it down quickly.  The finishers can get on it sooner andyou get a slab that will likely exceed the expected yeld stength.

Most of us don't have the luxurey curring a slab properly for 28 days.  We need to get on it and start framing asap. I try to pour on a Thursday so that I can get at least 4 days to wet cure on a new slab before any other trade gets on it.  By wet curing I mean keeping the slab suface wet or damp by covering it with another layer of poly,, a layer of wet burlap, or even running lawn sprinklers on it.  This assures the slab does not loose the moisture it needs for curring to evaporation.  The longer you can keep the slab wet the better your cure rate and stronger the slab will become. It also helps reduce the amount of shrink cracking you get.  The intial hydration reaction in concrete is exothermic.  It produces a lot of heat, which means that the slap expands a little like msot other material when it gets hot.  As the reaction slows down, the slab cools quickly and because the suface has set and exposed to water lose through eveaporation, you get shrink cracking (fine hair lines cracks).  Keeping the surface wet those first couple of days greatly reduces the amony of shrink cracking and the severity of it.

Thanks! (post #207345, reply #2 of 2)

That is an awesome response, and just make plan sense... I appreciate it a ton!