Plastic above or below insulation for slab?

[Tomtint]

When I built our current home, I used 14mill Polly under the slab , on top of the gravel. I did not use any rigid insulation as we are not in a very cold climate and the Polly was primarily to eliminate and moisture from the ground as well as radon mitigation. .. Our basement is bone dry and even after 8 years there is absolutely no "basement" smell. We actually have a giant room finished that is nicer than most living rooms.

 

[mikester]

From bottom up:
clear pea gravel
poly
rigid insulation
concrete

Lately I've been seeing guys using 2" closed cell spray foam on the gravel base to the walls. I think I'd do that next time

 

[majorwager]

The higher the zone #, the colder the winter climate. Would use an extruded insulation product like DOW or Formular by O.C. (not expanded type) Also would use High Density, min 60 psi. Otherwise you will break the sheets when pouring concrete even if you use a pump. Important that the subgrade, stone/gravel is uniform and level. Built a Large Head House for the USDA, years ago, they required the insulation over crushed limestone base, the insulation, then 12 inches of course concrete sand. Somewhat of a heat sink but they did NOT require radiant piping imbedded in the sand. Guess they thought heat would not permeate the slab. Did result in a true floating slab and it never did display a crack. Vapor barrier under slab, not insulation. Pouring concrete over sand directly, would allow moisture from concrete to be absorbed by sand, The bottom of the slab would cure faster than the top, the slab would likely suffer a strength failure, by interrupting the chemical reaction of the Portland cement component. (separation)

Note, Head house is the staging/equipment monitoring, and operations area, Structure for large attached green houses. All masonry construction. The green houses they later attached were 150 ft long in three directions. Tax $ at work.

EDIT: Pea gravel is unstable, will swim all over while working on top, does not compact. Never seen closed cell spray below grade? wonder the density, would it crack and separate from weight of concrete buggy, or old school wheel borrow? Long term performance of closed cell spray below grade? Generally agree VB should be under insulation, the proper insulation.

 

[dmccarty]

Slab Edge Insulation | Building Science Corporation

This site has hundreds of articles on all types of building construction principles.

We used one of the building books from that link to specify many details on our house, including the slab, which has 4 inches of insulation and a plastic barrier.

Later,
Dan

 

[JFoy]

Much appreciate all the advice.