Every so often when developing construction details for a new ground up project, a question tends to arise that sparks debate within the confines of the office.
WHERE IS THE PROPER LOCATION FOR THE UNDERSLAB VAPOR BARRIER?
LET THE SLAB CURE
Before answering that let's talk about what leads to the question. Shrinkage! Concrete Shrinkage to be specific.
Humidity, temperature, airflow etc. contribute to the curing of concrete. So when concrete is in direct contact with a vapor barrier it loses NO moisture; resulting in the top drying rapidly causing the slab to shrink which pulls the edges upwards.
Placing the concrete on a sand blotter layer "is expected to help reduce curling by minimizing the difference in moisture content between the top and bottom of the slab". ** The argument seems to make sense. Especially in concrete mix designs that have high water content.
VAPOR BE GONE
One of the main reasons architects specify a vapor barrier is to stop water and vapor from migrating through the slab into the finished space.
Also keeping the slab dryer maintains the stability of installed finished floor materials. Another reason is the benefit of the subgrade friction effect. This subgrade friction effect reduces friction between the slab and subgrade allows more shrinkage contraction to occur resulting in fewer cracks forming.
NO SHRINKAGE & NO MOISTURE
So let's get back to the question at hand. Where should the vapor barrier be placed?
In the opinion of your humble writer, the arguments on both sides are intriguing. However, taking the conservative approach by placing the vapor barrier under a sand blotter layer ensures protection from water and vapor making its way into the concrete while also allowing the slab to cure more evenly by not trapping water contained in the slab mix.
So here's to minimizing concrete shrinkage!
**As described in the American Concrete Institutes (ACI) Committee 302 "Guide for Concrete Floor and Slab Construction"