A lot of customers contact me asking whether a vapor retarder is really necessary when glueing down an engineered floor to a concrete subfloor. In my opinion, absolutely.
In general, manufacture warranties require a vapor retarder only if the calcium chloride reading is greater than 3lbs. However, only a tiny percentage of homes in North America feedback calcium chloride readings within these requirements. Moreover, concrete emits moisture throughout its life as it adjusts to the changing climate that surrounds it. As such, a reading taken one season may be significantly different the following season. For these reasons, it is strongly recommended that a vapor retarder is applied.
To further this point, in February of this year I attended a NWFA course titled Introduction to Wood Flooring lead by Steve Marley, Technical Director at Johnson Premium Hardwood Flooring. The course was geared towards beginners but was attended by installers ranging in experience of 1-15 years. Steve himself has been involved with hardwood flooring in one capacity or another for over two decades. Even though the NWFA does not require a vapor retarder for concrete with a calcium chloride reading greater than 3lbs, he won’t install over concrete without one and strongly advised to all in attendance to do the same. Good enough for me.