Expert: Dan Griffin - 2/26/2008

Question
I will pouring concrete floors for the main and upper floors of my house. There will radiant tubing in both floors. The upper floor framing is exposed beams w/car decking.The upper floor will be 1.5 inches of lightweight concrete, the main will be 2 inches of standard concrete. Do you recomend putting a vapor barrier between the concrete and the plywood sub floor as to isolate the slab from the wood structure to avoid the concrete cracking as the wood settles or will this only bring all the water to the surface and create a weak floor?

Answer
David,

I don't feel qualified to answer your question.  I have used light weight on 2nd floor wood framing with 1 1/8 decking.  We used it as a sound deadner primarily, and it is still in good shape as far as I know.  I have read and heard of other situations where the lightweight has problems about breaking up years later.  I don't know whether the problems stemmed from insufficient framing or decking allowing the lightweight to flex too much - it was always my assumption.  Visqueen won't stop the floor from cracking.  It will make the bleed water all exit the top of the slab.  The bleed water does not weaken the concrete, the danger is allowing the finishers on too soon and sealing the bleed water into the slab which will cause delamination and fracture planes.  I assume you are working with an architect or supplier with better knowledge about radiant tubing in a very thin slab section.  I may have suggested self leveling gypsum.

I assume the lower level is a concrete slab and you adding a topping slab with the tubing.  The system I know of uses a sand bed, rigid foam isolation/thermal break, wire mesh with tubing tied to it, full depth slab poured over the tubing.  I know other systems exist, but I am not familiar with them.