Does glueing 3/4" solid hardwood (quartersawn rustic grade white oak) to a 3/4" ply subfloor over radiant help or cause problems with wood flooring expansion/contraction and heat transfer ? I have a 5/8 subfloor with sleepers and attached PEX and then 1-1/2" of poured concrete on top. On top of this there is another 3/4" ply subfloor. This floor structure has had about 8 months to dry out. I would now like to install the 3/4" solid hardwood on top of the 3/4 ply. In addition to nailing the flooring to the 3/4 subfloor, could I ask for advice on benefits or pitfalls for additionally gluing to the subfloor. And if a good option, any recommendations on type of glue?

Two worries about glue over radiant: it fails due to expansion and contraction and it expedites moisture transfer from subfloor to hardwood. No worries on the first if you are nailing besides, except for wasted time and money of course. This company claims that its hardwood glue is highly elastic and a moisture retardant as well. https://usa.sika.com/dms/.../pds-cpd-SikaBond-T21-us.pdf I can vouch for the elasticity of sika caulking on metal roofs, which is a much harsher environment. Sika's only caveat for radiant is a tight temp range for application: 60 -70 degrees.

I am disappointed at the lack of knowledge about radiant heat in the flooring industry. My remaining mystery: does nailing through cork subflooring negate its sound proofing qualities? Stay tuned.

One piece of advice that pops up routinely in searches is to ease into the heating season to avoid subjecting floors to abrupt drying.

Plywood is already layers of wood glued together. What's one more layer....

One more layer is chancy unless it is subjected to high heat and high pressure as plywood is.

Posted By pfoley on 11 Aug 2015 09:21 AM
Does glueing 3/4" solid hardwood (quartersawn rustic grade white oak) to a 3/4" ply subfloor over radiant help or cause problems with wood flooring expansion/contraction and heat transfer ? I have a 5/8 subfloor with sleepers and attached PEX and then 1-1/2" of poured concrete on top. On top of this there is another 3/4" ply subfloor. This floor structure has had about 8 months to dry out. I would now like to install the 3/4" solid hardwood on top of the 3/4 ply. In addition to nailing the flooring to the 3/4 subfloor, could I ask for advice on benefits or pitfalls for additionally gluing to the subfloor. And if a good option, any recommendations on type of glue?

I think instead of gluing one more layer it is better to add a rug or a carpet which will not only be affordable but also protect the floor underneath it.