Hardwood Over Radiant Slab – Another Vapor Barrier On Top?? – GreenBuildingAdvisor

Hi, long time reader and practitioner, but 1st post 🙂 I’m hoping for some of the talented wisdom I often see here…

QUESTION:  

1) do i break rule of no double vapor barriers or the rule of need a vapor barrier on top of slab to protect natural wood flooring

2) Is my tapered sleepers & plywood good or do I remove and revert to.. WHAT?

CONTEXT: Full gut rehab and expansion of former 1850s schoolhouse in Vermont (climate zone 5a) as single family.  Both former crawlspace under kitchen and new living room addition (together 1,500 sf) have R15 XPS insulated 4″ slab floors over gravel with in-slab radiant heat.  ICF foundation walls to below frost line.  On top of crushed stone/below concrete 10 mil vapor barrier placed, not taped, but overlapping joints. Likely decent but certainly not meticulous.

Importantly, slab laid 1 year ago so fully dried.

We will now install natural strip wood flooring (5/4 black walnut locally milled to just under 1″ with T&G. Widths 3-8″. I can control milling spec)

DISCUSSION: I understand the general rule not to have dual vapor barriers so as not to trap moisture with no escape path.  I also understand natural wood on slab sensitivity to moisture and frequent cry for top-of-slab VB.

Because slab and surrounding old-house areas of joisted floor are not perfectly flat, in Kitchen zone (was crawlspace, now slab) we glued tapered pressure treated sleepers tapering from 1.5″ to 0″.  Then glued/screwed 3/4″ continuous plywood on top of sleepers/slab.  Where sleepers are 0″, PT plywood glued to slab.  Where >0, plywood glued & screwed to sleepers 

In new living room addition, we have not yet added any subfloor system (waiting for guidance here)

OPTIONS (likely others):

1) Keep as done in kitchen and repeat in living room addition (no 2nd VB, continuous plywood subfloor either glued to radiant slab, or glued/screwed to sleepers, which are glued to slab)

2) Keep Kitchen subfloor system described, but add VB on top of plywood, below finish flooring. Same or different in living room addition.

3) Remove subfloor system described and add VB below plywood or below sleepers. if below sleepers and/or where sleepers taper to 0″ how to affix system to slab (to protect radiant tubes)

4) If because need VB directly on slab but cant penetrate slab, or because flaw in existing tapered-sleepers-with-plywood system: remove subfloor system and (add VB?) and then fully “floating” subfloor system of interlocked sleepers and plywood not affixed to slab, but “floating” on slab (still no penetrations into slab to protect radiant).  If so, VB or not?

[5. revert to manufactured flooring. This is my absolute last resort option and will be a huge personal failure to do this. I hate it.]

IN SHORT:

1) do i break rule of no double VBs, or rule of VB on top of slab to protect wood.  

2) Is my tapered sleepers & plywood good or do I remove and revert to WHAT?

Thanks in advance for your smart thinking.  Sorry for a long post…

-Caleb