Cork Underlayment and Radiant Heat

Q: We are ready to buy an engineered wood floating floor, but have gotten stuck on an issue.
I have a single story house built in 1951 with slab radiant heat as the sole heat. Most of the house currently has glued down cork tiles about 4mm thick, the rest is carpet (which has been ripped out down to the slab) and peel and stick linoleum/?vinyl tiles. The system heats fine with this setup (old boiler in garage at 120deg F). We plan on installing a floating engineered oak floor. To keep the floor “sounding” the same throughout, I had planned on leaving the old cork tiles as an underlayment (adding vapor barrier on top), using a vapor barrier and adding 3mm cork underlayment in the exposed slab (where the carpet was) and putting a vapor barrier +/- 3mm cork or a combo underlayment on top of the vinyl tiles (a little worried about height differences).
Now I am worried that the cork will be too much insulation (too much R-value). Should I scrape up the old cork and vinyl? What is the best, or at least acceptable, underlayment system in this situation? I have read that for radiant heat, you should keep the total R-value of the system (floor and underlayment) to less than 2. The engineered wood would be about 0.5-0.6, but I can’t find numbers for 3mm cork. Also, the Sound Barrier 6 R-value of 3.19 seems whoppingly too much for this application, if that number is correct.
I am in Alabama, and the heating demands are not too great, but I don’t have any calculations, etc. for the house or 50 year old system.
Thanks,
Michael Jacobs
A: I have had many conversations about the impact of certain underlayments, specifically cork in most cases, on radiant heat flooring. A while back Steve mentioned to me a statistic I was unaware of: 3mm cork reduces the amount of heat generated by a radiant system by 3-5%.
Since you currently have a 4mm or so cork floor, you can expect the heat amount produced to be roughly the same. There will be a small difference which is insulated by the wood, by over all this should be a minimal difference.
For your tiled area, it may not be necessary to remove the vinyl, assuming it is in good condition, but if you have height worries, then I would remove the vinyl. Make sure you get all of the adhesive off of your subfloor before installing your vapor barrier and underlayment. I would shy away from foam based combination underlayments as they don’t insulate sound very well and will put a bit more spring into your floor as you walk over it.
Overall I think you are headed in the right direction here. Just make sure you take your time during your installation and get everything right.