Retrofitting Plank Floors With Subfloor Radiant Heat



I wonder if you have any info/opinions on retrofitting existing plank floors with subfloor radiant heat?

I have come across many references on the pros and cons of retrofitting radiant heat under wood floors, most particularly with concerns about plank flooring, of which we have lots.

Here are some particulars:

1. The floor has been in place for 40-50 years and is in good condition, no warping, etc.

2. Boards vary in width from 3″ – 9″.

3. A variety of unidentified woods were used. They may include koa, walnut, oak, teak, pine, and ash.

2. There are currently gaps (cracks) between the boards of up to 1/8″, mostly about 1/16″ inch or less.

3. We are in fog-laden Coastal California, where the temperature stays between 45 and 65 degrees pretty much year-round, with extreme winter lows in the 30’s and the summer highs occasionally hitting the 80’s. Our humidity is pretty high with the sea air and regular fog (we regularly get mildew on outer walls and in the bathroom).

4. We have an unfinished (vented, dirt floor) crawlspace under the house.

The installation we are currently considering would have radiant heat pipes suspended under the subfloor with insulation beneath. I’d really appreciate any information might have.


– Emily


Dear Emily

I normally give advise on what species and sizes of wood floor to choose when installing new wood floors on these hydronic heating systems. But that has been done for you already. I was raised in the SF bay area, so I know what that costal climate is like. The damp air might not affect the wood as much as you think, as this is just the season that we heat our houses, minimizing the amount of moisture that the boards will absorb. And you already have some gaps between the boards, that are normal for wood that has not (I presume) been screwed and pegged down. As long as the boards have a steady dose of humidity and the RANGE of humidity they encounter is narrow the wood will not move much.

But that said, now if you were to introduce a heating system that WILL tend to dry the boards out, maybe more than they ever have before, you will get some initial shrinking on the widest boards. But I’m not sure that will do them much harm either, as long as you raise the temp slowly in the Fall and lower it slowly in the Spring. Keep the pipe temp to no more than 80F and if you are careful you will have no warping. That’s the real danger in this situation, that the wood will have such a difference in moisture content from face to face, that the wide boards will warp.

Normally I advise a narrow 2 1/4″ white oak quarter sawn strip floor to be nailed on 1″ plywood in this situation. And finished with 2 coats of poly on the under side of the strip floor before it’s installed and 3 coats on the finished surface. What I want to do is slow the moisture migration in and out of the wood, to prevent shrinking and warping.

So I would say you WOULD be taking a chance on using this heating system on these wide boards, but if the boards started to show some warping and further gaping you could always remove this valuable lumber, clean it up, under coat it, install a thicker underlay (to slow moisture migration from under the floor) and reinstall the numbered pieces. But do it with screws and pegs this time.

But there is no way to know for sure what will happen. Best to contact a neighbor that has installed the same system on a similar floor.