Avoid the amazing winter shrinking floor by following the detailed guidelines I set out in this article.
Oh for sure there are a lot of considerations before you install a solid wood floor over a hydronic heating system. First I should state honestly and truly that these heating systems work better when you have a real thermal mass like ceramic or stone over top of them. Then the dense floor material will absorb the heat slowly and hold it, letting it more gently and evenly heat the room. Wood on the other hand will act as a bit of an insulator, blocking the heat from reaching you. But that said, even a wood floor will warm up and heat you and the room quite well. A good compromise is to install the hydronic system right in a concrete slab and install the wood on the concrete.
When you are installing wood floors over top of concrete the double layer (two layers of 1/2 ply) floating plywood method will be the best method. I give the details of this method in my Wood Floors on Concrete article. The reason for this thick plywood subfloor is to prevent the 2" angled flooring nails from penetrating all the way through the subfloor. This is important because you do not want any excess moisture in a concrete slab to ever get into the wood. The flooring nails (never use floor staples) will only penetrate 7/8 ", thus providing an unbroken vapor barrier under the floor. It's the moisture difference between the top and bottom of wood floors that causes cupping and crowning.
But most folks seem to want a strip floor installed on the hydronic heating system that runs under a wooden subfloor. And again I would suggest a heavy 1" plywood subfloor be installed. Or add a layer of ply so the total thickness of the subfloor is one inch . The reason for this thick plywood subfloor is also valid when you have wooden joists with a basement or crawl space under the floor. The thick waterproof plywood subfloor will not be penetrated by the flooring nails. This is the only way to have a really unbroken vapor barrier under the floor. A simple layer of roofing felt laid on a thin subfloor with a thousand flooring nails punched in it, will not do. The one inch plywood subfloor will hold the flooring nails better, and is your only chance to keep the top and bottom of the hardwood strip moisture stable.
But it also is the high temperature of the wood that will tend to dry out the wood more than usual and more quickly in the Fall when you turn on a hydronic heating system. You have to take steps to slow the moisture migration in and out of the wood, when the heating season changes. You have to be aware of the hygroscopic nature of wood.
It would be best to stick with the 2 1/4" size of 3/4" strip floor, this is the most stable sized wood available. I mean moisture stable. When this size wood shrinks and expands with indoor humidity changes, because of it's narrow configuration, it will move half as much as the 5 wide boards, thus forming the smallest gaps in the long run.
I think peoples greatest disappointment with wood floors is the large gaps they form, when there is a great change in relative indoor humidity. Avoid the wide plank floors unless you dont mind large, unsightly, dirt collecting gaps.
Be sure to use the 3/4 depth of strip floor, these floors will resist warping. If it does warp it will be minimal. This is a good place to use a satin finish on the surface of the wood. A low luster finish will show any slight cupping or crowning a lot less than a high gloss finish will.
The best wood for this hydronic application would be a quarter sawn CLEAR grade of white or red oak. Because of the way this wood is selected and milled it will have the best chance to resist surface warping. I would strongly consider having these unfinished strips coated at least once on the underside a week or more before they are installed (with oil based polyurethane). They also have to be stored in an environment that is most like how you live, for about 2 weeks before they are installed. Best to do the acclimatizing, and installation during the heating season, with the heat in the hydronic heating system and the subfloor never exceeding 80 F. Don't store the wood in your own house if you are still plastering or painting, or any cement is curing. Concrete slabs will take at least 60-90 days to cure. Wood is hygroscopic it will take on this moisture and expand along its width.
You must moisture test the hardwood and the subfloor with an electronic probe, to make sure both fall within the EMC range of indoor wood in your region. You can find out what the proper EMC (equilibrium moisture content) for indoor wood is by contacting a wood floor or cabinet making shop.
But if its a glue down floor you want, try a square edged parquet (not the tongue and groove parquet commonly found at the big floor stores) or maybe a real 5 ply laminated wood floor. You will get better performance in the long run with either of these. There will only be minor shrinkage and gapping with the parquet. And the 5 ply laminated wood plank will keep the same dimensions though out its life. Be sure to read about the laminated prefinished floors in my Prefinished Wood Floors : How to Choose ? article free in this web site also. The most important consideration here is the adhesive. A poor flooring adhesive will fail all too quickly when subjected to the prolonged under floor heating conditions.
I have been using the rubber based mastics and specifically the Dri Tac 6200 brand for an number of years now, with great results. This is the glue holding down thousands of square feet of parquet that I have laid in the past 5 years. We just went through a particularly hot and humid summer in the North East and just about all of these installations have no air conditioning, and some don't even bother dehumidifying their basements (big mistake). But only 2-3 small areas (less than a sq. ft. each) in all that flooring buckled up that summer. And once the humidity dropped, the pieces of parquet were simply pressed back in place, into the still tacky glue. Hence the name Dri Tac.
In fact I talked to the vice president of the company and she said they have samples of this glue that they have checked after 45 years, and the tack and flexibility is still there. This stuff just keeps going and going, and will give you almost unlimited life to your parquet floors. This is the only glue that I trust to my parquet and laminated wood floors. Dri Tac is also what I use for all the parquet repairs I do. It has what they call "memory", so that when the glued pieces shift out of place in humid once conditions dry, the wood will be drawn back into position, by the strong tack in the glue itself.
There is no doubt that this memory will also come in handy with the hydronic heating system, keeping the wood slats flexible but firmly glued to the floor. And even the laminated wood floor benefit from the proven high tack and longevity of this glue. I like this glue so much I am going to be writing a separate article on this amazing stuff and the company behind it. Also I do have a separate (but paid) article, for the parquet aficionados. This article will explain in more detail the distinct advantages of this age old material. Yes, finished parquetry predates by centuries the finished tongue and groove strip floor.
Be sure to sand a solid wood floor nicely and finish it with 3-4 coats of Oil modified Polyurethane. This finish has proven to be the most moisture resistant finish. Never use water based finishes, and never use prefinished floors, for these types of applications. Water based finishes in my opinion are still in the infancy of their development, and have yet to prove good performance over the long run. And prefinished solid wood floors are just inviting wood movement and shrinkage with the open seams between the boards. As I said before the exception would be the laminated prefinished floors, which are the most stable in this type of application, due to their waterproof glue layers. But the thinner the ply layers the better, as explained in my Prefinished floor article (free).
Oh, you could also use the floating laminated wood floor, but I have to say these will sound rather drum like when you walk upon them with hard shoes. And the foam pad they generally employ will degrade quickly in this high temperature application. But try instead use cork as a under pad in this case. You need an exceptionally level and smooth subfloor for floating floors to avoid popping of boards that span dips. This advice is contrary to what you will hear at your big flooring show room. Just let logic guide you in this case.
After the wood floor has been finished, make sure that you dont allow the wood itself to reach over 80 F. This should be fine as a 80 F temperature at your feet should make for a very comfortable 70 F temperature at your head. You will need to raise the temperature gradually during the fall, and let it down gradually in the spring. It would be a good idea to have as many as three thermostats, like they have in commercial buildings. One inside and one outside, measuring air temperature. This will give the system a chance to adapt slowly during the season change. And then one more measuring the temperature of the water in the system as close to the floor as possible, so that it doesnt exceed 80 F.
And most importantly buy a wet-dry bulb hygrometer and keep the room at between 40-60 % relative humidity all year round. 50 % would be best, but the main thing is to keep the RANGE of indoor relative humidity as narrow as possible. If there is a crawl space or basement under this floor, it would be best to keep this area also within this humidity range if possible. Sounds like a lot of trouble to me, and like I said ceramic makes a better thermal mass. But these methods I have described here will work for wood floors over these hydronic heating systems. At least you know now what you are up against, I dont tend to sugar coat the truth.