Ash is about 3% more stable than red oak, between it's stability and 4" width it may not be the best choice. In other words you may experience some shrinking especially in cold months.
Note: the 1/2" plywood isn't sufficient, a minimum 5/8" plywood subfloor is recommended for naildown installations.
As you may know, some wood species are better candidates for radiant heat (RH) installations that others, if you prefer solid wood floors, choose a #species known for its *stability. Here is the order for best floor type selections as recommended by the National Wood Flooring Association (NWFA)
Floating wood floors and laminates - are highly recommended because they tend to move as a total unit better accommodating moisture and temperature changes.
Engineered flooring - works well with radiant heat because of its dimensionally stable plywood construction. This construction allows each wood layer to run perpendicular to the other thereby resisting movement and moisture fluctuations.
Quarter sawn or rift-sawn flooring – this cut or style of flooring is preferable to Plain sawn because it will expand upwards and downwards from elevated moisture- not side to side, producing less gapping and board edge cupping.
Strip flooring - is also a better choice than plank flooring, because narrow boards expand and contract less than wide boards do because there are more seams in a floor to take up or allow for any excessive board movement.
If you choose to use solid boards or planks wider than 3" make sure the board moisture content is dry to around 6-9% before and maintained well after installation. For any wood application over radiant heat, the newly installed system should be operating some two weeks prior to the floor installation, this helps to further dry out any unknown pockets of trapped moisture. Acclimation is critical with radiant installations to prevent board shock. In no case should radiant heat surface exceed 85 degrees. Maintain in-door 30%-50% humidity and temperatures of 60˚- 80˚
Common red and white oak have been used successfully over radiant heat. Ideally, species 15% to 20% higher in stability than red oak are considered best choices. See our Stability chart for boards listed on the higher stability spectrum. Brazilian Cherry is more stable than red oak, however we do not recommend this species for radiant heat applications. In some instances sustained high heat can have a negative impact on this species. The natural oils, calcification and other properties of this species can slowly migrate to the surface just under the clear top finishes manifested as small whitish oxidation spots. Calcification is a species characteristic, not a defect. Calcification cannot be sanded out. Therefore after installation, individual boards manifesting this condition can cut out and be replaced with new boards.
Good species for radiant applications are;
Australian Cypress, Bolivian Rosewood, Koa, Mesquite, Teaks, Blood wood, and Mahogany. Poor performers are Maple, Beech and Birch.
*Wood stability refers to a measured amount of movement that can be expected in some species after installation. Species stability is directly related to local humidity, temperature and surface moisture, therefore acclimation for radiant heat in very important for a successful installation.
#Note that even when using an ideal species for Radiant applications actual wood performance can still vary due to the Radiant system construction, installation Techniques, as well as your region of the country.
Answered on 2/9/2012 by Tech & Install