I am doing an radiant heated system using hardi-backer siding to provide spacing/ thermal mass over an Advantax sub- floor. Above that will be aluminum reflecting plates bent to accept the Pex tubeing and the Ash flooring directly above that. I am measuring the moisture contents of all the materials and they are within 1% at approx. 8%. I am also planning on sealing the back of the 3/4" Ash as well. Also much of the Ash is long length. I would appreciate any feedback /advice on this plan. I am located in the mid-Huson valley of NY State. Thanks

Are you talking about the cement fiber siding made by Hardi or do they have another product? Google doesn't show one. I have used Hardi products and consider them excellent options-for siding. Sounds like a good idea for themal mass too. The problem with it though(I've used it on two houses), it doesn't hold nails well at all. Papaer and cement, it is somewhat flaky. I would not think of hoping it to hold 7/16" worth of my flooring fastener.
Maybe I'm not understanding something?

I suppose you could think it was cementatious but I don't think it is; its' a proprietary process of Hardi- backer- the same material they use as tile backer board. I have done a mock up with 2" staples which protrude 1/2" into the Avantex sub flloor but even just into the hardi-backer the staples seem to hold real well. My main concern is the heat from the radiant tubes with the aluminum reflecting plates directly against the flooring adversely affecting the Ash flooring. I'm told it will be a low heat - 80- 90 f - does anyone have any experience with this type of flooring and this type heat source? Mike L

You can plan on that floor shrinking during the heating months if it is a solid 3/4" floor. I am doing a project very similar and decided on a 5/8" thick engineered plank with a 3/16" solid sawn face, very similar to Owen's PlankFloor. I will be nailing this directly over a 3/4" CDX plywood underlayment that will be screwed down to a radiant panel subfloor system. Some of the radiant panel system manufacterers have said solid over radiant works fine but others caution it. Personally, unless it was a bevel-edged 3" plank that was a medium brown or darker to hide the gapping, I wouldn't touch it or I'd get a waiver. No comment on Hardibacker. I don't believe it is an approved subfloor or underlayment unless one is gluing the floor down. About temps; everything I've read has said the subfloor should not exceed 82 degrees f.