It's a good point, but PEX is installed above the sub-floor to circulate the heat and then the mud floor for the tile is installed on top of it. It shouldn't add any height, since the mud floor already has to be 1" to 1-1/2" thick.

Here is what I would do if I was you. You don't need that many btu's to heat up them two rooms. Plus you want to get hot water also. haved a certified takagi tankless water rep. come to your home. ( this will be free ) tell him you want a tk-3 pro. ( better heat exchange in the unit itself) it can fire up to 199,000 btu's and down to 11,000 btus. then you want to use a bronze bump with a heat exchanger on one side, plus boiler trim. (1/6 horse power. with a mixing valve for your domestic water. you want to then put two zones on other sided of a heat exchanger with a zone board for 2 zones with 2 pumps one for the baseboard. Ask for high trim baseboard so you can set the water temp down and get max btu's per lininer foot. set the temp for 150 degress durring the winter cold month. Now for the radiant floor get a viega dealer in ur neighborhood. get climate panels that you can put cement nails in and the put a flooring on top. If you wanted to know more about this rout just leave a post. you can have all 3 comming off one unit. The best thing about this type of set up is that you dont need 180 degree water like a boiler. and in summer time you can set the water temp down to 120 or 115. and when your away on vacation you use no gas. The down side is hat the climate panels can be expensive, the tankless water heater has to have a nice size gas line. the equipment to do this job can get pricy. I mean really pricey. with nice equipment comes a good price. Takagi & viega are my brand of choice. but you could use navian and wirlsboro if you wish. all 4 manufactors will be happy to come to your home and sit dowwn and talk to you. ( I sell all four)

we live in MN and it was 68F around dinner time. the indoor temp was 78F according to the remote control thermostat i saw on the counter. pretty cool unit. almost like a fancy remote for his home theater. i could feel the air coming from the high velocity ceiling vents and wondered what was up? why so warm? a cloudy windy day yet the house was very warm.

Wide width is subject to warping anyway, and there are already a host of requirements in place when installing it like the use of humdifiers and AC in their appropriate seasons. If you do all of that already, and choose a hydrionic system that is designed for low temp loops, then it's doable. If you choose a system that runs above 80 degree water in it, then most wood floor manufacturers will not warrant it. Most will tell you though that they ONLY recommend engineered wood floors with radiant heat. Not solid wood. Solid wood just moves too much with the change of temperature and humidity, even with using all of the humidifier and AC.