Very basic reply based on a very generic rule of thumb. For generic construction, you can figure a WAG of 30-35BTU/Hr/ft needed in your bath. Again, that's a generic WAG. If your house is older and the bathroom has exterior walls, windows, a poorly insulated crawlspace below and a cold attic above, you may need more. If your house it tight and well insulated, then you may need less. Thus, the 30-35BTU WAG. You say the bb is 1520BTU, for your 100sqft bathroom that equates to 15BTU. Most floor mats give off roughly 12-15BTU on average. Check the specs and you might find one with a higher rating. So if you add 100sqt of 15BTU mat to your bathroom, then you'll be at about 15.2 + 15 = 30.2BTU/Hr/sqft. That might be sufficient, but that's also using the output of the RFH mat as an actual heat source versus simply a source of under-foot comfort. And it's also using both sources, the BB and the RFH mat, at 100% of their rated outputs. If the RFH gets overstressed, CL&P may send you a thank you card along with your monthly bill. Again...a WAG.

I love the electric radiant heat under my kitchen tile. My only regret is that I didn't put it in my master bathroom. Here's what I love about tile (mine runs from the back door through a bathroom, laundry room, front entry, kitchen, dining and office area.) 1. Its easy to keep clean. I vacuum several times during the week and stick mop once a week. Hand scrub once a year (the goal is more often, but you know...). SEAL the grout. I've resealed it twice in 11 years. 2. It still looks as good as it did when I put it in 11 years ago. No scratches, dents, scuffs etc. It's indestructible. I've had one broken tile that they repaired and I can't even find the spot anymore. I have a husband, four boys and live on a working dairy farm. No way I would put in anything that didn't hold up as well as tile. Here's what I don't like: It's hard and the unheated sections are cold. (I have a concrete subfloor so no matter what flooring I put in it would be hard.) I just wear good tennis shoes on long kitchen days...

Forced air is a furnace with fan running to force the air. I love my hot water rads they heat well and hold the temp. well and I would never go back to forced air with all the hot and cold spots . As for infloor heating that is lovely but to add it to an upper floor is costly and you will probably need a new boiler to handle the new addition . What you need is a really good HVAC person to advise on what you can have and what the price is .

I'm looking for anyone that has installed warmboard radiant floor heat system in their home. How has it gone for you? Is the floor solid? Is there any squeak or creaking you believe may be coming from the warmboard subfloor? https://www.warmboard.com/