Wow. Thanks everyone for your suggestions. Bishop8 - it's a fire hazard to have towels too close to a baseboard heater. I believe the distance must be 8" between the top of the heater and the bottom of the towels, which should be fine, except for people and kids who fling the towel over the bar so that it slides down almost to the floor. Billl - yes, that's exactly what I want to do, heat the floor surface not so much the air. You said infrared heater. I think of those when I think of the old-fashioned bathroom ceiling heaters you still see occasionally in motels. Are you saying they make infrared baseboard or wall heaters? Do you know any brands you could point me to? Davidro - as I mentioned in my OP, I am not going to pull up floor tile, because I would never be able to re-do the marble floor tile to match the existing marble tub surround and shower walls. Also, this bathroom is an interior ensuite to the master bedroom - i.e. they share the floor structure, and there are no outside walls in the bathroom. But the bedroom floor feels fine, because it is not marble. It is the marble that is the issue, not the floor structure underneath the marble. Birdgardner - I thought of the attic too, which is actually little more than a crawl space above the bathroom. I checked up there again, and it is quite well-insulated. As I mentioned in my OP, you can feel the marble through slippers, and I should have mentioned they are the thick slippers which are normally warm! I am thinking of chucking the whole idea of a heater (and the cost) and getting a thick rug. Maybe I'll post a new thread on this Forum to ask for suggestions for bathroom rugs. Only problems with a rug is that it won't cover the entire standing area, and I am not a fan of rugs in bathrooms because they are dust/hair/powder/sprays traps. I guess there is no ideal solution, but I must weigh the pros and cons of all the solutions provided here. Thanks so much!

I am using a product that is series of heat mats connected together that goes underneath the tile. There are a few different vendors that make them. I was a bit surprised at the price for this stuff - I thought it would be inexpensive for the material but it wasn't. The cost to install was 3 dollars a square foot - they lay the mats in a base of thin set mortar and then tile over it. I looked at a hydro radiant heating system which I thought I could do after we moved in since it goes under the subfloor on the basement side. Turns out this approach would cost a little more and I had access issues. So I bit the bullet and put the mats in. There were too many conflicting opinions on what type of system is better. I went with the product my builder had used it many times before and had good results.

Creative tile, can nu heat always work as a heat source was I miss lead to buy a radiator. I have nu heat and was told it was not sufficient for the volume of my space...I trust creative tile on this more than my plumber for those reading this looking for advice. He has helped solve problems I have had with my tile, I defer to his opinion on anything tile and floor related, my GC, who I respect and will use again, appreciated and listened to creative tiles advice when I had some issues. Creative's expertise is superb in my mind!

Do not put heat under the shower floor: 1) Electric mats can develop defects and the presence of water can lead to a shock hazard. If you think this is far-fetched, there is more than one thread in these forums about such problems. 2) Hydronic heat (warm water circulated in tubes) does not present a shock hazard, but it is also of little use in a shower. Radiant heat only makes the floor warm enough to keep the ambient air at a decent room temperature...as soon as you turn on the shower, the initial blast of water overwhelms any contribution of warmth from the floor in a second or two.