What is the current heating source in the bathroom--forced hot air, baseboard???? You may have to increase the heat output of theexisting system as well as insulate.
PyramidDG.com

We have a combination fan, light, and heater in the ceiling, here in Australia.

Judy m suggestion is a good one if you are thinking of a major renovation (to replace the floor you would have to remove toilet, vanity, existing floor...) if you are not planning a large renovation then install an electric baseboard heater with a thermostst.

We have central heat and air, but it still stays cold. We will be replacing the tub and floor within the next year, but the expense if radiant heat floors is not in the budget. There used to be an in wall electric heater , we took it out because it was old.

If you are replacing the floor, then electric radiant should not "break the bank" (it is hot water radiant that is expensive to install).

chook, That is a excellent idea. Wonder if such a unit is available in Canada. My bathroom is also quite cold in the winter. No heat source. Condo was built before the building code says every room needs a heat source.

You might also be able to add the wall mounted heaters that also heat towels.

Here is a photo with one type of wall heater, there are other styles and sizes available.
http://houzz.com/photos/167553

By insulation under the house, I assume you have a crawl space. There is generally plumbing under there. The perimeter walls of the crawl space should be well insulated to save money on heating your entire house. The first 16" of the floor joists next to the outside wall should also be insulated. You could insulate some additional floor space under the bathroom, but in general, you should not cover the plumbing or it may freeze. You may also have vents for your crawl space. These should be closed in the winter and opened back up in the summer to get rid of excess moisture. If you have noticeable moisture in your crawl space, you should cover the ground with black plastic.

We have a crawl space. Previous owners insulated the floors with fiberglass insulation. Not a good idea. The fiberglass either fell apart due to moisture or by animals, i don't know for sure

Our plumbing is located in the crawl space. Prev owner wrapped it up using electrical tape to prevent pipes from icing.

In Russia, many people put some kind of external insulation on their houses. It looks like styrofoam, but is designed for houses and to last. I don't know what it is called, but I'm sure this kind of product exists in America. Google.

In the short term, our bathroom floor was greatly improved with a few area rugs/bath mats.

If you have any long term needs for heat, putting heat in the ceiling will be frustrating, as heat rises and it can take a lot of output (ie high electricity bill) to get heat down to the floor.

@Sigrid Re: External insulation
Adding R value to external walls? See http://enduramaxwallsystem.com/

How about putting in a new in wall electric heater where you took out the old one?

Cold feet. At Ocean state job lot we got foam with a cool looking pic on it

@Shirley Barton-Runyon & @condomary: Hi! Why not using electrical underfloor heating? Google "Warmup underfloor heating" and discover us. We'll be pleased to discuss about your projects if you give us a call... and we don't like getting cold feet neither! Our solutions are affordable. We are located in the US and in Canada (among others). Have a nice day!