Once fermentation begins, the yeast generate their own heat. Keep in mind, the ambient temperature of your house is less important than the temperature inside your carboy or bucket. You can get a thermometer strip to place on the outside of the fermenter that can give you an idea of what the temperature is inside. Have you tried placing the fermenter next to a heat vent, or in a small room that has a heat vent.
I put my my fermenting bucket into a small bathroom (its a half-bath), and close the door. It actually gets quite warm in there, and I don't keep my house that warm (62 at night).
Brody's Brew House
Primary: Spiced ale
Planned: Butternut squash & gala apple harvest ale; brown ale or porter; barley wine
In the fridge: Dogfish 60 minute; Imperial Stout