What to wear under a sleepsack?

I'm not sure if this is the right forum for this question, but I was wondering what people put under the sleepsacks? Now that it's starting to get cold in Connecticut and oil prices keep going up I'm having a dilemma. My ped says to keep the room 70-74 degrees. That's way too hot for us (and would be crazy expensive--we don't even keep the house that warm in the daytime), so we've kept it at 68 at night. I haven't put DD (5 weeks old) in a sleepsack yet, just footies, but what do you do when it gets cold out? Footies under a sleepsack? Onesie? How do I know if she's too hot or too cold? The only sleepsacks I have are the Carters fleece ones with sleeves, so I don't know if they're warmer or cooler than the Halo ones. I'd like to turn the temp down more (to, say, 62-65) so we don't sweat or go bankrupt, but still keep her warm. Is that possible? We haven't been swaddling her because she seems to do fine without it. Thanks!

Hi,
My DS is almost 11 months old and we still use the Halo Sleep Sacs (in just a much bigger size!). Since it has gotten colder here recently, I have been putting him in a regular Carter's sleeper with footies and then putting the Halo sleep sac over that. He doesn't seem to be too hot and he is usually a really hot kid. Hope that helps!