More info would help. For example what kind of lighting are you using? How close is it to the terrarium, is the terrarium open or mostly sealed what is it near? A window? Or just in a room or a cool basement?
I have found for my indoor plants the easiest thing you can do is work with your lights. What I mean by this is let's say you have a 2ft. 4 tube t5 h.o. Flourescent fixture. In order to get the desired light intensity the light must be X amount from your plants. This light throws heat, thus warming your terrarium. I usually adjust my terrarium once I figure out how to set up my light source etc. I have a 40"x20 indoor greenhouse set up that I have various plants in. The lights can be no higher than about 4 inches from the top of this in order to provide enough light to the plants. This is fine in the winter as these plants are in my basement. It heats up the greenhouse but the cool basement temps counter some of the heat.. the cool basement provides a night time drop as well when the lights go off. I have two holes cut in the top for ventilation, during winter one is covered allowing it to heat up right to where I want it. During summer both are uncovered providing a bit more hot air release. Because the basement is warmer. I'm sorry if this reads a bit incoherent I have a headache today but I'm trying my best to make this make sense.
. You have to find the balance. Get that terrarium up to about 68 (not ideal but what I consider a bare minimum)during the day otherwise your nepenthes will grow glacially slow and not be very robust. With that being said I keep my set up in such a way so that it's about 75 during the day and dips down to about 58-62 at night.
Anyhow give us more info and it might be easier to help. On a side note in one of my grow spaces I have a seedling heat mat under a 1020 tray with a thin layer of pumice in the tray and some distilled water and it turns on an hour before the lights come on and it adds about 3-4 degrees to a 24x24x18 terrarium and some humidity.