We have had to use a space heater at night for our chicks. We have them in a plastic box in our bathroom with two lamps (in case one burns out, that way there is always a back up). As long as we don't open the door too much the temperature stays ok in there, but our house does not have a central heating system, just a wood stove, which does not heat the bathroom at all. I've got the space heater in there on a low setting so that if it gets really cold, it kicks on for a while. It's hard to keep the temperature stable in a fireplace heated house, plus the bathroom is all stone tile (holds cold really well!) with no windows, and the temperatures in our area outside fluctuate wildly from day to night. It can be 80 degrees in the day and then go down to 35 at night.
You could turn the space heater on for a few hours each day maybe, or just at night, to help boost the temperature. Also, trying a cover of some type over the brooder can help to hold heat in. Keeping the birds in as small a room as you can with the door shut--like a bathroom-- can also help, because then you have a smaller area to heat.