Indoor "loft" grow. Keeping temps up?

Hi there I am about to set off on my first proper grow. I am going using a loft in the top of a building I own, however it is a large old open building (no heating etc) and winter is setting in here with temps dropping,

I am looking for any tips on how to keep the temps up in the growbox. I was thinking of lining it with insulation that is used in cavity walls (white styrofoam sheets) I am going using CFL for the most part would I need to put a higher heating light in the corner of my box to try get the temps up?

use an hps it will give you a nice warmth

heat mats to keep roots warm.

insulation?

Thankyou all for the replies, I will add an HPS I think and maybe a Heatmat it will get very cold here soon i.e. -5 some nights. As for insulation I plan to use cavity wall insulation on the box.

mylar helps keep the heat in somewhat. heating pads do work well for keeping roots warm. the hps is the only way to go when it comes to keeping the heat up. i normally close the heat vent to my room in the winter and just use the hps to heat up my room like a personal heater. the furnace heats up the rest of the house better without having to heat my room. i just have to keep my door open when my hps is off. peace.

run the lights when the coldest temp is around and shut them off during the day or when it gets hot.