If your going to build something that size and you can afford to insulate it, well insulate it.
It would be a good time for more specs of the building materials you plan on using, including wall covering inside and/or out. If you use 2x6 lumber you could use R19 fiberglass insulation w/a vapor barrier and that would probably be enough. If not then wrap the outside with a 1" or 2" board insulation.
Does your attic have insul in the floors or ceiling? If it doesn't have insul in the ceiling and you plan on doing so then make sure you leave a gap between the insul and the actual roof. It needs the gap to vent.
It sounds like a small addition by the time you get done with all the steps in building some thing like this. If you are handy it will be easy though. Just think it out first with some thought about what you really want. Sketches help. Stuff like different parts to it for clone/seedlings, veg area, blooming and the lighting for each, venting (cooling/heating) each (you don't want light leaks from one to the next), etc. Also soil verses hydro, power safety and consumption and on and on.
I think most find a way to grow (soil/hydro), figure the lighting, venting(cooling/heating) and plan the area as those two things are usually big space consumers, besides your blooming girls.
If you have a grow journal leave a link.