I am in the process of walling 1/4 my garage. It will cost about $30 for the 2x4's, $30-40 for the sheetrock, $50 for a door, and $40 for the insullation. Since we keep our laundry room door shut at all times, I am running the AC/HEAT duct from there to this small room I am building. The room is about 10' x 12' so we are looking at 100-120 sq feet. I live in Dallas so the weather is hot in the summer and not too cold in the winter. The AC vent should bring the temps to at least 80-83 in the summer and 70 in the winter. My thermostat and heat tape should take care of keeping the temps where they should be. I am only worried that the room will stay at 70 in the winter and the ambient temps will be too low.