I am building my coop and I am trying to plan ahead for next winter. Coop is 4x6 and 4' at the peak and 3' at the eave. It will be insulted with R13 on floor,ceiling, and walls. The coop will house 6 RIR,s. I will have a 4"x 8" adjustable vent at each end. I live in central WI, will I need a heat lamp in the winter, or will a 100 Watt light bulb provide enough heat?