Ideas for heating

Still doing some reading on solar heating and came up with a few ideas. 

One is for the greenhouse. Since the greenhouse faces east/west, if I place black plastic on the north side of the greenhouse, it should help capture and retain heat. If I lay down black plastic on all the unplanted areas, it will turn the entire greenhouse into a solar heat collector. In the spring, I can just roll up the black plastic to throw away or store for next winter, depending on condition. That will also help kill off some weeds and compost the soil beneath the plastic, if left on all winter. 

I also came up with an idea for using solar heat for the house for under $50 (when I have $50) and no structural changes. Kind of a large heat grabber for the bathroom window, which would draw cold air from beneath the house. More on that in the future.