This past spring I wrote about our building of a chicken coop greenhouse. Anyone that has read through any of the permaculture literature has probably seen this combination structure mentioned. The idea is that the chickens share their night-time warmth and carbon dioxide with the greenhouse and during the day the warmth of the greenhouse helps to heat the coop. Another benefit would be the proximity of coop to greenhouse during chores.
I've seen doubts posted about the actual, real-world benefits of this design so was curious to see what results we would have. There is no doubt that there are benefits in proximity in terms of chores. For example, a compost pile inside the greenhouse or just outside greatly benefits from the manure of the chickens as well as the scratching of the chickens. Having it in or near the greenhouse makes using the compost that much easier. Also when disposing of plant material from the greenhouse is easier as it can be thrown to the compost with little effort and the chickens will munch and scratch-whatever they don't eat is tilled into the compost.
So far I'd say the heat and gas benefits are hard to pinpoint. Part of the problem is that I need to increase the size of the current vent as well as add a bottom vent and probably a small fan to aid in the exchange. I will say that the greenhouse is definitely warming up nicely. On a sunny day it is easily 30 degrees warmer than outside temperatures. With proper venting and a small fan I'm pretty sure this warmth would help keep the coop warmer during the day. The question is whether the warmth of the chickens would keep the coop any warmer at night. I've read that each chicken produces about 40 watts worth of heat but have come to doubt this. We've got 15 chickens and 4 guineas which would add up to at least 600 watts. I'm not real sure how to accurately measure the effect in terms of heat but it does not seem that we're getting that much heat from them. Short of kicking them outside for a night (which I'm obviously not going to do!) and taking temperature to measure I don't know how I could get any kind of accurate reading to compare.
That said, I've had the water freeze up if I don't run a light at night if temps are below 20. Anything above that and the water tends to stay water so that might indicate about 12 degrees of generated warmth. I'm thinking the real benefit in terms of temperature exchange is from greenhouse to chicken coop. If I could get a bit more thermal mass in the greenhouse via a few more straw bales I might see more night time heat retention that could be shared with the chickens to eliminate the need for heat lamps. I was planning on putting in at least two more bales anyway to form a work table for spring seedlings. Perhaps I can fit in 4 more which would be a total of 8. The straw bales seem to collect and hold more heat than anything else in the greenhouse. In fact, I'm thinking that next fall I might use straw bales as the base of the planting bed which would not only add thermal mass but would keep the lettuce and other winter greens up off the colder ground. In early summer all of these straw bales could then by cycled out to the garden and food forests for mulching or to the chicken coop for bedding.