We built a sun room on the barn several years ago. It was built out of necesity to protect our cats from coyote at night and yet still give them some air and a place to look out at the views. We love out cats.
When winter came, we put heavy plastic over the wire box of the sun porch and presto (!) we had a solar heat collector that heats the whole barn! It does not make the barn "warm" enough for working inside when it is extremely cold but it keeps the temps above freezing.
Take a look here if you want to see more photos:
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