The best advice is often the advice you hear every time you ask a question. In the case of solar energy, the best advice is always "fix everything you can fix before you start planning your project". If you plug all your leaks, improve your energy efficiency, and insulate your home properly, your solar energy installation can be smaller and will cost you less.
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One of the best innovations for passive solar homes are ICFs – Insulating Concrete Forms. At first glance, they look like a lazy way to build foundations because they are simple and lightweight, they snap together, and it's more like playing with Lego than building a house. The good news is that, despite their apparent ease, they create one of the best insulated foundations possible.