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.

ICFs are foam-insulated forms used to build concrete foundations. There are many varieties manufactured by dozens of different companies (over 50 in the US alone!), but in general they are foam forms that are assembled on a footing. They snap together to create a container into which concrete is poured to make the foundation.

Some ICFs are hollow foam blocks with interlocking edges. Others are longer foam panels which lock together. Others are polysteel panels with standard sizes ranging from 8 to 32 square feet in surface area. Whatever their design, the foam is reinforced with steel or steel rebar and cross-bridges so that it resists blowout when the concrete is poured inside.

Unlike traditional forms, which are usually made of wood or steel, ICFs are extremely light and do not need to be removed once the concrete has set in place (cured). You just leave them there instead of tearing them down. The foam acts as extra insulation throughout the building's foundation and creates a massive R30 value (approximately) – far higher than any other type of foundation insulation currently available.

As an extra bonus, ICFs use less concrete but are stronger than other types. An 8-inch wall built with ICFs can use 25% less concrete than one built with traditional forms, but can be an enormous 50% stronger!