Made from magnesium oxide cement mixed with water, frothed with air, and pumped into cavities, it's efficient, naturally fireproof, and resists mold and pests.

Polyicynene and polyurethane
Both of these foams are made with an oil-derived chemical, polyisocyanate, but does that necessarily erase any green tint? Maybe not, considering the energy these products save and the fact that their blowing agents—water for polyicynene, a non-ozone-depleting chemical for polyurethane—are environmentally benign. Different formulations produce two types, open-cell and closed-cell, the latter delivering a higher R-value and price.

In some formulations of polyurethane, petroleum-based ingredients are partially replaced with those from agricultural resources like soybean oil, sugar cane, and corn. Environmental benefits aside, oil's recent price trajectory makes these products even more appealing. Available as sprayed foam and, in the case of soy-based, a rigid board.
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