Insulation helps reduces the cost of household heating and cooling throughout the year. For more than a century, most new homes were built with fiberglass insulation, but it is associated with several health issues. Healthier alternative types of insulation include sheep's wool, which is fire retardant and can help keep a home warm in winter and cool in the summer. Compressed wool fibers form tiny air pockets, making the outer layer resistant to water while the inner layer absorbs moisture. Another alternative is cotton in the form of leftover blue jean material, which is shredded and treated with a borate solution so it is not flammable. Icynene is a spray foam made out of castor oil that expands approximately 100 times its volume when sprayed into a wall or ceiling. However, Icynene's sealing powers are so strong that after it hardens it requires the installation of a ventilation system. Polystyrene is also considered "green" because it helps save energy when used as a spray foam or as rigid foam boards. Meanwhile, Aerogel is about 90 percent air, but does not less much heat pass through. It is made by removing the liquid from silica under high pressure and temperature. Other insulation options include ThermaCork, which is natural, free of toxins, renewable, recyclable, and biodegradable as well as noise-absorbing, while cellulose is made from recycled newsprint and other paper, and is safe to install.