The mainstays of modern home insulation are fiberglass, rigid foam and cellulose. But there are natural alternatives to these which are worth considering, especially if you're going for super-green construction!
Surprisingly, cotton insulation has the same R-value as cellulose at 3.2 to 3.5 per inch. It is supplied in two forms – batts and loose fill – although it's not widely available, so you may have to hunt around for a manufacturer.
Cotton insulation usually comprises around 75% recycled cotton fibre with the other 25% made up of synthetics like polyester. The mix is treated with a flame retardant and often costs more than a fiberglass equivalent (though the cotton insulation does not use a formaldehyde binder).
On the down-side, although it looks like a very environmentally-friendly option, cotton insulation relies on the cotton industry. As you may already know, this is one of the most chemically-intensive and world-damaging areas of agriculture, with massive use of both herbicides and insecticides – chemicals which can end up in your walls and pose a problem for sensitive individuals.
Wool is a wonderful insulator – ask any sheep! It is extremely friendly to the environment, as the only significant problem associated with sheep rearing is overgrazing and the wool itself is produced with virtually no fossil-fuel use.
Wool insulation is slightly more effective than standard fiberglass, providing a marginally better R-rating, but its real value comes from the fact that it works when it's wet – something that most other forms of insulation cannot boast. Wool is also naturally flame-resistant. Some manufacturers add chemicals to wool insulation to deter insects (boric acid is the usual choice since it also adds fire resistance) and improve the insulation's "loft", which is its ability to spring back after compression.
On the down-side, wool is food for certain insects, notably moths. It does naturally contain lanolin to deter them, but some manufacturers put cedar shavings or moth balls into the mix to keep moths away. This obviously affects the insulation and could be a potential problem for people sensitive to the chemicals in moth balls.
The easiest natural insulation to obtain is, without a doubt, straw bale insulation. Straw is extremely common in most countries, is pretty cheap, and is surprisingly effective as home insulation. Three-string bales can be slotted into wall and ceiling cavities to provide excellent heat retention. Bales should be treated with a fire-retardant chemical such as boric acid (which also deters insects) to reduce fire risk.
It is possible to use loose straw as insulation, but the result provides much lower R-values and increases fire risk. At the same time, loose straw in ceilings is a better choice in earthquake-prone areas, as it is much lighter if the ceiling cracks and the insulation falls on your head!
An alternative to simple straw is a material called "straw-clay", which is made by mixing straw with a watery solution of clay and dirt. This solution is known as "clay slip" and is a natural fire retardant, as well as slowing down