Options for insulating a green house have increased tremendously over the last several years, with many new products and improved versions on older products. There are may types of fiberglass, foam, and even recycled jeans (see http://www.nrdc.org/business/design/denim.asp)
A tiny home poses a few unique problems, but the biggest concern is the thickness of floor, ceiling/roof and walls, or should I say lack of thickness. In a conventional foundation-home, each of these parts of the house envelope can be made as thick as necessary to accommodate whatever thickness is required of the particular materials. The most extreme example is the straw-bael house walls that are two feet thick. Every inch allocated to make room for insulation has a very dramatic effect on the interior space, and of course, effect the restricted width (8.5 feet wide and 13.5 feet tall) a trailer must be to be road legal in most states.
The material we chose for our Tiny Dawg House is in the latter category and is a fairly new revised form of foam insulation called Icynene. Its older cousin was a polyurethane foam that was a great insulator but had some environmental issues in both the manufacture, disposal and even in the outgassing (typically related to Volatile organic compounds (VOCs)). Here is a great article that discusses this product as well as the broader discussion of insulation in general: http://home.howstuffworks.com/home-improvement/heating-and-cooling/icynene.htm . This product meets the criteria for relatively environmentally sound and most importantly, it has a very high R-value per inch of thickness.
Full disclosure here, our decision was made very easy by the very generous offer of a local applicator (yes, this is not a do-it-yourself job) to apply the Icynene for the cost of labor only. The local vendor/contractor for this product is Southern Foam & Coating Insulation North GA, Inc. (http://www.southernfoamcoatinginsulation.com/). Would we have chosen this product without the generous donation, it’s hard to say for sure, but not because it is not an excellent product for a tiny house (or any house for that matter), but because it is more expensive than alternatives. Spray foam does deliver on R-value, compactness, and air infiltration. Once again, the balance of cost, quality and “greenness” comes to a nexus of sorts on the building of a tiny house.