Most cotton futons are treated with boric acid in the form of borate powder, the same flame retardant used in cotton insulation. Borate also increases mold and mildew resistance. Unless you have a natural, untreated futon, the fill might be comparable to cotton insulation.
However, getting approval from a building department may be tricky. The amount of borate in your futon is probably unknown, and the material did not undergo the strict testing protocols that commercial insulation products typically require. The Uniform Building Code sets limits for flame-spread and smoke density (UBC 707.3), and a building inspector might call you on those. The UBC has provisions for alternative materials, but you would have to demonstrate to a building department official that the material is suitable. You could get yourself a sample of cotton insulation and compare the flammability to your futon fill.
It?s a good idea. If it works, somebody should turn it into a business.
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UltraTouch recycled cotton insulation is an easy-to-install product with no chemical irritants, great for DIY insulation projects.