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I'm attempting to find a green and cheap insulation for the roof of a school bus conversion I am building. Kapok is easily sourced for me as it literally grows on trees.

The dimensions of the roof are approximately 40' by 8.5'.

But what is the R-value of Kapok? Are there other cons I should be aware of?

  • Generally anything that grows is a bad idea for insulation. Like the horsehair insulation of old, it will interact very, very badly with water -- it will invite mold growth. And relevant to you it will retain water, which means rusting out the bus body. – Harper - Reinstate Monica Jun 25 at 17:59
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    Also, since it is a conversion (to a place people are when not in motion), consider flammability - it could spread fire too fast to escape. A common source of fire is whatever you use to heat the space. Another is poisonous smoke which could hurt your escape chances by incapacitating you or doing long-term health damage. Aviation people pay very close attention to this. Granted organic things are less likely to be toxic, but far more likely to be flammable. – Harper - Reinstate Monica Jun 25 at 18:03
  • Most insulation is cheap, usually the expense is down to fitting it... – Solar Mike Jun 25 at 19:18
  • as @Harper states, flammability is a problem, but can be helped by treatment, e.g. applying borates. Kapok is cellulose, so see this reference: insulationinstitute.org/wp-content/uploads/2015/12/… – DrMoishe Pippik Jun 25 at 23:53
  • @DrMoishePippik That resource and clarification was super helpful. Please post it as an answer – Joe B Jun 26 at 16:47
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As @Harper states, flammability is a problem, but can be helped by treatment, e.g. applying borates. Kapok is cellulose, so see this reference. However, the amount of fire-retardant used is ~20% by weight.

That said, kapok was the standard filler (in separate compartments) for life vests for many years.

According the the link, the R-value of cellulose is between 3.2 and 3.8 per inch.

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Scientific paper on thermophysical properties of kapok

Research Journal of Applied Sciences, Engineering and Technology

  • You should put the relevant info in your answer, as links can suffer from link-rot... – Solar Mike Jun 25 at 20:56
  • Point well taken - Thanks – user1946891 Jun 26 at 21:48

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