Hi Bri,
Many have experimented with
straw bales as both ceiling and floor insulation, and both are now widely discouraged. Buildings lose most of their heat through ceilings because warm air rises, so it's no surprise that building code minimum R-values for ceilings are often twice or more than what's required for floors. Here in S. Oregon the minimum wall insulation is R-21, and ceilings are R-49. In colder places more insulation in both places is a good idea, and may be required.
We know that straw bales are rated at R-1.55 or R-1.85 per inch, which is based on their density--a minimum of 6.5 lbs./cubic foot. If your required ceiling insulation is R-49 (it may be higher where you live), divide 49 b 1.85 or 1. 55, and you’ll see how many inches of densely compressed straw bales you'll need above the ceiling--between 26" and 31." Not only do bales not come in those dimensions, but that's a lot of weight! So you'll need to use extra lumber in the ceiling framing to support the weight. While we know straw bales burn about as well as phone
books, unless you plaster the tops of the bales in the attic they would potentially pose a fire risk as they are otherwise untreated, and that adds even more weight.
Straw bales under the floor, if in contact with the ground, also fare poorly--they decompose. If stuffed between floor joists in a crawl space there may be ways to keep them dry and less likely to be a fire hazard, but that's where other types of insulation work better.
There are just some places where more natural materials like straw don't do well, and we need to substitute things like blown-in cellulose for ceiling insulation, or foam insulations that aren't damaged by moisture for below grade applications, or use recycled cotton batt, fiberglass, or rockwool for crawlspace insulation.
It's a trade-off. It’s very difficult to build a high-performance building to code with entirely natural materials. I try to make reasonable compromises, with the goal of building with “mostly” natural materials.
