Insulation to Retrofit Transitional Off-Grid House

Hi, all! This is a bit of a weird, convoluted question, but please bear with me as I make it as clear as I can. I'm hoping for some creative brainstorming! I'm very much not a builder, and although we work with the land around us, imitate what we observe wherever we can, and live quite light, our place is certainly transitional. We're fully off-grid and are working on a small shelter, slowly but surely. It's pretty conventional: 2x4 framing with plywood as the primary structure, and then corrugated metal siding (same stuff as the roof). For the inside walls, we're thinking of using pallet wood as the interior surface (in place of drywall or what-have-you), and we're debating what to use as insulation inside the walls and ceiling.

Cost is our primary limiting factor, but we're also thinking long-term, i.e. after we're dead and gone and the house starts to deteriorate in the sun and wind, we don't want weird materials like plastic, etc. blowing around this beautiful desert. Building a straw bale house or an earth ship or something like that isn't an option for a number of reasons (ask me how much I want to build a wofati!!!), but we'd like to be responsible as well as frugal in our choice of materials for the project.

What are folks' thoughts about the R-value and other properties of potential insulation material that could be recycled, scavenged, or purchased inexpensively? We can find some pretty amazing things to reuse if we wait long enough and find the right places to look and ask -- it's that kind of region. We wondered for a while about collecting used plastic grocery bags, but we balked at the eventual consequences as mentioned above (there are already plastic bags blowing around the desert -- no need to add to that). We like the idea of getting good use out of something that people throw away, though. We thought about a modified dirt bag idea ala Nader Khalili. We're still mulling.

We get true winters here, with freezing temperatures and some snow, but nothing too intense and the snow doesn't last long. We'd bless the insulation most in the summer, when the sun beating down on an uninsulated metal roof on 100-plus degree days can make staying inside an unpleasant notion from around 1pm until sunset. (Luckily we spend a lot of time outside, but still.)

We'd be so appreciative of any thoughts, recommended resources, etc. Thank you all in advance!

The Ideal structure for your spot in Arizona would be adobe, it is both an insulator and a heat sink.
I've been inside an adobe house when it was well over 110f and I've been in the same house when it was below 0f, both times I was comfortable.
The owner had built it all, from making the adobe blocks to doing the roof, and he was no builder type.

When it comes to insulation, other than straw bales, I don't know of any natural materials that work really well.
In a cold winter environment you will want at least an R value of 22 for the walls and an R value of 33 for the roof, that will keep you from needing a lot of fuel costs to heat the interior.

Thanks, Bryant! I agree, one ideal structure for this area is adobe. They really are lovely, and last for generations when maintained -- and there may still be some experts in the area (probably mostly south of the border) to approach for guidance and help getting started. Unfortunately it's not an option at this point for this structure, although it's another structure type I'd like to try in the future.

As for other natural materials, another thought I had was cotton rags (T-shirts, etc.). Or how about raw cotton gleaned from the local fields after harvest? Would these things be too much of a fire risk? What about soil, in the form of modified earth bags or something else like that? Obviously not for the roof, but for the walls? What about papercrete, maybe in combination with earth bags for their thermal mass? What about cinder-type rocks like scoria (this page has some really interesting information: http://www.earthbagbuilding.com/faqs/insulation.htm). Has anyone else here looked into such things? No big deal if not -- I can continue to research -- but I just wanted to try tapping the resource of the knowledgable people on this forum as well.

We're not too concerned about fuel costs, just as a note in passing. Experience shows that our little wood stove will cook our food and keep the place quite warm in winter without needing to burn a huge amount of wood. But like I said, insulation would be especially welcome in the heat of summer. Thanks again!

With the basic structure you describe, and the climate, I think packing earth between the exterior sheathing and the interior pallet wood might be a viable plan. Moistening the earth before installing, packing it firmly, and going up a few boards at a time on the interior, would make it easy to do, and would give you some of the benefits of adobe without any masonry skills needed. I don't think you would have too many problems with rot since it seems to be a dry environment. I would use a double layer of pallet wood so there are no gaps where dirt can fall through.

Great ideas, Glenn, thanks!

I found this useful list of R-values of some common natural building materials on p. 211 of Jessi Bloom & Dave Boehnlein's Practical Permaculture for Home Landscapes, Your Community, and the Whole Earth and am posting in hopes that it's useful to others as well:

straw bales: 1.45 per inch

straw-clay: 0.9-1.8 per inch

wood chip-clay: 1.0-2.0 per inch

granite: 0.05 per inch

brick: 0.2 per inch

blown cellulose: 3.8 per inch

wood: 1.0 per inch

wool: 3.5-3.8 per inch

This has me wondering if there's an efficient, effective way for us to make something like blown cellulose from used materials on site and use that -- in combination with earth and wood chips from our woodpile if need be (i.e. to stretch it) -- to fill the walls.

Also, on the same page of this book is a picture of a metal roof insulated on the ceiling with caña brava (Arundo donax) to keep the interior space from becoming unbearable on sunny days and to reduce the noise of rainstorms (but why on earth would we want to do that? we love the sound of the monsoon rains on the metal roof). It turns out that Arundo donax is an invasive weed in our area and my partner has seen a thicket of it not far from here, so we could potentially use it to eradicate it.

Edited/updated to add: We had a brainwave after I turned the page to pp. 212-215 and read and saw more examples (including the section on straw clay and wood-chip clay, which notes, "This can be a very good retrofit for houses with poorly insulated walls"). We're very slowly hand-digging a pond in an area of our property that has a lot of clay (we've been using the earth we dig out for earthworks to redirect monsoon floodwater to our gardens, etc.). We realized we can use this as a cob-making pit: dig and loosen more clay-heavy soil in there, add in wood chips and shredded recycled paper materials and such, pour in water from the nearby livestock tank, and stomp and mix in there before carting off to fill in walls (or making into adobe bricks to use to fill in walls -- this appeals more to my partner because we'd be sure they're fully dry before putting them in the walls). Once we've used it for that for a while, it'll be further along towards a pond around which we can plant fruit and nut trees! Any thoughts/warnings/guidance/advice on this? Neither of us has worked with adobe or cob before. Off to look for local workshops... Thanks!