Wood is close to the most eco-friendly building material around:
- It's renewable.
- It's production overall pulls carbon out of the atmosphere.
- Material tied up in construction is carbon that isn't in the atmosphere.
- A lower fraction of its cost is transport since the building grade wood is sourced locally when possible.
The issues people have with the growth and harvest of wood:
- Logging is ugly.
- Done badly on steep slopes it causes erosion.
- It's disruptive to the local wildlife.
But let's take your question at face value: To be be more eco-friendly than wood, it requires the following attributes:
- Even more local than your neighbourhood forest.
- Renewable, or be from a very large resource not currently in signficant demand.
- Low in energy to gather, process and use.
Option 1. Dirt.
You are digging a hole for the basement anyway. Use that dirt to make adobe, or cinva-ram blocks, or cob. These materials give you high thermal mass, which in hot dry climates averages out the high daytime and low night time temperature swings. All of these have to be protected from rain. See option 2.
Option 2. Strawbale.
Pioneered in Nebraska shortly after the first horse drawn balers appeared, the oldest strawbale buildings are over a century old. Currently big in the alternative construction crowd, it is the easiest and best (IMHO) construction method for small buildings in dry climates. In much of North America building material is within 50 miles. Walls are coated with lime, earth, or cement based plasters on inside and outside walls. Google 'strawbale construction' for details.
Option 3. Leichtlehm.
This is a sensible infill system for post and beam construction, so it doesn't eliminate wood, but reduces both the amount, and the processing required. Leichtlehm uses straw dampened with a clay slip, and packed into forms made between the posts. Traditionally a small amount of barley is added when filling the forms. The process is fairly rapid. The forms can be moved to the next bay immediately. One level a day is easily done.
The barley sprouts, drawing water out of the straw. Eventually the barley dies, but its mat of roots helps hold the straw together. The wall is finished with lime plaster.
Option 4 Stone
Go look at Scotland, especially the highlands. These people take a nuisance (stones in field) and turn it into a resource.
Stone construction is slow, requires mortar (high embodied energy) but the buildings last.
Stone is also a bad insulator. Even a 2 foot wall does little more than average a chill day with a cold night. Most stone buildings have an interior wood lining which uses nearly as much wood as a conventional stick built house.
While we pride ourselves on no longer living with dirt floors, a earth based floor can be quite workable.
Option 1. Clay on grade.
Mix up using typical adobe mixes. Mix as dry as possible, and pack HARD into place. Screed, and fill to get a flat surface. Allow to dry. This can take a couple of months. Oil with boiled linseed oil. Repeat, allowing each coat to polymerize before adding the next coat.
In some cases the floor has cracked -- too much clay in the mix. Apply a contrasting colour of clay mix as a liquid slurry and wipe. The net effect will be of an incredibly even job of laying grouted flagstone.
Option 2. Clay on on insulated slab.
The above doesn't work well in cold climates. The floor stays too cold.
Lay down 2-6" of closed cell foamboard first, followed by 4" of crushed stone "road