Walls have the biggest effect on a building's overall environmental impact. They are also responsible for most of the heat loss from a house.
Walls can have from a low to a very high intrisic energy embodiment, depending on the amount of energy it takes to make and transport the materials used.
You might also want to consider the breathability of the walls as breathable walls can help to offset the highs and lows of humidity caused by modern living, helping to promote a healthy internal environment.
Modern brick and block walls, complete with plasterboard and internal plaster skim, painted with vinyl paint, have very low or zero breathability. The problems caused include condensation, mould growth and the need for tricle vents, air bricks, etc which defeats the object when it comes to keeping the house warm!
If you choose to build build a breathable wall, you must make sure that the moisture is always drawn outwards, not directed inwards into the building! This can be acheived by using different renders on the internal and external walls, choosing a more water resistant render on the internal wall and a lower moisture resistant render on the external surface.
These vary from hand-clamp handmade bricks to factory produced bricks. The hand-clamp bricks were usually held together with lime mortar which falls away easily when demolished. Bricks which are cemented together are more difficult to clean, but time and effort is all that is required.
These are not load bearing and can only be used for internal 'partition' walls. The clay is dried naturally without heat and the bricks are breathable with good thermal qualities.
Straw is widely available in almost all areas of the UK. Until recently, when it became illegal, it was burned on the field after harvest and is a true 'waste' product. Straw walls are easy and quick to build and have excellant insulating properties. They are ideal for countries like the UK that have an abundance of straw.
This is subsoil which is rammed between shuttering boards and allowed to dry before the boards are removed. The walls are not load bearing unless cement is included in the mix. Please see Greenspec rammed earth info.
Cob is a very old traditional building material as is composed of a mixture of earth and straw, usually puddled by foot and compacted by hand. Sometimes, local stones were added to the build. The walls need to be very thick - 1 metre is not uncommon for cob walls, but the insulation properties are good.
Wood is available locally almost everywhere and the build is easy and cheap. It is also load bearing and highly insulative. It also helps to reduce cabon emissions as, like straw, it removes carbon dioxide as it grows.