If you live in a cold or in a mixed climate walls need more insulation than the one provided by just filling the 2x4 walls or 2x6 stud cavities.
One possible way of getting it is by using double 2x4 walls (it’s cheaper than 2x8 walls). About 80% of American homes are built from wood, using 2x4s and 2x6s studs.
Double 2x4 walls aren't difficult to build: instead of a single wall, you just have to build two parallel exterior walls with a space between them – and, obviously, to line up the window and door openings (the space between the two walls is expressly designed to be filled with insulation material and to solve the problem of thermal bridging through the studs).
Double 2x4 walls with a space of 5 inches/13 cm can yield an insulation of R-40 (U-0.14), which is excellent for a super-insulation approach in a very cold climate. The wider the space the more insulation you can install.
Advantages of Wood Frame Construction
»» Cost competitiveness;
»» double 2x4 and 2x6 stud walls provide high levels of home insulation;
»» Fast construction and short on-site times
»» Significant environmental benefits (natural resource, low embodied energy)
2x6 walls can also provide very high levels of insulation - as long as you apply a thick layer of insulation to their exterior.
2x6 walls with 4 inches (100 mm) of mineral wool sheathing also yield an insulation of R-40 (U-0.14).
You may also choose 2x6 walls with all of the insulation on the outside of the framing (no stud cavity insulation). 9 inches (230 mm) of very dense mineral wool yields about R-40 (Metric: U-0.14). This solution allows the framing to be kept dry and warm (minimizing possible moisture problems within the walls) and simplifies the work of electricians and plumbers.
A 2x6 wall partially or totally insulated on the outside of the frame is also the best and easiest way of fixing the problem of thermal bridging through framing.
Wood frame vs. Masonry Walls
Wood-frame walls are the most common walls in North America. Masonry walls – brick, blocks, ICF, concrete - come a distant second. But in most parts of the world the situation is reversed.
Hence the question: What’s the best type of walls? What to choose?
Wood frame walls allow good insulation and low costs and fast building times in the regions where they are common; masonry walls provide structural strength, longer lifespan and more water-resistance.
Anyway, the best depends largely on getting the details right.
The climate and local conditions - and the costs, and the know-how of local builders and designers - matter a lot. There isn't an ideal type of walls.
SIP panels are an “engineered wood” alternative to traditional wood frame construction. SIP (Structural Insulated Panels) involve large-prefabricated cores of insulation material between two engineered wood panels (see: Insulated Panels for Walls Guide).
See also, on wall moisture control issues:
Preventing Exterior Wall Moisture Damages