Q: I am interested in building a natural home in central Vermont. I am very interested in cob
but am confused as to whether it is a good choice for the climate of central Vermont. What do you suggest?
A: Like other earthen wall systems, cob does not have a very high insulation value per thickness (less than R=0.5 per inch.). One way to get a higher R-value is to build a very thick wall.
The traditional 3-foot-thick cob walls in Devon may have the equivalent of R=15 or so. The problem is that in a very cold climate such as yours, all that mass will be constantly losing heat to the outside, especially on the north side of the building and other parts that never receive direct sun in the winter. (If the wall gets regular sun on the outside, solar
warming will make up for some of the heat loss.)
Generally I would not recommend using cob for exterior walls where winter temperatures stay below freezing during the day. However, there can still be a place for cob in every climate. As interior walls, surrounded by a highly insulating envelope (such as straw
bale), a massive material like cob helps to maintain a consistent interior temperature, preventing both overheating and overcooling. An especially efficient place to put your thermal mass is close to your heat source, so build a sculptural cob hearth and mass wall around your wood stove
That's my suggestion for very cold winter areas: cob for interior walls (and possibly south-facing exterior walls if you get very regular sun through the winter). The only exceptions would be for buildings that don't need to be heated or if you are building a very tiny building, in which case the amount of heat lost through the walls may be negligible anyway.