Higher heat flow rate than predicted

We found that heat flowed through the cob wall at a rate of about 2.7 inches per hour. This is a significantly higher rate than predicted by our hypothesis, which stated that Heat flows through cob at the rate of an inch per hour.

Possible reasons for the higher heat flow rate

The higher rate may be partially explained by the high indoor temperature during the heating period. The indoor temperature was typically from 90 to 100 degrees Fahrenheit. The heat flow rate is dependent on the conductivity of the material as well as the temperature difference between the two sides of the material. In this case, the average temperature difference between indoor and outdoor temperatures was about 40-45 degrees Fahrenheit. The heat flow rate of 2.7 inches per hour may be characteristic for situations with an equivalent temperature differential, for instance an indoor temperature of 70 degrees Fahrenheit and an outdoor temperature of 25-30 degrees Fahrenheit.

Slow heat release

We also found that the heat release from the wall was detectable for a period of 40 hours following the 12 hour heating period. About 18 of these 40 hours recorded temperatures higher than 68 degrees Fahrenheit, a typical indoor temperature and the lower end of the ASHRAE comfort zone (which goes from about 68 degrees to 82 degrees Fahrenheit for humidity levels between 20 and 50 percent). Again, this result is somewhat artificial as the indoor temperature during the heating period was higher than a typical indoor temperature.

Questions for Future Studies

  • In which situations is a traditional (monolithic) cob wall desirable?
  • In which situations is additional insulation desirable?
  • What is the difference in heating cost between an uninsulated cob structure, and an equivalent structure that combines thermal mass and insulation?
  • How can solar gain be maximized in a cob structure, and to what extent can it compensate for lack of insulative value?
  • Does the age of the structure have an effect on the thermal qualities?
  • Are there uncomfortable temperature swings when heating with a wood stove?
  • Do the cob walls moderate the indoor humidity?
  • How comfortable is it to live in a cob structure compared to other structures?
  • Are there certain patterns of behavior that are more suitable to living in a cob structure (for instance, working from home as opposed to being away during the daytime hours)?

home | abstract | intro | hypothesis | methods | data
analysis | conclusions | appendix | acknowledgments

Cob Comfort - a study of heat transfer through cob walls
Kathy Bevers | Per Kielland-Lund | Arch 591 Fall 2003 | GTF: Sara Goenner