Getting Cob to Meet the Energy Code
Getting Cob to Meet the Energy Code
As anyone who has lived in a cob home or tried to get a building permit for one knows, cob is a very poor insulator. At around R-0.2-.25/inch, even 16” thick cob walls only offer an R-value in the range of 3 or 4, well below energy code standards that require at least R-13 (or higher in colder climates). Some of this deficit can be made up using substitution, by adding extra insulation in the roof and/or flooring and/or traditionally framed exterior walls (if there are any). Many US county jurisdictions accept a Rescheck analysis rather than standard code requirements for meeting the energy code. This analysis can be done online for determining appropriate insulation substitutions. Inquire with your local code enforcement official prior to beginning to ensure your efforts will be acceptable.
An ideal combination for an external wall is one with high thermal mass on the interior and insulation on the exterior, and that is weather-resistant and with a tight building envelope. This is what Joel Lstiburek and the Building Science Corp have referred to as “The Perfect Wall”; i.e. from exterior to interior there is a rain control layer; an air control layer; a vapor control layer; and finally a thermal control layer. Achieving these aims with conventional construction will, unsurprisingly, add much additional cost/ft2 to the construction project. However, the same results can be achieved for much lower cost by utilizing a natural building combination of cob structural walls surrounded by hempcrete, finished with a lime plaster render on both sides, that will provide a tight building envelope with an extremely high decrement delay (Kappa).
Decrement delay is a poorly understood concept in the U.S. construction industry. It is the amount of time it takes for heat to pass through an external building element. It is dependent on both thermal mass and insulation. Wall systems with a combination of low thermal conductivity (high insulation), and high specific heat and high density (high thermal mass) will have a high decrement delay. Decrement delay acts a means of evening out temperatures inside the home to greatly reduce loads on building systems such as furnaces and air conditioners. Conventionally framed wall systems and most building code in the US does not take it into account.
I wouldn’t go trying to school your local building code official on decrement delay unless they seem open to it. Regardless, you’re going to have to meet the energy code that doesn’t (yet) take it into account. Your local building plans reviewer is going to want your exterior wall assemblies to meet a certain U-value criteria to give you a building permit. Hempcrete on the exterior of your cob walls can get you there. I was able to use the research conducted in the following two papers (at bottom) to convince my plans reviewer that hempcrete has a R-value of 2.1/inch. By adding 3.5” of hempcrete to the exterior of my cob, I was able to get to a roughly R-10 wall assembly value. By making up for this with some additional roof and floor insulation and decent windows, I produced a Rescheck energy code approval document that was acceptable to my plans reviewer here in North Carolina. This energy code approval is the second big hurdle to getting a cob building permit after getting the okay for using cob as a structural wall component, based on a structural engineer’s analysis (or possibly the new IRC cob codes) and results from compression tests on cob brick samples using your intended locally-sourced clay binder.
I hope these thoughts help you in your endeavors to get a cob building permit!
Hygrothermal Performance of Hempcrete for Ontario Buildings
Thermal Conductivity of Hemp Concretes
This paper is good for better understanding hempcrete in the context of high thermal mass building applications (decremental delay):
Sustainability of Solid Brick Walls with Retrofitted External Hemp-Lime Insulation