Hi there Aliyah,
Cob, hempcrete (and
straw bale, straw panel, blown-in-straw, straw-clay, et. al.) can be used in building designs and wall assemblies that will last for at least a hundred years, be sturdy, and be more "sustainable" than more conventional building material alternatives.
The problem is that cob, hempcrete (and straw bale, straw panel, blown-in-straw, straw-clay, et. al.) can also be used in building designs and wall assemblies that won't last more than a dozen years, feel more sketchy than sturdy, and are far from sustainable if the materials come from far away and don't create
energy efficient and healthy structures.
In other words, it's possible to use "sustainable" natural building materials in a poorly designed and poorly built structure that's doesn't last very long and uses a lot of energy to heat and cool.
If the building is well designed for it's site and climate, uses as many locally or at least domestically sourced materials, is structurally sound (appropriate foundation, appropriate lateral force resistance systems to handle
local wind and/or seismic forces and snow loads), and well detailed so it's insulative and thermal mass features perform as designed, you'll be far better off than if the building design doesn't fit the site or the climate, many of the building materials are shipped a great distance and poorly assembled on a foundation unsuited to the local soils and frost conditions, and the walls can't stand up to lateral forces expected for that area.
Hempcrete is much better at insulating, and cob is much better at being a thermal mass (both materials can work well together in the same building!). Both have a range of R-values depending on their respective mixes and densities. Hempcrete is considerably more insulative--according to IRC Hempcrete Building Code, Appendix BA, hempcrete insulates at R 2.1 per inch. A 12" hempcrete wall would be around R-25. According to the IRC's Cob Building Code (monolithic adobe), Appendix AU, cob's R-value is .22 per inch. A 12" wall has an overall R-value of 2.64.
Jim
Many Hands Builders
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