There's been lots of discussion in blogs and social media about 'mold-free' building and 'mold-free' products. Here's my review of hempcrete:
What is HEMPCRETE?
Hempcrete is a mixture of hemp hurds (shives) and lime (possibly including natural hydraulic lime, sand, pozzolans or cement) used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, and Isochanvre. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints.
The typical compressive strength is around 1 MPa, around 1/20 that of residential grade concrete. Hempcrete walls must be used together with a frame of another material that supports the vertical load in building construction, as hempcrete's density is 15% that of traditional concrete.
Know that hempcrete is NOT structural. Therefore, structural sheathing such as plywood or OSB will need to be replaced with alternative lateral bracing. Hempcrete cannot support any loads from gravity, a roof, snow, wind or a second floor. Wood/timber framing is recommended to carry loads and provide lateral bracing to prevent racking and collapse. Steel studs and a steel moment frame are also another option.
Hempcrete is actually very similar to wood in many characteristics, except that wood can be structural. The sustainability of growing hemp quickly and easily is a major benefit. The hemp plant is apparently resistant to mold, although it is highly absorbent of water. Mold can grow on the surface of hempcrete, just like other materials, when the conditions are right... as the food source can be dirt or dust, but due to the lime component and natural fungal resistance of the hemp, mold will not feed on the hempcrete itself.
Hempcrete is very fire resistant and testing suggests that it will not contribute to a fire. Hempcrete will char on the surface when burned, much like heavy timber, with the char effectively extinguishing the fire once a source has been removed. Torching hempcrete to remove mold is not likely to be a good idea. Hempcrete is also very breathable, so care must be taken to make sure the other materials applied are either vented or as breathable as the hempcrete. Perm ratings have yet to be established but European values converted suggest a highly breathable system.
Hempcrete cannot be used as a floor slab or foundation system and requires 4-6 weeks to cure after installation. The installation process requires pouring the hempcrete in 2-foot lifts, which can be done consecutively. With an R value of 35 for hempcrete panels and R2.4 per inch for poured hempcrete, the possibility of a very tight, well insulated and yet breathable wall seems achievable. Costs for hempcrete are still quite high, but as manufacturing moves to North America and awareness and use increases, prices will most certainly go down.
When it comes to mold, controlling moisture is still the only real solution. Hempcrete is an exciting new material to consider for sustainable building, as long as moisture management is the top priority.