Hempcrete: You Can Build a House with Hemp

Hempcrete: You Can Build a House with Hemp

You can do anything with hemp! You may already know that you can make fabrics, plastics, ropes, and paper with hemp, but did you know that you can build a building with hemp? Well, you can! This is possible because of a building material called hempcrete and from its name you might have already guessed what it is.

History of Hempcrete

Hempcrete was discovered in a bridge abutment I France built I the 6th century.  Since it has been rediscovered, hempcrete buildings are popping up all over Europe.

For over 60 years, growing hemp in America has been illegal, the reason being is its close resemblance to its cousin marijuana. Fortunately, the US has recently “opened up” to growing hemp, and many US states, including Florida, have begun to allow the cultivation of the hemp plant. 

However, hemp has grown legally in Europe, the UK, and Canada for a very long time.  Fortunately, this has been possible through a system of licensed growers producing seed certified low in THC. Unfortunately, these farmers are subject to strict regulations. These hemp farmers are forced to purchase their seed through these certified seed producers and their farms are subjected to vigorous inspections.

What is Hempcrete?

Hempcrete is a suitable and sustainable alternative to concrete. It is a bio composite made of the inner woody core of the hemp plant mixed with a lime-based binder. The core of the hemp plant has a high silica content that allows it to bind well with lime. The result of mixing these two ingredients together is a lightweight cementitious insulating material weighing about a 7th or 8th of the weight of concrete but is just as strong. Fully cured hempcrete blocks float in a bucket of water.

Hempcrete is more of an insulating material than it is a structural.  As a result, it shouldn’t be used as a load bearing material, the load created by the hempcrete should be carried by internal framing. Wood studs make a great material for the load bearing frame. However, this makes a great structure for the insulating infill between the frames of the walls of a structural frame.

The material is mixed in mortar mixers for 1-2 minutes and stuffed by hand into the wall cavities between the frame. The wall is slipped formed with a temporary wooden or plastic mold forming the inner/outer surface forms. The hempcrete material is lightweight. Because of this, it can be transported around the site easily in tubs and passed up bucket brigade fashion to workers filling the cavities.

The mold being taken off of the hempcrete wall during construction.

The material is finished on the outside with a hard render coating about 20mm thick to protect it with a final colored top coat finished added. The result appears like any stucco finished building. The builder can chose to leave the finish natural or finished with lime plaster for a traditional look.

Advantages of Using Hempcrete

Hopefully, you consider benefits to the environment and advantage.  Hempcrete is better for the environment than using concrete. It is so good, it is carbon negative. The production of cement produces too much CO2, more than any other man-made material. Lime production is a lower in temperature process that produces much less CO2 which it reabsorbs during curing as it mineralizes back to calcium carbonate. So, the lime used to bind the hemp material absorbs a great deal of CO2.  Not to mention, during the growing process of the hemp plant it also absorbs carbon. The result is a building material that absorbs more CO2 than it produces.

More Advantages of Using Hempcrete:

  • It has great energy efficiency
  • If maintains a steady temperature
  • Doesn’t shrink, so there are no crack lines
  • Breathable material
  • It provides a healthy environment
  • It’s very simple to use if you follow the process correctly
  • Gains in strength over time
  • Can also be applied by spraying or plastering
  • Used in floors and roofs as well
  • It’s hygroscopic, so absorbs moisture and humidity within the building

Other things to Know About Hempcrete

  • Not used as a load bearing material
  • Can be used to make Hempcrete blocks, however these generally aren’t recommended as some of the strength is lost by having to use mortar to fix them together
  • Prefabricated hempcrete panels are available, however they lose the advantage of having a continuous airtight layer
  • It probably wouldn’t be a good choice of material for footings as the hemp could potentially rot if permanently wet
  • It is a site-mixed material so is ideal for self-builders, but it makes it less likely for mainstream contractors to adopt
Learn more about hemp!

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