The ingredients for making hempcrete are not common building materials, and as such they do not benefit from the volume price breaks of other insulation options. Still, even prior to wide market availability and the cost reductions this will bring, the cost of hempcrete is comparable with other insulation options, while bringing advantages over those options in other ways. The variability in hempcrete pricing is reflected in the chart below, showing that proper attention must be paid to sourcing affordable materials. Hemp sourced from Canadian producers is considerably less expensive than that imported from Europe.

Hempcrete is unique among the plant-fiber insulation materials (cellulose, wood fiber, straw bale, straw/clay, cotton) in its ability to maintain integrity in humid conditions. Like all of the plant-fiber insulation options, hemp hurds are able to store a great deal of moisture because of their porous structure; the moisture is adsorbed onto the large internal surface area of the plant fibers and absorbed into the cellular structure. This storage capacity is very helpful in allowing the material to take on moisture when it exists and to release it when conditions allow. A study performed in France found that up to 596 kilograms (1314 pounds) of water vapor could be stored in 1 cubic meter (35.3 ft3) of hempcrete, providing storage capacity for a sustained elevated relative humidity of 93% without overwhelming the capacity of the material to adsorb moisture.

The advantage of hempcrete over other plant-fiber materials and conventional insulation types is found in the properties of the lime binder. Lime has a high pH and is inherently antimicrobial and antifungal, and the lime coating around each piece of hemp hurd in the mix creates a surface that resists the development of mold even when the humidity and temperature conditions would cause mold to occur on other insulation materials. This resilience in the presence of humidity or even liquid moisture makes hempcrete unique among insulation materials and a desirable choice in both cold and hot climates and anywhere where humidity levels are high. Good structural qualities Hempcrete has a density that allows it to play a minor structural role in the building — unlike batt, loose fill and spray insulation materials in the cost chart above. Hempcrete insulation does not have the structural capacity to fully support roof loads, but cast around conventional wall framing or double-stud framing, it can help restrain the studs from bending or buckling under loads, thereby increasing the load that can be carried by each framing member. Testing at Queen’s University in Canada showed that a 2×6 wood stud with 313 kg/m3 (19.5 lb/ft3) hempcrete infill could support three to four times the compressive loading of a standard stud wall due to the support the hempcrete provides to the wood stud in weak axis bending. The rigidity of hempcrete insulation and the textured surface it presents on the face of the wall makes an excellent substrate for plaster finishes without any need for mesh or other bonding agents.

Hemp is an agricultural crop that has particularly high yields. A study by the US Department of Agriculture found worldwide yields ranged between 2.5 to 8.7 tons of dry straw per acre. This compares favorably to yields for wheat straw of 1.25 to 2.5 tons per acre. In terms of the amount of biomass available for use from a single crop, no other plant provides as much volume as hemp.

The hemp plant is typically grown for either the strong fiber it produces or for seed (rarely for both at the same time). In either type of hemp production, the hurd is not the primary use and is considered a by-product. It has some market value as animal bedding and can be compressed into fuel pellets, but large-scale hemp production can generate tons of hurd for the insulation market as producers supply fiber or seed to their primary markets.

According to a 2003 study, 716.6 pounds (325 kg) of CO2 are stored in one tonne of dried hemp. Tradical, a hempcrete manufacturer in the UK, cites a study showing that their hempcrete product sequesters 110 kg of CO2 for every cubic meter of material (6.88 pounds per cubic foot)6 when the carbon emissions from producing the lime binder are taken into account.

In Canada, about 200,000 new homes are built each year, with an average footprint of 2,000 square feet (185 m2). If they were all insulated to code minimum requirements with hempcrete, a total of 990,718 tons of carbon could be sequestered annually. If the same homes had walls with fiberglass insulation, 207,345 tons of carbon would be emitted to create that insulation, so the total net carbon savings for the planet is significant.

Hempcrete is quite a benign material. The farming process uses far fewer pesticides and herbicides than other grain or fiber crops, creating much less environmental damage due to the use of toxins on the fields. The crop does, however, require liberal use of fertilizer, which can have negative ecosystem impacts. Harvesting and processing take place without the input of heat or chemicals.

The dry, powdered lime binder can generate a lot of dust during mixing, and is highly caustic. Adequate breathing protection must be worn by anybody handling the dry ingredients and working around the mixing station. When wet, the lime binder is mildly caustic to skin, so rubber gloves and fully covered skin are required.

Once placed in the wall and fully cured and dry, hempcrete does not off gas or release any toxins into the indoor environment. The lime is antimicrobial and antifungal, and the material is generally thought to have no ill effects on the indoor environment. The excellent moisture-handling abilities of the material can reduce the chances of a poor indoor environment due to excessively moist or dry air in the building.

Hempcrete is an insulation material, and as such its thermal performance is important. One of the primary difficulties in introducing hempcrete to the building industry is the vagary that currently exists around quantifying the thermal performance values of the material. A thorough literature review turns up 19 thermal tests on hempcrete conducted at research facilities around the world. The insulation ratings found by these tests vary widely, from R-1.25 per inch to R-2.3 per inch for low- (200 kg/m3) to medium- (400 kg/m3) density wall insulation mixes. Even mixes with the same density vary in the test results. To meet minimum code requirements of R-24 in much of Canada, these results could make the difference between needing a wall that is over 19 inches deep (488 mm) to one that is a more reasonable 10.5 inches (266 mm).

To compound the issue, several in situ tests have shown that the actual thermal performance of hempcrete walls is considerably better than the R-values would indicate. Hempcrete has some properties that are unique among insulation materials. As one very comprehensive test of hygrothermal properties of hempcrete states: “The reasonably low thermal conductivity of hemp lime, combined with phase shift, phase change effects, high internal thermal comfort, low initial energy transfer rates, passive humidity control and lower energy requirement for ventilation, all contribute to the reduction of [energy use].” These aspects of hempcrete thermal performance will be explored in the Building Science Notes chapter of this book.

An average of all the test results of mixes in the 275 to 350 kg/m3 range gives a value of R-1.9 per inch (requiring a 12.5-inch wall to reach R-24). The only North American tests performed to date have been at Ryerson University, and are summarized in the table shown here. We build our hempcrete walls in the range of 12 to 16 inches (300 to 400 mm) wide for northern climate use (climate zone 6), and achieve actual performance results that exceed code expectations.

Although the Internet is full of videos of people aiming blowtorches at hempcrete samples, there is not a great deal of certified testing done on the fire resistance of hempcrete walls. As the homemade videos indicate, the mineral coating of the lime binder around each piece of hemp hurd adds a high degree of flame resistance to the plant material.

A 2009 fire test was conducted by BRE Global in the UK to meet the BS EN1365– 1:1999 standard.10 This test subjected a 3×3 meter (10×10 feet) wall of hempcrete that was 300 mm thick (12 inches) to temperatures of 800 to 1,000° Celsius (1800° Fahrenheit), while also subjecting it to a vertical load of 135kN (30,349 lb). The test showed that the wall met all requirements for integrity, insulation and loadbearing capacity for 73 minutes. During this test, the mean temperature on the side of the wall unexposed to the flames stayed under 60°C (140°F), and for the first 15 minutes stayed under 30°C (86°F). This test was performed on a hempcrete wall with no plaster or other finish on the insulation, so real-world performance would be enhanced by protective plaster or other wallboard.

A Canadian fire test was undertaken in 2015 to ASTM E119-14 and CAN/ULC S101-07 standards. The test report concludes that “The test specimen successfully met the conditions of acceptance for a 68-minute Fire Resistance rating,” and included a successful hose stream test.

It is worth noting that the lack of chemical content in hempcrete means that the small amount of smoke generated has none of the highly toxic compounds generated when petrochemical wall insulation and components burn.

There has not been a great deal of testing of hempcrete’s ability to dampen sound transmission from outside the house or between rooms. In 2002, a test in the UK was performed on a pair of 6-inch (150 mm) walls with a 3-inch (75 mm) cavity between them, which is a standard arrangement for walls separating units within a building. The hempcrete walls offered sound reduction of 57 to 58 dB, exceeding the 53 dB code requirement.

Currently, hempcrete is a material choice for owners and builders who wish to create a building with the following qualities:

• Low- or zero-carbon footprint
• Nontoxic materials and high indoor air quality
• Excellent moisture-handling properties
• Durability
• Fire resistance
• Good thermal performance and very stable indoor temperatures

In exchange for these positive qualities, the builder will face slightly higher initial material costs and additional effort to source the materials from non-standard channels.

Current trends in North America are toward carbon reductions, less toxic materials, moisture resilience and durability; a builder who establishes an early foothold in the hempcrete market is likely to be rewarded as more people make choices based on the qualities that hempcrete has to offer.

One of the key advantages of hempcrete over other natural insulation materials is that it fits well with conventional framing techniques, so although it requires a different installation process than conventional insulation, it does not necessarily require experienced builders to change their approach to framing. And the formwork used for installing hempcrete is a lightweight version of the formwork familiar to any builder who has worked with poured concrete. Forming hempcrete in a frame-walled building results in perfectly straight and flat walls, so that the aesthetic result also closely matches mainstream expectations.

In particular, hempcrete is a good choice for those who live in extreme climates, either cold or hot, and in places where humidity levels are high for sustained periods of time. The antimicrobial and antifungal qualities of hempcrete make it very stable and safe when highly loaded with moisture, and can help to prevent mold and deterioration in such conditions.

There is an underlying tension among hempcrete advocates between those with a do-it-yourself philosophy and those aligned with the companies that have proprietary formulations for hempcrete.There are some real advantages to working with materials that have been tested and developed by a company with some history and knowledge and customer support. At the same time, the materials are simple, straightforward and accessible to those who have a desire to formulate and mix their own hempcrete. There is no right or wrong tactic to take, and it is the aim of this book to cover both options fairly and thoroughly.

As the hemp industry begins to take hold in North America, there will be improved supply chains and reductions in cost that will make hempcrete even more attractive. Early adopters using the material today are helping to set the stage for an increase in the use of hempcrete in the near future.