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Hemp: The Healthy Building Material

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Hemp: The Healthy Building Material

By Jackie Smith

In September 2022, hemp took a huge step toward being used in building in the United States. Hempcrete has been approved by the International Code Council (ICC) in the appendix for the 2024 International Residential Code governing building codes in 49 of the 50 states. The U.S. Hemp Building Association worked with civil engineers, architects and other experts to submit data to meet building standards for review and approval. (1)

Why is this important? Builders all over the United States will be able to utilize a new building material – hempcrete, which will improve indoor air quality and present other great features in residences. 

Indoor air quality, according to EPA studies, contains two to five times more chemical pollutants than outside air. (2)

Why? It has to do with how airtight our homes have become. There’s a reason we all want to “air out” our homes every spring. But hemp building materials can prevent the building envelope from creating Sick Building Syndrome.

What is Sick Building Syndrome (SBS) and How to Prevent It
We spend almost 90% of our time indoors, so it’s important to have toxin-free, healthy air. Sick Building Syndrome (SBS) is the result of airtight buildings with poor air circulation. (3)

Wrapping our houses in plastic air and vapor barriers traps moisture and pollutants inside the structure.

Not everyone will suffer from SBS, but for the elderly, children and those who already suffer from an illness, that airtight building can pose serious health risks. The symptoms usually associated with SBS are headaches, nausea, eye and throat irritation and difficulty focusing.  

There are many sources of indoor pollution: cleaning products, off-gassing from interior objects (especially plastics) and mold spores from moisture trapped in the building.

Our buildings need to breathe to release pollutants and moisture from the interior to the outside environment. That can be accomplished without sacrificing winter warmth or summer coolness. Using hemp as a building material reduces the need to “air out” our homes in the spring.

How is Hemp Wall Construction Different From Conventional Construction?
There are a number of different fabrication methods for hemp walls. They’re made of hemp hurd, the inner woody core of the hemp stalks, lime and water. The mix, called hempcrete, is a thick, chunky slurry that can be poured into a wooden framework at a factory, formed into blocks, or mixed and applied with a blown or tamped process on-site.

Hempcrete walls are denser and thicker than conventional walls. According to the U.S. Department of Energy, hemp’s R-value is similar to other conventional insulation at about R-3.5 per inch. Hemp walls are 9-12 inches thick for thermal mass. When a hempcrete wall is completely dry, it acts like rock and stores heat acquired throughout the day and passively releases it when the air cools. 

A hemp wall isn’t load bearing, so framing is necessary for structural support. For most construction, the support will be wood. It’s encased in hemp, creating a seal against air leaks. There are no vapor barriers in a hemp wall, the biggest difference between hemp construction and conventional.  

Hempcrete’s mixture of hemp and lime is hygroscopic, allowing water vapor to move through the wall. It can also hold water vapor when humidity is high and release it when the humidity goes down. The water vapor content of the inside air is high after a family completes their morning (or evening) showers. What happens to that water vapor? In a hemp construction, it is stored or expired into the outside.

In a conventional construction, that water vapor accumulates in dry wall, wood and insulation. Over time, moisture buildup deteriorates the structural components of the building and leads to the development of mold. According to Doug Hastings, ASHE certified inspector, “approximately 90% of all structural building failures are caused by moisture.” (4)

An airtight building has a high R-Value at the beginning of its life. But over time, the features that favored that construction method may become detrimental to the entire structure and its inhabitants. 

R-Value and U-Value
In the construction industry, R-value is used to measure the insulating ability of a building material. The higher the R rating, the warmer the interior in the winter and cooler in the summer. But the rating on the package of the insulating material doesn’t tell the whole story.

Usually, we talk about R-value for insulation and U-value for windows. It’s not as simple as that. Heat moves through a wall or roof based on its U-value, not just the R-value. That’s because the R-value is measured with only the single material. For example, insulation with a fiberglass batt on the interior wall and foam board on the exterior wall has multiple non-homogenous layers. An exterior wall could be as complex as 15 layers.

Each component has a different R-value. There are air spaces, gaps and seams in between each of the layers, except perhaps the paint. The more complicated the wall construction, the greater the difference between the insulation R-value stated on the packaging and the actual R-value of the completed wall.

The insulating value of the entire wall as a system is the U-value. It’s a measure of the amount of heat transmitted over a time period from one side of the wall to the other. There are 14 layers in the example above, each with an R-value (even the paint). The formula for determining the U-value is:

U-Value = 1/(Sum of all R-value)

The goal for a conventional wall or roof construction is to have as little air flow as possible. And vapor barriers are installed on the interior and exterior assemblies to keep water vapor from penetrating the insulating layers. This creates a tight seal, sometimes too tight. Moisture from cooking, baths and other human activities accumulates just as in a greenhouse. Over time, accumulated moisture leads to a number of issues including structural rot and mold.

Mold, Mildew, Pests and Fire
Mold and mildew are not problems with hempcrete. The hygroscopic nature of hempcrete even makes it a good choice for humid environments. The lime binder creates an alkaline wall that naturally repels insects.

Hempcrete is a dense material, although light in weight. That density, along with alkalinity, deters pests, such as rodents, from nesting. This nesting is a recurring problem in conventional insulating products such as fiberglass and cellulose. When nesting occurs, the R-value goes down and organic material builds up in the wall. Mold grows on organic matter. The accumulation of moisture and organic matter creates the perfect environment for unhealthy mold and mildew growth.

Fire tests have been conducted on hempcrete in accordance with IRC building codes. Hempcrete isn’t flammable because of the lime in the mix. Jean Lotus reported in “Hemp Building” that a bush fire in Australia burned down a hempcrete factory but the cured hempcrete blocks survived undamaged. (5)

Is Your Builder Ready for Hempcrete?
Many contractors shy away from hempcrete because they may only have heard about the tamp or blown in-site construction methods. Both are time consuming and require special skills and equipment. But the hemp building industry has evolved and products are now available that use conventional building skills with minor adaptations.

Hempcrete panel systems are custom made off-site. All curing is done in the factory, decreasing on-site time by up to 60%. All plumbing, wiring, and window and door placements are set up at the factory from blueprints. The panels arrive at the job site. With the aid of a skid-steer loader or forklift, they can be set into place on a foundation of cured cement or wood pillar-and-beam.

Most blocks made from hempcrete are also made off-site and delivered to construction sites and require masonry as well as carpentry skills. The mortar is lime-based so it maintains the wall’s breathability.

It is important to note that hempcrete blocks aren’t load-bearing and require wooden framing for structural support. Research and development are underway to make hempcrete blocks load bearing.  It’s only a matter of time before hemp walls, whether blocks, panels, blown or tamped in, will be load-bearing. What a difference that will make in the construction industry. Think how many trees we can save!

Both panels and hemp blocks decrease the on-site time dramatically and produce a healthy, comfortable structure with no off gassing.  Hemp is non-toxic for both the inhabitants of the structure and the crew installing the walls. Volatile organic compounds (VOCs) are what give a structure that “new home” smell. For some people, those VOCs cause asthma, chronic respiratory problems and other long-lasting medical problems.

Exterior hemp walls can be finished with either wooden siding or a lime plaster so they continue to breathe. Obviously, it would be counter-productive to use vinyl siding on the exterior or drywall on the interior of a hemp structure. Neither of these materials breathes.

Acrylic and oil-based paints also act as vapor barriers. Tinted lime plaster creates a beautiful appearance. Lime plaster can be very fine for a smooth wall or gritty to add texture. There are also paints that breathe, such as milk or clay paints. Both are used by Natural Builders.

The embodied energy of building materials and their life cycle are important for building material decisions. Polyurethane spray foam requires 14 times more energy to produce than hempcrete because it is sourced from petroleum. Fiberglass and mineral wool, two major insulating materials, contribute more than 30% of the GHG that are destroying the ozone layer, according to the EPA. (8)

A Hemp Home is Carbon Negative
It takes at least 120 years for a tree to create the same amount of building materials as a few acres of hemp in one growing season. Both sequester carbon during growth and for their lifetimes in a structure. Mature living trees sequester 3.09 tons of CO2/hectare per year depending on species and growing conditions. On the other hand, hemp sequesters 15 tons of CO2/hectare per growing season. (6,7)

The vast majority of conventional building materials end up in landfills. Some break down more quickly than others, but all have retardants that are toxic to the environment and subject to leaching.

Hemp, on the other hand, is completely recyclable. Lime, a natural binder, is a soil amendment and hemp is broken down by microbial activity. A hemp wall, with glass and any metal removed, literally becomes soil at the end of its long life, or if it is all not used in the building.

Does Hemp Construction Sound Interesting to You?
This article discusses only the health benefits of a hemp structure. Stay tuned for a more in-depth article on the construction skills needed on-site. The fabrication method used to create a hemp building component determines which skills are needed.

A hemp structure can be built using conventional construction skills, with a few minor adaptations. It’s safe enough for a DIYer. To find out more about industrial hemp as a sustainable building material, watch the building materials video on our YouTube channel. Stay tuned, there’s more to come!

Jackie Smith is a marketing strategist for industrial hemp businesses. She can be reached at This article was contributed by the Global Hemp Association.

1. Hempcrete Approved for US Residential Building Codes — HempBuild Magazine








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