Frequently Asked Questions

Where is Hempstone Based?

We are located in Northampton, Massachusetts

What is HempStone’s Service Area?

North America for Consulting, Training, and Project Management but our installation and supply is limited to the Northeast and Midatlantic regions

What Services does HempStone Provide?

HempStone provides Construction, Advisory, Research, Education, and Supply. [CARES] We Install hempcrete, lime plaster, and hemp insulation batts and board, Advise on design, construction, and supply, Research materials and assemblies, Educate professionals, students, and the public, and supply material and equipment for installation.

Can you teach me to build with hempcrete?

Absolutely! HempStone offers beginning, intermediate, and advanced trainings geared toward building professionals and homeowner enthusiasts who are looking to use natural building materials. We specialize in helping construction crews and design teams learn to specify and use hempcrete and are willing to travel and train on-site as needed to help build networks of hempcrete building professionals across the country.

What is Hempcrete?

Hempcrete (aka Hemplime) is a bio-composite insulating and walling material used in building assemblies. Hempcrete is made from combining the pithy inner core of the industrial hemp plant or HURD (natural bio-aggregate) with a mineral binder (which is most often LIME or similar pozzolanic binder) and setting the mix using WATER as the catalyst.

How is it different than concrete?

Concrete is a composite material composed of stone and sand aggregate bonded together with a fluid cement that is catalyzed by water. Concrete has many structural properties and can be used below grade in ground contact but has limited vapor permeability. At 150 lbs/cf hempcrete is almost 8 times the mass of hempcrete at roughly 18 lbs/cf.

Hempcrete is a bio-composite material composed of hemp hurd bio-aggregate bonded together with a fluid binder that is catalyzed by water and air, most often lime.

Both binder materials are usually made in part from a calcined (kiln) limestone heated to differing temperatures with many additives in the case of cement. Both can be installed in-situ into or against formwork or premanufactured into blocks or panels to be installed on site.

Hempcrete does not require any control or expansion joints and does not need to be vibrated during install.

What depth do I need to meet code?

We recommend 12-14” (R24-28) for Zones 5-7 and as little as 8” for Southern regions and 6” for Tiny houses on wheels. Specific microclimates and program parameters may alter this.

Can Hempcrete be Loadbearing?

Concrete can be structural or load bearing BUT Hempcrete is typically only relied on to be self-supporting. There are a few exceptions to this most notably the Just BioFibre (lego-like) block system from Canada.

Can we build foundations with Hempcrete?

Hempcrete cannot be used below ground for foundations. Cellulosic material (even coated in lime) exposed to below ground moisture for extended periods will deteriorate just like any plant material.

Where can Hempcrete be Installed?

Hempcrete can be installed in Walls (both exterior and interior), roofs, and floors. In exterior walls it is both insulation and walling material, substituting for the many layers that make up more traditionally built walls. In roofs and floors it is utilized simply as an insulating layer and the structural framing may need to be increased to accomodate hempcrete’s heavier mass per square foot.

Can a roof be done with hempcrete?

Yes the roof can be done with Hempcrete. Making sure the roof assembly is both engineered structurally to support the additional weight of hempcrete AND proper detailing of the air and vapor control layers is critical. We typically counsel to leave a 1" ventilation space between the top side of the Hempcrete and the underside of the roof sheathing material if no other continuous insulation is being installed first.

How is Hempcrete finished on the inside?

Hempcrete is almost always finished on the interior with a natural lime or clay plaster finish.  Hempcrete is the perfect substrate for plaster as it is rigid, textured, and readily absorbs moisture for installation.

Does Hempcrete need weather protection?

Hempcrete exteriors can be finished with either a lime render or covered with a rainscreen cladding to provide a durable finish. Plaster for the exterior of hempcrete requires more care than interior applications in the formulation, exterior detailing, and installation to ensure long term durability.

What is Rainscreen Cladding?

Rainscreen cladding is an option if you want a more traditional exterior finish depending on the location. This involves placing a ventilated supportive structure over the outer hempcrete or insulation layer and typically integrating a smart membrane for bulk water and air sealing control that will work with the vapor open nature of the hempcrete.

Is it for bathroom or interior walls?

Yes it can be used for any interior wall or for baths and kitchens where moisture is present. Hempcrete with lime or clay plaster finish are perfect for bathrooms as the walls can assist in regulating moisture. Hempcrete is an excellent sound dampener and is great for sound proofing interior partition walls.

Can Hempcrete be used to renovate buildings?

Yes hempcrete can be used in retrofit scenarios to replace infill or to wrap the exterior or interior in continuous insulative material.

How is Hempcrete Installed?

Hempcrete can be installed in four different ways. It can be mixed and installed on site by either casting by hand into forms or sprayed against a one sided form from the inside or outside. These forms may be temporary or, if the right material, left in place permanently to become one of the interior or exterior substrates. Alternatively, hempcrete can be pre-manufactured in block or panel form and transported to the site to be installed. The advantage of pre-manufacture is the ability to better control the material quality and be able to close-in the structure quickly to protect against weather.

is it better to cast or spray hempcrete?

Casting-in-place is typically used for smaller projects, self-builders, or for workshops as it is more “hands-on” (pun intended). Hempcrete spray application can cut down on labor for larger projects and speed up install slightly but this requires a professional crew and sometimes proprietary materials.

How soon after casting can you remove forms? 

For Cast-in-Place you can remove the form work once it has set up either immediately after casting or the next morning, and then allow the drying process to begin. For Spray applications with one-sided forms we typically wait at least one day past when the entire wall section is done. In general, Hempcrete SETS up from a couple hours to a day depending on the mix. Drying out takes longer.

How long does Hempcrete take to dry out?

The general rule of thumb is roughly 1 week per inch of thickness to fully dry to 10-15% moisture content. It is usually around 6-8 weeks (depending on climate/season) to dry to around 20-25% moisture content to be ready for plastering. If you try to apply lime plaster prior to proper drying you may see brown stains in the plaster from lignins leaching thru from the Hempcrete.

Any Special Framing Considerations?

Hempcrete is regarded as a non-structural, monolithic infill material, cast or sprayed around the structural frame. We usually specify some amount of “anchorage framing” to help keep the hempcrete mated to the structure. Main framing can be reduced to 2x4 and/or 24” o/c as long as structural load requirements are met. While hempcrete likely adds some racking strength this has not yet documented fully in North America.

Is there a warranty?

Any reputable contractor should be providing a warranty covering the replacement of any defective material as well as a limited warranty for installing the Hempcrete.

What makes Hempcrete so High Performing?

Hempcrete does nine things exceptionally well:

• REGULATES temperature, moisture and sound.

• RESISTS fire, pests, and mold.

• RESPONSIBLE as hemp is renewable, is recyclable or compostable at the end of it’s useful life, and hempcrete is net carbon storing.

What is the R-value of hempcrete?

Hempcrete has a conservative R-value of 2. Although this “Static” R-value isn't incredibly high compared to some other insulating materials, hempcrete generally performs better than modeled due to some of its other attributes that almost no other insulating materials has. A few of these added high performing characteristics include being highly vapor open allowing the building to naturally regulate its relative humidity and having a medium mass which can further delay heat transfer. With the new building codes requiring continuous insulation in wall assemblies in most climate zones, hempcrete already is installed this way and further, the more material the more carbon beneficial.

How does Hempcrete Manage Moisture?

Hempcrete is highly vapor open (> 10 perms) and can easily absorb and deabsorb moisture. This allows the hempcrete to act as a moisture stabilization buffer significantly enhancing occupant comfort.

How does Hempcrete handle sound?

Hempcrete is a great sound dampener. Some studies have shown a reduction around 40 DBA with 12” (300mm) thick walls.

Is Hempcrete Fire Resistant?

Hempcrete will not ignite, smoke, or burn. In dozens of European and North American tests to date, hempcrete materials have scored Zero (0) flame spread and Zero (0) smoke development

Is hempcrete pest resistant?

Hempcrete is inhospitable to insects and vermin due to both the density of the matrix and the high ph of the typically lime binder.

Is Hempcrete Mold Resistant?

Hempcrete is made with lime which has a pH in excess of 12. Stringent household cleaners are around 13. The highly alkaline nature of hempcrete means that it is naturally antimicobial.

How much carbon does Hempcrete Store?

Around 7 lbs of carbon equivalent stored per cubic foot. The storage comes from the carbon uptake of the hemp hurd used as the aggregate. This net beneficial amount (negative carbon) factors in (and overcomes) the planting, harvesting, processing and transportation of the hemp as well as the extraction/mining, processing and transportation of the binder.

Is Hempcrete Airtight?

On its own NO - Anyone who tells you this has never blower-door tested a hempcrete building or is using portland cement and installing to densities that are not appropriate and that will compromise the high performance of the assembly. Hempcrete requires a detailed air barrier to complete the envelope. Typically, properly taped and sealed plaster on the interior and either plaster or a smart membrane on the exterior.

How Dense is Hempcrete?

Wall mixes range between 16-22 lbs/cf (18-20 more typically) while roofs can be lighter around 12-16 lbs/cf and floors can vary widely up to 25lbs/cf.

How Strong is Hempcrete?

Compressive strength = 116 - 145 pounds per square inch (52.7 - 66 kg). (From Hemp Tech Global - Tradical)

How Durable is Hempcrete?

In some parts of Europe (Switzerland & Belgium) there are some examples of buildings without any plaster or cladding being left exposed for decades without any noticeable degradation. Hempstone built a wall that is unrendered and has been left exposed outside since 2018 and looks like the day it was built. What’s more, the exposed framing and non pressure-treated plywood it rests on is also completely intact. This is because every time it rains or snow is plowed onto it, the hempcrete wicks the moisture up and away and stabilizes the entire assembly as it dries out, now through hundreds of cycles.

Is Hempcrete HEalthy?

There is documented evidence that Hemp/Lime has contributed to more than 50% reduction in sick leave and absenteeism in Europe where they have been building with Hempcrete for 30+ years now. (from Hemp Tech Global)