What is hempcrete?

<h3>The basics of hempcrete</h3>

<p><strong>Hempcrete</strong> is a biocomposite building material made from three ingredients: <strong>hemp hurd</strong> (the woody inner core of the hemp stalk), a <strong>lime-based binder</strong>, and <strong>water</strong>.</p>

<p>It's mixed on-site, poured or tamped into wall forms around a timber frame, and cured into a lightweight, insulating wall mass.</p>

<p>It is <em>not</em> a structural material on its own, which is the most common misconception about it.</p>

<p>The timber frame carries the building's weight, while hempcrete fills the wall cavity and provides insulation, thermal mass, fire resistance, and a vapor-permeable envelope.</p>

<h3>What it's actually made of</h3>

<p>The hemp portion is specifically the <strong>hurd</strong> (also called <em>shiv</em>), the chopped woody inner stalk left over after fiber is removed during decortication.</p>

<p>Hurd is roughly <strong>50 percent of the hemp stalk by weight</strong> and was historically a low-value byproduct of fiber processing, which is part of why hempcrete pencils out economically as the hemp fiber industry scales.</p>

<p>The binder is typically <strong>hydrated lime</strong> (calcium hydroxide), sometimes blended with hydraulic lime or small amounts of pozzolans like metakaolin to control set time.</p>

<p>The lime is what gives hempcrete its longevity, fire resistance, and mold resistance, since the high pH (around <strong>12 to 13</strong> in fresh mix) is hostile to fungi, insects, and rodents.</p>

<h3>Performance numbers</h3>

<p>Hempcrete typically has a <strong>density of around 15 lb per cubic foot</strong> (240 kg/m³), about <em>one-tenth the density of conventional concrete</em>.</p>

<p>Its insulation value is approximately <strong>R-2 per inch</strong>, which is lower per inch than fiberglass batt insulation (around R-3.5 per inch) but is paired with significant thermal mass.</p>

<p>That thermal mass matters: a 12-inch hempcrete wall performs in real-world conditions closer to a much higher effective R-value because it dampens daily temperature swings rather than just resisting heat flow.</p>

<p>Hempcrete is also <strong>vapor permeable</strong>, allowing moisture to move through the wall and dry out rather than getting trapped (the trapped-moisture problem is what causes rot in conventional sealed-wall systems).</p>

<h3>The carbon story</h3>

<p>Hempcrete is one of the few <strong>genuinely carbon-negative building materials</strong> currently in commercial use.</p>

<p>The hemp plant absorbs <em>roughly 1.6 tons of CO2 per ton of dried hemp</em> during its 90 to 120 day growth cycle.</p>

<p>The lime binder also absorbs CO2 over its lifetime as it carbonates back into limestone, which is the process that gradually hardens the wall over years.</p>

<p>Net result: a typical hempcrete wall sequesters roughly <strong>110 kg of CO2 per cubic meter</strong> over its life, even after accounting for production and transport emissions.</p>

<h3>Where you can build with it</h3>

<p>In the United States, hempcrete became <strong>federally legal</strong> as a building material when the <strong>2018 Farm Bill</strong> removed industrial hemp from the Controlled Substances Act.</p>

<p>It was added to the <strong>2024 International Residential Code (IRC) Appendix BA</strong>, which means jurisdictions adopting that code can permit hempcrete construction without requiring a custom engineer-stamped variance for every project.</p>

<p>This was a major milestone, since prior to 2024 every hempcrete house in the US required individual code approval, which made permits expensive and slow.</p>

<p>In Europe, where hempcrete has been used commercially since the late 1980s, it is well-established in countries like France, the UK, and Belgium.</p>

<h3>Tradeoffs to know before you build</h3>

<ul>
<li><strong>Higher upfront cost</strong>: typically <em>10 to 20 percent more expensive</em> than conventional framed-and-insulated walls</li>
<li><strong>Long cure time</strong>: walls need <strong>6 to 12 weeks</strong> to fully dry before finishing, which extends build schedules</li>
<li><strong>Limited contractor pool</strong>: most general contractors have not built with hempcrete, so finding skilled labor adds time and cost</li>
<li><strong>Not load-bearing</strong>: requires a separate structural frame (timber, steel, or masonry), which is why retrofits into existing framed buildings are sometimes easier than new ground-up builds</li>
</ul>

<p>The offsetting benefits are <em>significantly lower lifetime energy costs</em>, an expected service life of <strong>over 100 years</strong>, Class A fire resistance in most tests, and a building envelope that doesn't degrade the way fiberglass and foam insulations do over decades.</p>