Thermal performance is arguably the most important practical consideration when choosing any wall material or insulation system. For hempcrete, the picture is more nuanced than a single number can convey, and consistently more favourable in real buildings than theoretical models predict. Understanding why requires looking beyond U-values alone.
What the U-value figures actually show
A U-value measures the rate at which heat passes through a building element, expressed in watts per square metre per degree kelvin (W/m²K). The lower the number, the better the insulation. For hempcrete walls, the figures are as follows:
| Wall thickness (mm) | U-value (W/m²K) |
|---|---|
| 250 | 0.23 |
| 300 | 0.20 |
| 350 | 0.17 |
| 400 | 0.15 |
These values are derived from laboratory testing, calculated from thermal conductivity measurements using probes on either side of a material sample to determine how quickly heat transfers through it. Treat them as approximate; the exact U-value achieved in practice varies depending on the specific binder used in the mix and the degree to which the hempcrete is compacted during application.
Even taken at face value, these figures are strong. A 300mm hempcrete wall achieving 0.20 W/m²K meets or exceeds the insulation standards required for new-build wall construction under UK Building Regulations, and a 400mm wall at 0.15 W/m²K comfortably surpasses them.
Hempcrete doesn't, thickness for thickness, insulate as well as lightweight insulation materials such as mineral wool, rigid foam boards, or aerogel panels. Compared on a like-for-like thickness basis, those materials outperform it. The reason hempcrete achieves competitive U-values overall is that cast hempcrete typically fills the entire wall thickness; there's no separate structural layer eating into the insulation depth. The wall is the insulation, and vice versa.
Why real-world performance exceeds theoretical predictions
Here is where hempcrete distinguishes itself from most conventional insulation systems. In-situ tests of hempcrete buildings have repeatedly shown that the material performs better in dynamic real-world conditions than steady-state U-value calculations would predict.
This matters because U-values and the steady-state heat loss models built around them are limited tools. They describe how a material behaves under constant, unchanging conditions, a useful approximation, but not an accurate model of how buildings actually experience heat gain and loss across a day, a season, or a year.
Hempcrete's advantage here comes from two complementary properties working together:
Thermal mass
Hempcrete has significant thermal mass, the ability to absorb, store, and slowly release heat. Where a lightweight insulation material simply resists heat flow, hempcrete also buffers it. During the day, the wall absorbs heat; at night, it releases it back gradually, dampening temperature swings inside the building and reducing the peaks of heating demand in winter.
Maalouf et al. (2014), studying a hemp concrete wall under French summer conditions, found this thermal inertia effect measurably helped moderate the impact of intense summer heat, an increasingly relevant consideration as climate patterns shift.
The interaction between density, moisture, and conductivity
Hempcrete's thermal conductivity isn't a fixed constant; it responds to its environment. Collet & Pretot (2014) measured this directly: thermal conductivity increases by around 54% when density rises by two-thirds, while moving from a fully dry state to 90% relative humidity increases it by only 15–20%. Density has a far greater influence on thermal performance than moisture content.
The practical implication for builders is clear: lower-density hempcrete mixes perform significantly better thermally than denser ones. Getting the mix right isn't just a structural consideration; it directly affects the wall's insulation value. Avoid over-compacting during application, which raises density and reduces thermal performance.
This also means hempcrete walls need to be detailed and maintained to remain breathable. The material's hygroscopic nature, its ability to absorb and release moisture vapour, is part of what allows it to moderate internal humidity, but saturated or moisture-trapped walls would see their thermal performance degrade. Correct detailing, breathable renders, and appropriate foundation design are essential to maintaining thermal performance over the building's life. See Moisture, breathability and vapour management for the full treatment of how hempcrete handles moisture, and why the two properties can't really be separated.
Bridging insulation and thermal mass
One way to understand hempcrete's thermal role is that it sits in a useful middle ground. Standard insulating panels offer low thermal conductivity but little thermal mass. Dense structural materials like brick, stone, or concrete offer thermal mass but poor insulation. Hempcrete combines moderate insulation performance with meaningful thermal mass, allowing it to leverage passive solar energy gain for thermal comfort in a way that lightweight insulation panels can't.
A 2024 review in Buildings (MDPI) found hempcrete's heat conductivity nearly on par with Autoclaved Aerated Concrete (AAC) blocks of identical density, while offering superior hygrothermal behaviour and the additional environmental benefits associated with bio-based materials.
Practical takeaways for builders and designers
Understanding hempcrete's thermal profile should shape how you specify and detail it:
- Don't rely on U-values alone. They're a useful starting point but underestimate hempcrete's real-world performance. When modelling energy use, consider supplementing steady-state calculations with dynamic thermal assessments.
- Specify the right thickness for your target U-value. A 300mm wall satisfies UK Building Regulations for new-build walls; 350–400mm gives comfortable headroom and better performance in colder climates or higher-performance projects.
- Keep density consistent and appropriate. Lower-density mixes insulate better. Avoid over-compacting during application.
- Detail for breathability. Hempcrete's thermal performance is intertwined with its moisture behaviour. Trap moisture in the wall and you degrade both.
- Consider the climate context. In climates with hot summers, hempcrete's thermal inertia is a real asset. In predominantly heating-dominated climates, the combination of insulation and mass still helps, but passive solar design becomes more important.
- Account for the binder. Different binders produce slightly different thermal outcomes. Where thermal performance is critical, confirm the properties of your specific mix rather than relying on generic figures.
The headline figures, 0.15 to 0.23 W/m²K across typical wall thicknesses, are a solid foundation. But hempcrete's real thermal story is one of consistent over-performance in the conditions buildings actually face, and that's increasingly supported by field evidence.
Sources
- Collet, F. & Pretot, S. (2014), "Thermal conductivity of hemp concretes: Variation with formulation, density and water content," Construction and Building Materials — sciencedirect.com
- Maalouf, C. et al. (2014), "Study of hygrothermal behaviour of a hemp concrete building envelope under summer conditions in France," Energy and Buildings, vol. 77, pp. 48–57
- "State of the Art Review of Attributes and Mechanical Properties of Hempcrete" (2024), Buildings (MDPI) — mdpi.com
- Stanwix, W. & Sparrow, A., The Hempcrete Book