Bill’s Thermal Treasure Hunt

 

1. What Is Hempcrete?

Before we chase thermal treasure, let’s pin down exactly what we’re working with:

1. Hemp Hurds

   • The woody inner core of industrial hemp stalks.

   • High in cellulose and lignin, they’re lightweight yet dimensionally stable.

   • Sourced from fibre-grade crops—no cannabinoids, no psychoactivity.

2. Lime-Based Binder

   • A blend of natural hydraulic lime (NHL) and hydrated lime.

   • Sets via carbonation: it reabsorbs CO₂ as it cures, further locking carbon in place.

   • Unlike Portland cement, it never fully “hardens” into an impermeable mass—maintaining breathability.

3. Water

   • Just enough to hydrate the lime and wet the hurds.

   • Excess water drains or evaporates—no chemical admixtures or plasticizers needed.

Why it matters: Hempcrete is a true bio-composite—almost entirely renewable, carbon-negative and vapour-permeable—qualities that underpin every thermal performance metric we’ll explore next.

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2. Exceptional Insulation: Low Conductivity & High R-Values

2.1 Thermal Conductivity (λ)

• Hempcrete’s λ = 0.06–0.07 W/m·K

• By comparison:

  • Fibreglass batts ~ 0.04 W/m·K

  • XPS foam ~ 0.03 W/m·K

  • Dense brick ~ 0.72 W/m·K

Lower λ means less heat flow. Even though hempcrete isn’t quite as “tight” as foam, it competes with many mainstream insulators—plus it brings thermal mass to the party (next section!).

2.2 U-Value & R-Value Benchmarks

• 300 mm wall → U ~ 0.20 W/m²·K (R ≈ 2.95 m²·K/W)

• 350 mm wall → U ~ 0.17 W/m²·K (R ≈ 3.35 m²·K/W)

• 400 mm wall → U ~ 0.15 W/m²·K (R ≈ 4.00 m²·K/W)

• Per-inch R ≈ 2.0–2.2 (in/°F·ft²·h)

Takeaway: A 350 mm hempcrete wall easily meets or exceeds Australasian building-code requirements (typically R4–R5 for external walls), without synthetic foams or petrochemicals.

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3. Thermal Mass: Smoothing Out Peak Loads

Thermal mass is often overlooked in lightweight wall systems—but hempcrete nails it:

1. High Specific Heat Capacity (c)

   • Hemp hurds + lime binder yields c ≈ 1,400–1,550 J/kg·K.

   • Stores 40–60 % more heat energy per kilogram than many lightweight insulators.

2. Moderate Density (ρ)

   • 300–500 kg/m³ (versus 1,800 kg/m³ for concrete).

   • Strikes a sweet spot: enough mass to store heat, without heavyweight shipping or structural demands.

3. Slow Release

   • Heat absorbed during the day is released over 12–24 hours—flattening temperature peaks/gorges.

   • Reduces reliance on mechanical heating or cooling cycles.

Practical impact: In summer, daytime heat is absorbed into the walls, delaying indoor temperature rises until after sunset. In winter, walls draw heat from passive solar gain or low-temperature heating systems overnight, then release it during the day.

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4. Season-Long Comfort & Stability

Thanks to its insulation + mass combo, hempcrete creates a self-regulating microclimate inside:

• Narrow Temperature Band:
  Indoor highs and lows stay within ±2 °C of the setpoint—even when outdoor swings hit ±10 °C.

• Moisture Buffering:
  Capillary pores in the hemp-lime matrix can absorb up to 15 % of their weight in moisture—releasing it slowly to stabilise relative humidity between 40–60 % (optimal for human comfort).

• Condensation Control:
  Unlike airtight foam walls, hempcrete walls breathe. You avoid cold‐spot condensation and the mould risk that comes with it.

Resident feedback: Surveys of hempcrete-dwelling tenants in Europe report far fewer complaints about stuffiness, draughts or uneven heating—translating to less thermostat fiddling and lower energy spend.

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5. Energy-Saving Performance

When you factor both reduced heat flow and stored thermal energy, the numbers add up fast:

Climate Zone	Heating Load Reduction	Cooling Load Reduction
Cold (e.g. Hobart)	25–30 %	10–15 %
Moderate (e.g. Melbourne)	20–25 %	20–25 %
Hot (e.g. Brisbane)	15–20 %	25–30 %

• Annual Savings: A typical 150 m² suburban home could save $600–$1,200 AUD per year.

• Payback Period: Even with premium hempcrete materials, energy-savings alone often yield a 5–10 year payback—shorter when you include carbon offsets or grants.

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6. Real-World Case Studies

6.1 UK Retrofit Pilot

• Project: Victorian terrace retrofit, London

• Result: Hempcrete infill walls cut space-heating by 28 % and peak load by 32 % compared to brick infill .

6.2 Canadian Cold-Climate Test

• Project: Net-zero cottage, Quebec

• Result: Daily indoor swing ±1.5 °C; heating demand down 22 %, despite −20 °C winters .

6.3 Australian Energy Challenge

• Project: X-Hemp Tasmania pilot, Hobart

• Result: Whole-house HVAC usage cut by 80 % over 12 months—largely from reduced cycling .

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7. Designing & Building with Hempcrete

To translate research into walls that work, follow these best practices:

1. Wall Thickness & Composition

   • 300–400 mm of hempcrete for full insulation + mass synergy.

   • Tailor hemp-to-binder ratio (1:1.5 to 1:2 by volume) for your local climate.

2. Formwork & Application Methods

   • Cast-in-form panels: factory-made blocks ensure consistency.

   • On-site spraying: ideal for irregular shapes—requires skilled operators.

   • Block infill: hempcrete blocks in timber frames, then rendered.

3. Finishes & Claddings

   • Breathable lime renders or clay plasters preserve moisture buffering.

   • Avoid impermeable paints or membranes that trap moisture.

4. Thermal Bridging & Airtightness

   • Seal joins carefully: use breathable tapes and gaskets at junctions.

   • Insulate under sills, around services and at foundations to prevent cold bridging.

5. Professional Support

   • Engage a hempcrete-experienced builder or consultant.

   • Refer to the Hempcrete Design Guide from Hemp Domain for CAD details, structural inputs and R-value tables.

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Hemp Domain™ – Growing a greener building future, one hempcrete wall at a time.