R value from thickness and conductivity

Thermal resistance of a uniform layer is R = thickness / thermal conductivity, and U = 1 / R. Measured hemp lime conductivity rises with density, from roughly 0.04 to 0.06 W/m·K in light lab samples to about 0.07 to 0.12 W/m·K for the mixes the 2024 International Residential Code (Appendix BL) lists, which work out to about R 1.2 to R 2.1 per inch.

This model uses a straight line teaching fit across that range: 0.068 W/m·K at 250 kg/m³ rising to 0.12 W/m·K at 500 kg/m³. It ignores studs, plaster and air films, so a real wall assembly will differ somewhat.

Drying and carbonation, simplified

Cast walls typically need about 2 to 8 weeks to dry before finishes go on. Moisture has to travel out through the wall, so the model scales drying time with the square of thickness, a rough diffusion rule set to about 4.5 weeks at 300 mm.

For CO2 uptake the model assumes the binder is 60 percent of the dry mass and holds 55 percent calcium oxide equivalent. Each kilogram of calcium oxide that becomes calcium carbonate binds 44/56, about 0.79 kg, of CO2. Multiply by your chosen carbonated share. These are teaching assumptions, not a life cycle assessment, and they leave out the carbon stored in the hemp itself and the CO2 released when the lime was made.