The growing demand for Heat Pulse Probes (HPPs) to estimate thermal properties and surrogate soil processes comes with a need for improved standards for calibration and validation in reference materials. This study proposed air-free ice as a calibration material to determine the apparent rod spacing (rc) for HPPs. The advantage of using air-free ice as a calibration standard over agar-stabilized water stems from the fact that at 0 °C ice provides a reference volumetric heat capacity (Cv = 1.93 MJ m−3 °C−1), less than half that of water (Cv = 4.22 MJ m−3 °C−1), while ice provides a four times larger reference thermal conductivity value (λ = 2.16 W m−1 oC−1) relative to that of water (λ = 0.56 W m−1 °C−1), which leads to a sharper peak temperature rise curve that may improve thermal property estimates in ice compared to the elongated peak obtained in water. We performed rc calibrations with air-free ice in a freezer at −21 °C and agar-stabilized water at 21 °C using both Infinite-Line Source (ILS) and Identical-Cylinders Perfect-Conductors (ICPC) models to fit λ and Cv to temperature rise data and compute thermal diffusivity (κ) from the fitted λ and Cv (i.e., κ = λ/Cv). Results showed that the ICPC model yielded similar rc values in both air-free ice and agar-stabilized water with reduced Total Error (TE) in estimating λ, Cv, and κ compared to results with the conventional ILS model. We suggest air-free ice as another standard for Heat Pulse Probe (HPP) rod spacing calibration and sensor-based thermal property validation.