A device suitable for conducting an on-site measurement of the density and/or R-value of a gas-permeable material is provided, comprising: a chamber comprising a first port and a second port; a diffuser in fluid communication with the second port of the chamber, wherein the diffuser comprises a diffuser port for conveying a gas flow to or from a gas-permeable material, and wherein the area of the diffuser port is greater than the area of the second port; a pressure sensor arranged to measure the pressure in the chamber; a temperature sensor arranged to measure the temperature of a gas in the chamber; and a relative humidity sensor arranged to measure the relative humidity in the chamber.