In a continuous medium where the mean free path is much smaller than gap size, the heat transfer is dominated by intermolecular collisions, transporting the heat between them conductively. The gas conduction heat transfer coefficient $H_{ct}$ for a continuous medium is defined as:

$$H_{ct} = { \frac{K_{n}}{t}}$$

where $K_{n}$ is the thermal conductivity of gas at 1 atmosphere and t is the gap size in meters.

The combined heat transfer coefficient $H_{g}$ (valid for distances < 10.000 MFP) is shown in figure below as a function of gas pressure and distance between 2 parallel plates for air.

The figure shows gas conduction heat transfer coefficient $H_{g}$ as a function of gas pressure and distance between 2 parallel plates. At 3.5 Pa and a wall separation of 1 mm the heat transfer coefficient is 3.56 W/m^{2 }K.