Bubble Insulating
Free air in the cavity has the highest conduction & radiative loss. The conduction through air is very low - one inch of "still" air would be R-40 - but it is the convection, or movement, of the air that causes heat transmission. Solids have true "conduction" values because molecules/atoms are fixed and cannot move around. Therefore there is a temperature gradient across the thickness of the solid. Liquids have so much random molecular movement that the energy is quickly transferred throughout the mass and the the volume will not have a temperature gradient. Air has low heat capacitance and a lower rate of transfer - that is why it will stratify and form zones of temperature difference. When this happens the density changes and the partial pressure of moisture in the air is affected and you have the complex factors of convection and precipitation (droplets forming in the air; fogging, raining) or condensation (droplets forming on surfaces such as the Transpiration Condensation) and so with all these variables we cannot say that an air space would have an R-Value. The insulation effect of multiple glazing is mostly due to the still "boundary layer" of air that is very thin and close to a surface. Wind or air circulation will disturb the boundary layer and reduce the apparent insulation factor of a glazing.
The bubbles are very insulating as compared to an empty space because they prevent the convention of air and prevent the diffusion of water vapor from the warm side to the cold side of the glazing cavity. Vapor must migrate from bubble wall to bubble wall - but the temperature difference across an individual bubble is very small and so the rate of transfer is low. The bubbles are also opaque to thermal radiation emitted from the building interior and warm glazing (long wave IR) and to the solar IR and so they greatly alter the transfer of energy by radiation, which is much more powerful then the conductive transmission. In fact there are so many errors in assumptions of thermal modeling of heat transfer that it is not possible to model the various modes of operation of the Sola Roof with any of the existing building energy simulation, even ones that have been adapted solar design. The bubble walls, which are a liquid, do provide a conductive path, but the rate of loss contributed by this is very small because of the high expansion of the bubbles (very little of the cavity space volume is liquid) and the long conductive path.
My studies over the years of research indicate that the primary thermal energy transfer mechanism is vapor migration across the bubble cavity. That is why cold bubbles are better insulators. Close to 0C you have no vapor within the bubbles; only air and so their cannot be any migration of vapor. That is also why the exterior glazing do not ice up on the interior. However, you will see some ice formation from vapour reaching a very cold glazing. Proper Bubble Regeneration cycles prevent the glazing from becoming very cold. A very cold outer glazing also means that the bubbles are old and "dry" and such condition indicates that the bubbles are not being used effectively for the dynamic insulation process which will regulate the temperature within the building. Renewal of the bubbles before they chill down is the Sola Roof mechanism to transport and distribute the stored thermal energy of the Liquid Thermal Mass system into the building envelope. To accomplish this it is important the the bubble glazing system has a good Bubble Flow Path so that all the old, cold bubbles are replaced with new, warm bubbles. Thus the interior layer of bubbles and the inner skin remains at (or near) the temperature of the Liquid Thermal Mass and the Soap Solution Tank that is part of this system for capture, storage and use of solar thermal gain. - Solaroofguy