An insulating glass unit is a flexible pressure chamber. It contains the pressure of the elevation that was in the manufacturing plant at the moment the unit was sealed. So, a Colorado replacement window should be manufactured at a suitable Colorado altitude.
When the glass is sealed at a low altitude and then installed at a higher altitude, such as Denver, the resulting increase in altitude causes the glass panes to bow out or take on a pillow shape appearance. If the pressure change is large, the insulating glass panes can fracture, and the sealant holding the glass panes can rupture causing premature seal failure.
Capillary tubes are necessary for windows coming to Denver from lower altitudes. Capillary tubes are used to equalize the air pressure between the sealed panes. A capillary tube is a small hollow metal tube with an inside diameter of .010 inches to .020 inches. The diameter of a capillary tube is significantly larger (one million times) than the diameter of a water molecule. Thus increasing the probability of moisture between the sealed panes of glass.
Windows with capillary tubes cannot be gas filled. The gas would diffuse easily.
Argon Gas Filled Windows
Argon gas replaces the air inside the glass unit. Since it is heavier than air and is not in continuous motion like air, the transference of heat and cold (convection) is greatly reduced. In essence, the Argon gas provides extra insulation. This glazing choice will meet or exceed most local building code requirements and provides ENERGY STAR® qualified performance.
Windows will perform better and last longer if they are installed at the same elevation as they were sealed.
If you live in Colorado, only buy Colorado replacement windows.