Contrary to popular belief, the space between the glass is not vacuumed. If it were, the unit would be under constant stress and be more prone to fractures. What dries the air between the lites of glass is the desiccant and what seals the unit is the back fill. There are many possible configurations for an insulated glass unit but what they all have in common is...
True Divided Lites (TDL)
As one can see from the multiple illustrations on this page, the spacer system separating the inner and outer lite of glass has a "Sight-Line" of from a minimum of 3/8" up to around 1/2". In other words, an IG unit should have between 3/8" and 1/2" of spacer buried out of sight in edges of the sash. The second illustration left shows the effect of not properly burying the spacer.
When considering "true divided lites" using standard IG units, the thinnest possible muntin will be 1 1/8", and that leaves no room for variations in sash or IG fabrication. Many IG manufacturers have a tolerance of 1/16" or more. That means that one needs to add at least another 1/16 to 1/8 to the muntin width to ensure the glass will fit into it's rebate. Now the muntin is approaching 1 1/8".
On a practical note, there are companies specializing in making "Low sight-line" IG units that can approach 5/16". However, the durability and life span of a unit is related very closely with the amount of sealant behind the spacer. Units with minimized sight lines are, in our opinion, flirting with disaster and the lineal perimeter is many times greater than a single IG unit, therefore exacerbating the problem. Furthermore, the per square foot price is often many times greater for smaller IG units since labor becomes a much larger factor in their manufacturing.
Simulated Divided Lites (SDL)
SDLs avoid all these challenges by constructing the sash with a single IG unit with muntins applied to the inside and outside surface of the glass. The primary reason for this approach is to maintain the original muntin width. When one compares a 3/4" muntin with one 1 1/4", the difference is striking. The larger muntin looks "chunky and clunky" versus the sleek appearance of the thinner muntin.
Two ancillary benefits of SDL is IG longevity and price. More sealant can be used and the glass is less expensive per square foot since it's much less labor intensive to make one larger unit rather than four or six smaller ones.
The spacer is a hollow piece of extruded aluminum with "micro-perforations" along the inside edge, which allows the spacer to breath the air trapped between the glass. It's filled with a granular desiccant (like in the small bag you find in some packaging) and inset from the edge of the glass approximately 1/8" - 3/16". The unit is assembled and "back-filled" with "hot-melt" polysulfide (or other sealing compound), which is heated to the consistency of peanut butter and injected behind the spacer.
The desiccant is now trapped in the sealed unit and "sucks" the moisture from the dead air-space that has been created.
With no moisture between the glass, changes in temperature can not cause condensation which results in "fogging" the glass.
In the case of the Magnum IG units (pictured above), a special dual seal system is employed. The primary seal is a "butyl rope" that is applied to either side of the spacer in it's specially formed concavaties. The unit is then ran through a special IG oven-press to heat the primary seal and press it to form a perfect seal between the glass and the spacer. This primary seal also serves the purpose of creating a "thermal break" between the glass and aluminum, to help prevent temperature transfer across the spacer.
Once the unit passes through the oven-press, the polysulfide secondary seal is then "shot" in behind the spacer to now form two seals for greater protection against moisture infiltration.
Means the IG unit has gone irreparably bad.
When an IG unit fails, or fogs, the seal has broken, allowing moisture laden air to enter between the glass. Eventually the desiccant will saturate and become inert. Then, the next time the temperature drops, condensation will form between the glass and cause the fogging, which is indicative of a failed seal. The unit is ruined and the only remedy is replacing the glass.
Unless otherwise requested, the Magnum comes standard with Low-E glass (low emissivity). A Low-E unit has a practically invisible, metallic coating on the inside surface of the exterior lite of glass. This coating helps reflect heat back into the room in the winter and back outside in the summer, creating a more comfortable atmosphere for you.
Both the interior and exterior lite of the Magnum IG unit is "double-strength" (1/8") glass unless tempered glass is mandated by code.