100% Warmer than Single Pane with Storm
Contrary to popular belief, the space between the glass is not vacuumed. 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 a spacer, a desiccant, and a sealant.
A spacer, whether it’s aluminum tube or aluminum channel, foam or butyl rubber stiffened with aluminum
A desiccant to absorb moisture from between the panes of glass
A sealant to enclose the spacer and desiccant in a separate atmosphere
You 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 its 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.
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: 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.