Insulated glazing
Possibly the earliest use of double glazing was in Siberia, where it was observed by Henry Seebohm in 1877 as an established necessity in the Yeniseysk area where the bitterly cold winter temperatures regularly fall below -50 °C, indicating how the concept may have started:[2] One of the peculiarities of this part of the country is that it is a land of dear glass.
Sometimes glass is dispensed with altogether, and pieces of semi-transparent fish-skin are stitched together and stretched across the window-frame.In winter double windows are absolutely necessary to prevent the inmates of the houses from being frozen to death.
On closer examination this extravagant sheet of plate-glass turns out to be a slab of ice carefully frozen into the framework with a mixture of snow and water in place of putty.Fitting a second pane of glass to improve insulation began in Scotland, Germany, and Switzerland in the 1870s.
Traditional double-hung windows used a single pane of glass to separate the interior and exterior spaces.
Insulated glazing (IG) forms a very compact multi-layer sandwich of air and glass, which eliminates the need for storm windows.
IG provides robust protection against the sun and keeps the house cool in the hot summer and warm in winter.
Soft coatings are vacuum-sputtered onto the glass surface and have higher performance but are easily oxidized and damaged, and thus have to be protected by an inert gas fill.
The first spacers were made primarily of steel and aluminum, which manufacturers thought provided more durability, and their lower price means that they remain common.
It may also result in water or ice forming at the bottom of the sealed unit because of the sharp temperature difference between the window and surrounding air.
A spacer made of aluminum that also contains a highly structural thermal barrier reduces condensation on the glass surface and improves insulation, as measured by the overall U-value.
An older and established way to improve insulation performance is to replace air in the space with a lower thermal conductivity gas.
[8] Argon comprises nearly 1% of the atmosphere and is industrially isolated at moderate cost, whereas krypton and xenon are only trace elements which are expensive to extract.
All particular noble gases are non-toxic, clear, odorless, chemically inert, and readily available because of their widespread application in industry.
The more modern technique is to use an online gas filler, which eliminates the need to drill holes in the spacer.
The desiccant will remove traces of humidity from the air space such that no condensation appears on the inside faces during cold weather.
Some manufacturers have developed specific processes which combine the spacer and desiccant into a single step application system.
Greater space increases the insulation value up to a point, but eventually with a large enough gap, convection currents begin to flow carrying heat between the panes within the unit.
Typically, most sealed units achieve maximum insulating values using a space of 16–19 mm (0.63–0.75 in) when measured at the centre of the IGU.
[12] IGU thickness is a compromise between maximizing insulating value and the ability of the framing system used to carry the unit.
The combination of thickness and weight results in units that are too unwieldy for most residential or commercial glazing systems, particularly if these panes are contained in moving frames or sashes.
[16] The required internal pillars exclude applications where an unobstructed view through the glazing unit is desired, i.e. most residential and commercial windows, and refrigerated food display cases.
Properly designed triple-glazed IGUs with low emissivity coatings on surfaces #2 and #4 and filled with argon gas in the cavities.
Double VIG units with warm edge spacer reach R-25 (center of glass) or more depending upon low-e coatings and other factors.
In these circumstances a large air space improves the noise insulation quality or sound transmission class.
Reviewing the glazing system components, including the air space material used in the insulating glass, can ensure overall sound transmission improvement.
Notably, many low-e glass and semi-reflective metalised coatings greatly attenuate Wi-Fi and cell phone signals.
For a standard construction IG unit, condensation collects between the layers of glass when the perimeter seal has failed and when the desiccant has become saturated, and can generally only be eliminated by replacing the IGU.
Atmospheric pressure changes combined with wet weather can, in rare cases, eventually lead to the gap filling with water.
This solution often reverses the visible condensation, but cannot clean the interior surface of the glass and staining that may have occurred after long-term exposure to moisture.
Typically cracks initiate and propagate from the narrow shaded cut edge where the glass is cooler and minute grooves and notches cause stress concentration.
