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What is window defogging?

Demystifying the facts about “window defogging”.

Tips on extending the lifetime of your windows and reducing their energy consumption by Simon Rolland, owner of Basco Calgary, Alberta.

We already discussed the importance of the sealed unit in a double or triple pane window in Part 1 of this blog series. Now let’s be more specific: How important is the seal in a sealed unit?

The seal is only there to keep the two interior surfaces of the glass unit clear and dry, and has very little to do with any insulating properties. Double glazing gets some of its insulation properties from the air-space between the glass panes, some from the coating applied on the glass, and very little from the glass itself, given the fact that glass is a heat conductor.

Each 3mm (1/8″) panes of glass has an R-factor (insulating value) of about 0.017. A half-inch airspace has an R-factor of 1.083, and the combined indoor and outdoor air against the window account for 0.966 when no airflow is present (that’s how the tests are calculated!) therefore an average sealed unit or IGU (insulating glass unit) has an R-Factor of about 2.083. Adding argon gas to the air-space adds about 0.115 R.

The airspace between the panes is sealed by two different types of sealant joining the glass panes to a spacer. The primary seal is usually made out of a type of butyl, and a second sealant, usually made of polyurethane, ensures a longer lasting seal. Despite the two layers, over time the seal will eventually be lost, and air circulation (although extremely minimal) will occur inside the no-longer sealed unit due to the daily changes in temperature of the unit.

But there is a third layer of protection to keep your window dry! The spacer to which the two panes of glass are attached contains desiccant, which will absorb moisture buildup that air normally carries. Over time however, even this third layer of protection may fail as it becomes saturated.

This is when you start to see moisture, or “fog” in between the layers of glass. Eventually the moisture will leave limestone deposits as well.

So from this point on, you can either change the whole window, or chose to restore it. Needless to say that restoring it will be less costly, while earning you points for saving the environment (believe it or not, all the glass from windows end up in the landfill and is not recycled due to a lack of cost effective technologies to reuse it).

One way to restore it is to replace the sealed unit, and we explained a lot about this in the previous section. Another, more cost effective way, is to “defog” it, meaning to permanently remove the moisture trapped inside the unit. The technique basically consists of drilling holes in the glass in order to be able to inject cleaning and drying solutions inside, and then plugging the holes with some microvalves. As explained in the calculations above, the microvalves should have an impact on the R-value of at most 0.117 given the minimal additional air flow allowed.

The venting process will permanently dry out a sealed unit as long as one condition exists…Less moisture gets in through the seal failure than gets out through the microvalve. We owe it to Boyle’s Law to explain that gas (water vapour) densities will try to equalize (the water vapour density inside the sealed unit with water vapour density in the outside atmosphere).

It is interesting to note that a renowned window manufacturer, Pella manufactures windows that look like they have a sealed unit but are actually a double-glazed unit with a removable interior pane (new ones can be tripled-glazed with a dual-pane sealed unit as the exterior glazing) but they have four “breather holes” or vents between the glass panels instead of a “seal”. The vent holes allow household moisture that gets into the space between the glass panels to escape to the outside atmosphere.

On rare occurrences, a “defogged” sealed unit may not stay dry. This is necessarily explained by the fact that more moisture gets in than escapes. The moisture inside the sealed unit almost always comes from inside the house although it can sometimes get in past old or poorly done exterior caulking and/or improperly installed sealed units.

Verifying the condition of the exterior caulking around the windows, and having it replaced by a professional is part of the basic maintenance that must be done on any building. Also to be noted, the standards organization for sealed units, IGMA (Insulating Glass Manufacturers Assoc.) specifies that there should be drain holes provided below an installed sealed unit to drain any possible accumulation of water. Many window manufacturers do not install drain holes because they detract from look of their window and then water collects where it can get into the IGU.

The defogging technique is a repair that saves money and is in line with sustainable environment preservation initiatives. One word of caution would be to ensure that you contract a reputable company to perform an evaluation (not every window can be defogged) as there have been improvised and non-qualified people trying to get into this business, only to disappear along with their warranty after a few years/months.
Learn more about Basco Defogging on their RenovationFind profile!


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