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Defogging Windows in Mesa

Any sealed enclosure acts as a self contained miniature environment but is still very much subject to the same laws of physics that exist externally. To comprehend what happens within the sealed enclosure, the window, there has to be some understanding of the basic laws of nature governing, temperature, pressure and effects of these elements on each other. For example, one of the purposes of the report discusses moisture removal within a sealed window. The air within that window is its atmosphere and atmospheres contain moisture. All of this is governed by natural laws. Given the right conditions, condensation (the Arizona of change from water vapour to liquid water from an atmosphere) on any surface will occur.

Why? And when will it occur?

Is it predictable?

If so, can it be eliminated?

How?

Where does it come from in the first place?

 

In some designs a single sealant provides both the seal and the structural strength to hold the unit together. In others, two different sealants are used, one for sealing the unit and another for structural support. The air in the space between the panes of glass is dried by purging with dry air before sealing. It is important to note that the desiccant (in assemblies containing desiccant) is NOT the primary drying agent but is an added insurance that any remaining moisture will be controlled by absorption into the desiccant.

In this report, condensation or misting, and the terms about to be defined, relate to the INSIDE environment of the window (condensation or misting between the two pieces of glass), although the exact principles described below can be used to explain condensation on the outside of the glass, both in the interior and exterior of a building.

This brings up the concept of humidity and relative humidity. Humidity is a measure of the amount of water vapour in the air. It can be defined as the ratio of water vapour contained in each “sample” of air- the sample of air in this case is the sealed window. It is expressed as the number of grams of water vapour in each kilogram of air. Obviously, one wants this number to be as small as possible. To do this, desiccants are added to remove the water from the air. Temperature does not come into play here. However, relative humidity (RH) is a measure of how much water vapour is actually in the air relative to (a percentage of) the air’s maximum capaMesa to hold water vapour.

Relative humidity is very dependant on the air’s capaMesa to hold water vapour. The air’s capaMesa to hold water vapour changes when the temperature changes. It follows that relative humidity depends on the temperature of the air AND the amount of water vapour in the air. Again, window designers add desiccant to minimize the internal relative humidity of a sealed window. Two more terms, to understand - ‘Condensation’ and ‘Dew Point Temperature’.

Condensation is the forming of water from water vapour. It takes place when warm, moisture-laden air comes in contact with a cold surface. In fact, condensation can only take place on a cold service. Cold, of course is a relative term.

The Dew Point Temperature is the temperature at which condensation first starts to form. If air that is not saturated with moisture starts to cool, its capaMesa to hold water will decrease as expected. (See the discussion on relative humidity.) The actual amount of water vapour in the air does not change. If the cooling continues, the capaMesa of the air to hold water will continue to decrease until it will hold no more moisture. At that point the air is saturated. The relative humidity reaches 100% and condensation to liquid water occurs.

The temperature at which the air is saturated is called the Dew Point Temperature. If air at 70ºF and 50% relative humidity is cooled to 50ºF, the relative humidity will reach 100% and condensation will occur at 50ºF. Using the Table 1, we can see that by reducing moisture in our window the dew point temperature falls. For example, the same window above with air at 70ºF, but with half the relative humidity (25%), has a dew point temperature of about 30ºF. Reducing the relative humidity to 10% will cause the dew point temperature to fall to 13ºF. So, controlling condensation obviously involves reducing the amount of water in the window environment.

Where does the moisture come from?

If the seal is intact, the moisture comes from the original air at the time of manufacturing and any moisture gain by the diffusion of water vapour through the sealing material. The ratio of desiccant to air is such that if sealed conditions are maintained, moisture will be controlled. Desiccants are chemically inert products. They do not fail, but like everything else they will obey the laws of nature. Like air, desiccants have their saturation points, and like air, the amount of water that a desiccant can hold, is affected by temperature.

A change in temperature could result in a desorption of moisture from desiccant to air. This would, of course, increase the humidity, increase the relative humidity, increase the dew point temperature, and if conditions are right, condensation will occur. But, in all likelihood, the manufacturer has taken these elements into consideration when building the units and calculated the amount of desiccant used such that there is little chance that moisture transfer by diffusion alone will cause failure within a five year period.

However, if seal leaks occur, the amount of moisture transferred by air flow through the failed seal will be in excess of the calculated value, excessive dew point temperatures will be attainable, and condensation will occur. Because of the desiccant in Type III windows, misting will be delayed but will occur when conditions are correct for such occurrences. Without desiccants to absorb moisture in Type I and Type II windows, the amount of moisture gain that can be tolerated is extremely small before condensation conditions are reached.

Organic sealants will lose their required properties over time. The glass surfaces themselves are under great stress. Differential temperatures over the surface can greatly vary producing high tensile stresses in the glass near the edge in contact with the sealant This is often exacerbated by under window heating units. Pressure changes due to fluctuations of temperature and barometric pressure distort the glass and stresses the sealant. Spaces between the glazing units and the sash can hold moisture and repeated frost action can damage the seal. In summary the sealant is subject to flexing and cyclical variations in temperature and pressure as well as water and sunlight which will eventually cause seal failure.

Moisture between panes of glass is more than a cosmetic irritant. Just like the moisture trapped on the outside of the glass, inside moisture will over a long term cause the window to fail. Repeated wetting and drying of common soda lime glass, for instance, results in scumming.

Scumming is the leaching of the sodium silicate salt from the glass, redeposited on the surface as a cloudy film. As well, the alkaline solutions from these silicates and the moisture will slowly attack the organic sealants. When this occurs on the inaccessible surfaces such as the inside layers of the windows, the window will normally require replacement.

The introduction of this report, describes a window as a ‘sealed enclosure’ that acts as a self contained miniature environment - but is still very much subject to the same laws of physics that exist externally to the enclosure.

Although the air between the glass, is referred to as ‘still’ air, the air within the environment is definitely not still. As the surface heats up or cools down, it affects the temperature of the air immediately adjacent. The air temperature then starts to rise or fall depending on whether the glass is hotter or colder. The resulting convection currents produce a flow resistance. This is known as air film resistance, and air film resistance increases the resistance of the material to the flow of heat.

Air film resistance is mentioned here for two reasons. In the experimental section of this report, thermograms will be used to show the effect of humidity on the insulation value or ‘R’ value of windows. These thermograms will show irregular temperature gradients over the window surface with cooler regions towards the center bottom. Air film resistance is one explanation for this phenomena. Air film resistance is an important factor in the thermal resistance values of glass and the insulation value of the window as a building material.

Resistance is usually given as an ‘R’ value which is the resistance of one square metre of the material (in this case - the window) subject to a one degree Kelvin temperature difference. Thus an R-Value of a typical window may be given as an R-Value of ‘R-2.4’. The R-Value of a typical insulated exterior wall is ‘R-20’ - the typical window loses about 10 times as much heat as a typical wall. R Values can be used to calculate heat loss. For example, the units for R Value m2K/Watt.

This means that if one takes the area of the window in square meters multiplied by the temperature difference in degrees Kelvin and divide by the R Value 2.4, one gets the heat flow in Watts. For example, 100 square meters of R-2.4 material, exposed to a 20ºK difference, will pass about 833 watts.

Next - the U-Value. It represents the air to air transmittance of an element or its conduction property. This refers to how well an element  conducts heat from one side to the other. By this definition U-Value is the reciprocal of its thermal resistance R-Value. U-Value = 1/RValue.

Its units are Watts per metre squared Kelvin (W/M2K). Window manufactures commonly use the U-Value to describe the rate of non-solar heat loss or gain through a window or skylight. The lower a windows U-Value, the greater are its resistance to heat flow and its insulating value.

The insulating value of a single pane glass is due to the insulating value of the glass itself and to the thin films of still air on the interior and exterior of the surfaces. In order to increase the insulating value of windows additional panes markedly increased the R-Value (reduced the U-Value) by creating ‘still’ airspaces. In addition to conventional double panes, manufacturers offer windows that incorporate new technologies aimed at decreasing U values even further. One such technology is low emittance (Iow-e) coatings. This coating is a microscopically thin, metal or oxide layer deposited on the glass surface. The coating limits radiative heat flow between panes reflecting heat back into the home during cold weather and back to the outdoors during warm weather. Less conductivity of heat, the lower the U-Value. Greater the resistance to heat flow, the greater the R-Value.

The insulating value of an entire window can be very different from that of the glazing alone. The whole window U-factor includes the effects of the glazing, the frame, and, if present, the spacer. (The spacer is the component in a window frame that separates glazing panes. It often reduces the insulating value at the glazing edges.) Since a singlepane window with a metal frame has about the same overall U-factor as a single glass pane alone, frame and glazing edge effects were not of great concern before multiple-pane, low-e, and gas-filled windows and skylights were widely used. With the recent expansion in glazing options offered by manufacturers, frame and spacer properties have amore pronounced influence on the U-factors of windows and skylights.

As a result, frame and spacer options have also multiplied. Window frames can be made of aluminium, steel, wood, vinyl, fibreglass, or composites of these materials. Wood and vinyl frames are far better insulators than metal. Insulated fibreglass can perform slightly better than either wood or vinyl. Some aluminium frames are designed with internal thermal breaks, nonmetal components that reduce heat flow through the frame. These thermally broken aluminium frames can resist heat flow considerably better than aluminium frames without thermal breaks. Composite frames have insulating values intermediate between those of the materials comprising them.

Frame geometry also strongly influences; energy performance. Spacers can be made of aluminium, steel, fibreglass, foam, or combinations of these materials. Spacer energy performance is as much a function of geometry as of composition. For example, some well-designed metal spacers insulate as well as foam.

 

 

 

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