Double-Pane Window Condensation Issues
Condensation is the accumulation of liquid water on relatively cold surfaces.
Almost all air contains water vapor, the gas phase of water composed of tiny water droplets. The molecules in warm air are far apart from one another and allow the containment of a relatively large quantity of water vapor. As air cools, its molecules get closer together and squeeze the tiny vapor droplets closer together, as well. A critical temperature, known as the dew point, exists where these water droplets will be forced so close together that they merge into visible liquid in a process called condensation.
Double-pane windows have a layer of gas (usually argon or air) trapped between two panes of glass that acts as insulation to reduce heat loss through the window. Other types of gas used in this space have various effects on heat gain or loss through the window. Some double-pane windows also have a thin film installed between panes that separates the space between the panes into two spaces, further reducing heat loss and heat gain through the double-pane window. If multiple-pane windows appear misty or foggy, it means that the seal protecting the double-pane window assembly has failed.
A desiccant is an absorbent material designed to maintain dryness in the space it protects. In a double-pane window, silica pellets inside the aluminum perimeter strip absorb moisture from any incoming air that enters the space between the panes. If not for the silica desiccant, any moisture in the space between the panes would condense on the glass as the glass cools below the dew point temperature.
Silica gel has an immense surface area, approximately 7,200 square feet per gram, which allows it to absorb large amounts of water vapor. As the sealant protecting this space fails over time, increasing amounts of moisture-containing air will enter the space between the panes, and the silica pellets will eventually become saturated and will no longer be able to prevent condensation from forming. A double-pane window that appears foggy or that has visible condensation has failed and needs to be repaired or replaced.
Why Double-Pane Windows Fail: Solar (Thermal) Pumping
Although double-pane windows appear to be stable, they actually experience a daily cycle of expansion and contraction caused by thermal pumping. Sunlight heats the airspace between the panes and causes the gas there to heat up and expand, pressurizing the space between the panes. At night, the double-pane window cools and the space between the panes contracts. This motion acts like the bellows of a forge and is called thermal pumping.
Over time, the constant pressure fluctuations caused by thermal pumping will stress the seal. Eventually, the seal will develop small fractures that will slowly grow in size, allowing increasing amounts of infiltration and ex filtration of air from the space between the panes.
Failure Factors in Double-Pane Windows
Double-pane windows on the sunny side of a home will experience larger temperature swings, resulting in greater amounts of thermal pumping, seal stress and failure rates.
Vinyl double-pane window frames have a higher coefficient of expansion resulting in greater long-term stress on the double-pane assembly, and a higher failure rate. Double-pane windows also experience batch failure, which describes production runs of windows, especially vinyl windows, that are defective, meaning that the double-pane assemblies have been manufactured with seals that have small defects that will cause the window to fail prematurely.
The Nature of Damage to Double-Pane Windows
If it’s allowed to continue, double-pane window condensation will inevitably lead to irreversible physical window damage. This damage can appear in the following two ways:
- riverbedding. Condensed vapor between the glass panes will form droplets that run down the length of the window. Water that descends in this fashion has the tendency to follow narrow paths and carve grooves into the glass surface. These grooves are formed in a process similar to canyon formation.
- silica haze. Once the silica gel has been saturated, it will be eroded by passing air currents and accumulate as white “snowflakes” on the window surface. It is believed that if this damage is present, the window must be replaced.
Detecting Failure in Double-Pane Windows
Double-pane window condensation is not always visible. If the failure is recent, a failed double-pane window may not be obvious, since condensation doesn’t usually form until the window is heated by direct sunlight. Double-pane windows in the shade may show no evidence of failure, so inspectors should disclaim responsibility for discovering failed double-pane windows.
Thermal Imaging as a Detection Tool
Under the right conditions, it’s possible to use an infrared (IR) camera to detect failed double-pane windows. IR cameras are designed to record differences in temperature.
Recommendations for Failed Double-Pane Windows
According to industry experts, the glazing assembly can be replaced approximately 75% of the time. Occasionally, the sashes must be replaced, and only about 5% of those cases require that the entire window be replaced.
There are companies that claim to be able to repair misty double-pane windows through a process known as “defogging.”
This repair method proceeds in the following order:
- A hole is drilled into the window, usually from the outside, and a cleaning solution is sprayed into the air chamber.
- The solution and any other moisture are sucked out through a vacuum.
- A defogger device is permanently inserted into the hole that will allow the release of moisture during thermal pumping.
There is currently a debate as to whether this process is a suitable repair for windows that have failed, or if it merely removes the symptom of this failure. Condensation appears between double-paned windows when the window is compromised, and removal of this water will not fix the seal itself. A window “repaired” in this manner, although absent of condensation, might not provide any additional insulation. This method is still fairly new and opinions about its effectiveness range widely. Regardless, “defogging” certainly allows for cosmetic improvement, which is of some value to homeowners. It may also reduce the potential for damage caused by condensation in the form of mold or rot. Some skepticism exists about the effectiveness and cost effectiveness of this method of repair.
Condensation in double-pane windows indicates that the glazing assembly has failed and needs repair or replacement. Visible condensation can damage glazing and is the main indication of sealant failure.
Building orientation is the practice of facing a building so as to maximize certain aspects of its surroundings, such as street appeal, to capture a scenic view, for drainage considerations, etc. With rising energy costs, it’s becoming increasingly important for building orientation to capitalize on the Sun’s free energy.
For developers and builders, building orientation of a new home to take advantage of the warmth of the Sun will increase the home’s appeal and marketability. For homeowners, building orientation will increase their indoor comfort and reduce their energy bills.
Thus, building orientation, along with day-lighting and thermal mass, are crucial considerations of passive solar construction that can be incorporated into virtually any new home design.
Facts and Figures about Building Orientation
- Many older homes’ designs were oriented through the use of a heliodon, which is a moveable light source used to mimic the Sun’s path that hovered over a small-scale model of a proposed building. Today, mathematical computer models calculate location-specific solar gain and seasonal thermal performance with precision, and have the added ability to rotate and animate a 3D color graphic model of a proposed building orientation design in relation to the Sun’s path.
- Homeowners may now tap into a specialty market of homes designed to spin on their axis in order to follow the hourly and seasonal path of the Sun. These UFO-shaped homes can spin a full 360 degrees in minutes and are built with unusually tall ceilings and windows for maximum efficiency in powering their solar energy system.
- While some passive solar features are relatively recent innovations, the practice of orienting a home to the path of the Sun is as old as civilization itself. Examples are numerous, from south-facing doors on Neolithic and ancient Ming Dynasty houses, to the astonishing Pueblo ruins in southwestern Colorado.
The Sun’s True Position and building orientation
Schoolchildren (and most homeowners) will tell you that the Sun rises in the east and sets in the west, and, if this were true, building orientation would be a fairly simple matter. In reality, the sun rises and sets in the east and west only on the autumnal and vernal equinoxes, and something very different happens during the remaining 363 days of the year. The Earth’s tilt causes the Sun to rise and set slightly south of east and west in the winter, and slightly north of east and west in the summer. This slight angle depends on the time of year and the observer’s distance from the equator.
As a result, the winter sun spends all of its time in the southern sky, and the summer sun spends all of its time in the northern sky. In the Southern Hemisphere, all of these directions are reversed, so the winter sun rises and sets in the northeast and northwest, respectively, and the summer sun rises and sets in the southeast and southwest, respectively.
How the Sun’s Variations in Position Can Affect Building Orientation and Design
The relative position of the Sun is a major factor in heat gain in buildings, which makes accurate building orientation of the building a fundamental consideration in passive solar construction.
Most importantly, a rectangular house’s ridgeline should run east-west to maximize the length of the southern side, which should also incorporate several windows in its design. For this reason, fewer windows should be located on the northern side of the house, where the summer sun can be intense. A deep roof overhang can shade the few windows in this area, as can different types of shade trees and bushes. Research supports an east-west ridge-line. Homes re-oriented toward the Sun without any additional solar features save between 10% and 20% and some can save up to 40% on home heating, according to the Bonneville Power Administration and the City of San Jose, California.
Builders should note that these directions are given in reference to the Sun and not magnetic north, which can vary significantly from the Sun’s actual position. Magnetic north, as read from a compass, can still be used as a reference if the builder adjusts the figure based on the location-specific magnetic variation, which can be found in publicly available maps.
Building Tips for New Construction and Building Orientation
The following tips will also assist homeowners and builders in maximizing heat gain through building orientation:
- Orient the floor plan – not merely the building’s profile – toward the Sun. Design the home so that frequently used rooms, such as the kitchen and living room, are on the southern side. Occupants will appreciate the sunrays in the winter and relief from the sun in the summer. Patios and decks should be built on the south side of the house, where direct sunlight will permit their use for more hours during the day and more days during the year. Likewise, the garage, laundry room and other areas that are less frequently used should be situated at the northern part of the house, where they will act as buffers against cold winter winds.
- Beware of mountains. The north/south sun differential is exaggerated in hilly and mountainous regions, where significant climatic differences can be seen over comparatively small areas. A passive solar house should be constructed on the south-facing slope of a mountain to avoid the extreme shading created where the low-angled sun is blocked by the mountain on the north side. Halfway up the slope is ideal, as the mountain’s peak is exposed to strong winds, while cold night-time air flows into the underlying valley, which is also a natural drainage point.
- Plan for tree shade. Trees are an important factor in passive solar design because they can both provide needed shade on a balmy summer day and starve the house of natural light when it is needed most. Deciduous trees planted on the south side will lose their leaves in the winter and allow natural light to enter the house, while evergreen trees planted on the north side will provide shade from the summer sun. Builders should carefully consider the age, species, growth rate and mature canopy cover of existing trees before deciding where to orient a structure on a building lot. Trees also pose unique danger.
- Install as many windows as possible, but not too many! The exact number of windows required is different for each house because it’s based on – among other considerations – the local climate. A “sun-tempered” house should include enough glazing to equal 5% of the conditioned square footage of the house. Remember, though, that windows allow heat transfer more easily than walls, so too many windows can actually drain heat from the house during the cold winter months. Read articles on Window Gas Fills and Window Films to learn how to insulate a house’s glazing.
- Stray from the rule on east-west building orientation, if needed. The east-west building orientation of the ridge-line may be adjusted to accommodate other factors by up to 20 degrees with only a minimal impact on heat gain.
- Driveways can get hot! Driveways and parking lots are made using gravel and asphalt – materials that heat up faster and reach higher temperatures than the rest of the yard. Excessive heat there can spill over to the adjacent house, which is why placement of the driveway or parking lot to the south or east of the building can reduce summer heat buildup in southern climates. During the cold winter months in northern climates, a south- or west-oriented driveway will melt snow faster and provide the home with greater warmth.
- Glass need not be vertical. Custom glass is available that may be tilted to match the angle of the sun and minimize reflection. Angling glass away from the vertical makes it less insulative, however, so builders should balance potential gains in sun exposure with loss of heat to the outdoors.
- Another environmental factor that should be considered in the equation of building orientation and positioning is prevailing winds, which are the winds that blow predominantly from a single, general direction over a particular point. Data for these winds can be used to design a building that can take advantage of summer breezes for passive cooling, as well as shield against adverse winds that can further chill the interior on an already cold winter day, or even prevent snow from piling up against windows and doors. Detailed information about prevailing winds for specific locations are plotted in a graphic tool called a wind rose, which is usually available from airports, larger libraries, Internet sources, and county agricultural extension offices. As a general rule of thumb, cold winter winds generally come from the north and west, which can be limited by using insulating glazing on these sides of the house. Also, remember that coastal areas typically experience breezes from an onshore direction, while cool breezes flow down valleys from mountain slopes.
Ultimately, factors such as street appeal and the property’s lot dimensions may restrict a builder’s ability for proper building orientation in strict accordance with passive solar techniques. Even while working under these constraints, however, a builder can still create an energy-efficient home through the implementation of energy-saving features, such as low-E windows, adequate insulation, air sealing, and cool roofs.