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.
Vapor barriers are an important part of moisture control for interiors.
Vapor barriers are a material, typically a plastic or foil sheet, which resists the diffusion of moisture through ceiling, wall and floor assemblies of a building. Vapor barriers or “Vapor-diffusion retarders” are also effective for controlling moisture in basements, crawlspaces, and slab-on-grade foundations.
The term “vapor barriers” is commonly used, but “vapor-diffusion retarder” is probably more accurate, since “barrier” implies that the material will stop all moisture transfer, but this is not actually the case. Any material will allow the passage of at least a small degree of water vapor.
The capability of a given material to resist the diffusion of water vapor is measured by units called “perms,” which quantify its permeability. A perm at 73.4° F (23° C) is a measure of the number of grains of water vapor passing through a square foot of material per hour at a differential vapor pressure equal to 1 inch of mercury (1-inch water column or WC). Any material with a perm rating of less than 1 is considered a vapor retarder.
Regional Applications of Vapor Barriers
Depending on the climate, vapor-diffusion retarders are used and installed in different ways. The number of “heating degree days” (or HDDs) for a given area is used to determine its appropriate application. A “heating degree day” is a unit that measures how often daily, outdoor dry-bulb temperatures fall below an assumed base, normally 65° F (18° C).
Pros and Cons of Different Vapor Barriers and Materials
Vapor barriers from vapor-retardant paint is a latex primer available for interior applications. It behaves and is applied the same way as standard latex primer, and has a perm rating of about 0.7. Vapor-retardant paint used as vapor barriers can be tinted, and it can be applied on new gypsum board and over painted surfaces. The cost per gallon is comparable to standard paint.
- Pros: The vapor-retardant function of these vapor barriers comes at virtually no additional cost in situations where interior primers alone can be used. Vapor-retardant paint is the simplest application of vapor barriers in an instance where it is not desirable to significantly alter the existing wallboard or plaster surface.
- Cons: The paint is only appropriate for vapor barriers on interior wall surface applications. Damage to paint can compromise the retarding ability of these vapor barriers, as can inadequate prep work before application. If all penetrations and material intersections on the interior wall surface are not fully caulked or otherwise sealed, the paint will not be completely effective.
Treated paper or foil Used as Vapor Barriers
Treated paper or foil used as vapor barriers typically come in the form of kraft or foil-faced batt insulation. It is useful in situations where the wall finish has been removed and new exterior wall insulation is being installed, as well as in new builds. This type is most effective in mixed climates with low humidity, since the amount of unsealed edge will allow a path for moisture-vapor migration.
- Pros: This is a very cost-effective vapor barriers option, since batt insulation and a vapor retarder can be installed in one step.
- Cons: These vapor barriers can be installed only during a new build or in a situation where the walls have been stripped to the rough framing. The number of joints and edges inherent in this installation don’t allow for an extremely efficient vapor retarder, though it is sufficient for mixed climates or heating climates where humidity is controlled.
Clear polyethylene used for vapor barriers is the most basic, plastic film available, as well as the most economical, and is best suited to interior wall applications over framing and insulation. It is also an environmentally friendly choice since it is comprised of up to 80% re-processed material, but this comes at a cost, as the quality can be uneven, making it prone to tearing and puncturing. This type of vapor-retardant material is not recommended for applications where it will be subjected to more than a limited amount of direct sunlight, as it will degrade over time.
- Pros: These vapor barriers are inexpensive and fairly easy to install. Because the material is translucent, it is easy to attach to framing members, and installing wallboard over the plastic is simple, as well. Clear polyethylene vapor barriers are most effective in severe heating climates.
- Cons: This material is fairly flimsy and can be easily damaged during installation. It incorporates limited resistance to punctures and tears. Any penetrations, such as for an electrical junction box, must be taped and sealed in order to form an effective barrier.
Black polyethylene used as vapor barriers solves the issue of degradation from sunlight exposure by adding carbon as an ultraviolet inhibitor. It is otherwise functionally identical to clear polyethylene.
- Pros: It can be used for exterior wall surface applications in hot and humid cooling climates where it may be exposed to sunlight.
- Cons: It has issues similar to clear polyethylene, such as flimsiness, in addition to losing the ease of installation afforded by clear plastic that allows framing members to be viewed while attaching the material.
Cross-laminated and fiber-reinforced polyethylene used as vapor barriers are specialty products for applications where higher strength may be required. For retrofitting over rough, irregular surfaces, such as solid-board sheathing, these products are less susceptible to tears and punctures by lifted nail heads, splinters, and exposed, sharp corner edges. Either product would also be appropriate where rough handling and adverse site conditions are expected.
- Pros: These materials used as vapor barriers can withstand rougher handling than standard plastic sheets, being less susceptible to punctures and tears. The reinforced and laminated products are typically rated for limited UV exposure for exterior use. Black reinforced and laminated poly can be used as the required weather barrier under exterior siding and cladding.
- Cons: These materials for vapor barriers are, again, similar to the other forms of plastic sheeting, with the added disadvantage of higher initial cost.
Vapor-diffusion retarders are in widespread use in many geographical regions. Knowledge of the advantages and disadvantages inherent in the various vapor barriers and materials can be helpful in determining vapor barriers that will be right for a specific application, whether it is for a new build or a retrofit.
Condensation, also called sweating, forms on building materials when the temperature drops below the dew point, which is the temperature at which droplets of water vapor are forced so closely together that they coalesce into liquid water.
Condensation can be a problem because water vapor droplets can pool and destroy building materials, such as when condensed water chronically drains from a toilet and weakens the bathroom tile floor and subfloor. Condensation can also pool and encourage the growth of mold, which is a serious health hazard. Dripping overhead pipes can be an extreme annoyance in a finished basement, as they may damage carpets, furniture and valuable electronics. Pooled condensation can even cause an electrical fire, or electrocute building occupants.
Where does condensation typically form?
- plumbing drains. Condensation may collect on cast-iron or copper drain piping if a leaking plumbing fixture sends cold water through the building’s drain piping system;
- cold water pipes. In humid environments, condensation will quickly form on water pipes that distribute cold water;
- water pressure tanks. Especially in colder or more humid climates, water pressure tanks may experience condensation when water is flowing at a high and steady rate;
- in windows, water vapor and condensation in double-paned windows; and
- plumbing fixtures that are in constant use or are defective and constantly refilling, especially toilets. Toilets commonly sweat in the summer due to high levels of humidity, unlike windows, which host condensate in the winter when the outside temperature is very low.
Metal that does not come into contact with cold water or air rarely exhibits excessive condensation, even though it’s exposed to the same moisture-laden air as everything else around it. A water pipe that carries only warm water, for instance, seldom cools below the dew point. And non-metal building materials that do come into contact with cold water or air (such as plastic drains and piping) often lack the thermal conductivity to become cold enough to be the source of condensation.
Water Vapor and Condensation
Metals also vary in their thermal conductivity. Thus, they tend to cause water vapor to condense. Inspectors can be aided by a rule of thumb that states that a metal’s ability to transfer heat (and, therefore, create condensation) is roughly equal to that metal’s electrical conductivity. Electricians and some inspectors may know that, of all metals, copper is the second-best conductor of heat and electricity, meaning that it’s more likely to respond to a brief burst of cool water or air than other metals, such as steel or lead. The metal with the greatest conductivity is silver, but it’s far too expensive to be used in ordinary construction.
Condensation is more of a problem in older homes, which often lack a vapor barrier or sealers in the concrete. In this instance, moisture in the ground is forced through the foundation and masonry, which is why condensation is commonly found in the basements of older buildings. The two strategies used to reduce condensation are to lower the relative humidity of the air and to keep surfaces from becoming cold.
Lowering relative humidity can be reached by using these strategies in the following ways:
- Use a dehumidifier. This is a simple, effective appliance used to lower the relative humidity of the air, as it forces water vapor to condense into a water tank so it cannot condense elsewhere. An air conditioner can also dehumidify the air.
- Remove plants from the home. This will decrease relative humidity, as their transpiration of moisture is a significant source of water vapor in homes.
- Insulate cold surfaces. Insulation will keep surfaces from becoming cold, and it can easily be applied to water pipes and water pressure tanks. Fiberglass insulation should be avoided, as it’s ineffective when wet and it can be the source of mold growth. Plastic foam wrap is an adequate material for this purpose. Keep in mind that by adding thermal insulation to water pipes, they can no longer be counted on to supply radiant heat to their surroundings.
- Ventilate the basement. However, this tactic might be counterproductive if the outside air is more humid than the indoor air.
- You can lower relative humidity by adding heat where condensation is a particular problem.
Also, keep in mind that what appears to be condensation may actually be a water leak. If insulation and dehumidification don’t seem to improve the condensation problem – especially if it’s appearing in only one place –- homeowners should contact a qualified plumber.