Condensation forms on a surface when the temperature of the surface is below the dew point of the air. During the winter, when the coldest surface in a room is often the window, it’s fairly common to see water droplets or ice on window glass — especially in a room with elevated indoor humidity.
Condensation is more likely to form when indoor relative humidity is high. That’s why it’s more common to see condensation on a bathroom window than a bedroom window.
Condensation is more likely to form on cold surfaces than warm surfaces. That’s why it’s more common to see condensation on a single-glazed window than a double-glazed window.
Older Americans who grew up in the days of single-glazed windows remember the joy of waking up on a cold winter morning to see beautiful frost patterns — swirls, vines, and lacy leaves — on their bedroom windows. And who can forget the frost-covered windows in the Hollywood version of Dr. Zhivago?
On today’s low-e windows, however, such frost patterns are extremely rare.
In theory, a window with a high R-value (that is, a low U-factor) should do a good job of resisting condensation. However, even a low-U window can have a condensation problem. That’s because a window’s U-factor is an area-weighted average of the U-factors of the different window components. Window condensation is a thermal bridging problem, not an average U-factor problem.
The coldest part of most modern windows is the bottom half-inch of glazing. (Although the width of the space between the panes of modern insulated glazing units has been optimized to minimize the effect of convective looping in the gas between the panes, convective looping still occurs. Naturally, the gas at the bottom of the gap — whether air or argon — tends to be a little colder than the gas at the top of the gap.)
The cold area at the bottom of the insulated glazing unit (IGU) is the thermal bridge — the weak link to the exterior — because heat is conducted by the spacer at the perimeter of the IGU. That’s where you’ll see condensation on very cold mornings.
Unless you live in an Alaskan Quonset hut with single-glazed windows, you generally won’t see condensation over the entire window surface.
Condensation complaints irk window manufacturers
For decades, window manufacturers have suffered from thousands of complaints from ill-informed homeowners who have concluded that their windows are defective because they sometimes have condensation on them. Window representatives are , “Condensation in the field can be a result of many variables. Thermal conductivity of surrounding building construction, interior/exterior trim, humidification control, and the method of heat distribution on the interior plane of the assembly will impact [a window’s] overall performance.”
Here’s a typical scenario: a homeowner invests in renovation work that includes air sealing, added attic insulation, and new replacement windows. After all the work is completed, the house is tighter than ever. But all of the usual moisture sources in the home — the damp basement, the 14 houseplants, and a husband who brews a 5-gallon batch of homemade beer in the kitchen every Saturday — are still in place.
Before the retrofit work, the house leaked like a sieve, so the air was dry. Now that the house has been tightened up, the indoor relative humidity skyrockets. The homeowners see condensation on their brand-new windows, so they start hollering at the window manufacturer. “We never had condensation on our old windows!” they shout. “These new windows are defective!”
It’s enough to drive a window rep bonkers. As Chris Mathis, the president of Mathis Consulting Company and a well-known window expert, points out, “Window buyers don’t understand condensation. They don’t realize that 99% of the issue revolves around their lifestyle choices. They’re cooking with gas without using a range hood, or they have a house full of ferns. Unfortunately, there isn’t enough money to educate the entire public. We can’t fix all the homeowners.”
Even though this type of condensation isn’t really the window manufacturer’s fault, it’s in a window manufacturer’s interest to improve windows in ways that reduce the chance of condensation — if only to slow down the phone calls to the company’s technical department. And that’s exactly what most window manufacturers have been doing.
Getting better all the time
Window manufacturers describe windows as having several (usually three) thermal zones: the frame, the center-of-glass area, and the edge-of-glass area. When it comes to frames, manufacturers have phased out the use of aluminum frames in colder areas of the U.S. The few manufacturers that still make frames out of aluminum have changed their frame designs to include thermal breaks.
Center-of-glass U-factors have been lowered (that is, improved) by switching from single glazing to double or triple glazing, by adding low-e coatings, and by introducing argon or krypton gas fills between the panes of insulated glazing units (IGUs).
The edge-of-glass thermal bridging problem has been addressed by phasing out aluminum spacers and replacing them with warm-edge spacers — that is, non-metal spacers or spacers made of a less conductive metal like stainless steel. According to Jim Larsen, director of technology marketing at Cardinal Glass Industries, between 95% and 97% of all windows sold in the U.S. now include warm-edge spacers.
Coming up with a way to rate windows for condensation resistance
Years ago, persistent condensation complaints (especially complaints about aluminum-framed windows) [no-glossary]led[/no-glossary] some consumer advocates to urge the window industry to come up with a method of rating windows for condensation resistance, and to provide that rating on window labels.
Two different methods for rating windows’ condensation resistance were developed. The first method was developed in 1972 by the American Architectural Manufacturers Association (AAMA). The AAMA rating is called the Condensation Resistance Factor (CRF). CRF numbers for windows range from 30 to 80; the higher the number, the better the window is at resisting condensation. (A table with CRF recommendations is reproduced as Image #3, below.)
Although information about the AAMA methodology used to rate windows is published on the Web, it’s very hard for even an intelligent window shopper to determine whether the test protocol yields results that are relevant to window performance in the field.
Scott Warner, an executive vice president at Architectural Testing (a window-testing laboratory), discussed the CRF in an article titled . Warner wrote, “The CRF for the product is determined by the lower of either the weighted frame temperature or the average glazing temperature. … CRF numbers are reported as whole numbers only. The surface temperatures utilized for determining the CRF are obtained under specified test conditions. The current AAMA test method (AAMA 1503-98) specifies a warm side temperature of 70°F and a cold side temperature of 0°F with a 15 mph exterior wind.”
“While not an absolute value, the CRF is a rating number obtained under specified test conditions to allow a relative comparison of the condensation performance of the product.”
AAMA has developed to estimate the CRF needed to prevent condensation at specific indoor relative humidity levels and outdoor temperatures.
A useful article on condensation resistance ratings has been published on the . The article notes, “The [AAMA] calculation is based on a series of empirical measures of frame temperatures, which are combined with weighting factors to determine a value for the assembly. There have been criticisms of the AAMA methodology used for testing windows. In particular, criticism has focused on the testing conditions (0 degree outdoor temperature may not be applicable for all climatic conditions) and how the calculation weights different areas of testing results. It is unclear to what extent the recent update of AAMA’s standard has responded to these criticisms. Users are cautioned that windows selected according to results from AAMA’s online calculator may not always prevent condensation, since the CRF is based on a weighting of indoor temperatures. Some parts of a window may be cold enough to allow condensation, though the weighted average of the whole window would satisfy AAMA’s requirements.”
A rival rating method
The National Fenestration Rating Council (NFRC) has developed its own independent method for rating the condensation resistance of windows; the NFRC metric is called condensation resistance (CR).
The NFRC methodology for rating windows for condensation resistance shares the same weakness as the AAMA methodology. It’s very hard for a window shopper to evaluate the relevance of the metric to window performance in the field.
According to , “The NFRC Condensation Resistance scale is 1 to 100, with a higher number representing more resistance to the formation of condensation. The Condensation Resistance rating is determined based on outside conditions of approximately -18°C (0°F) with a 6 m/s (15 mph) wind, and inside conditions of approximately 21°C (70°F) with relative humidities of 30%, 50%, and 70% taken into consideration. The Condensation Resistance rating is a value that considers the relative area under condensation at these three humidity levels, which are then normalized, and the degree to which the surface temperatures are below the dew point for the frame and for the glazing are taken into account. The Condensation Resistance rating specified in the NFRC rating is based on the lower of the frame, center-of-glazing, or edge-of-glazing values.”
includes a few caveats: “It should be noted that NFRC 500 only reports condensation formation on the inside surfaces of windows, and that in the real world, environmental conditions vary from the standardized environmental conditions used to determine Condensation Resistance. This standard (NFRC 500) is not meant to predict condensation; rather it is meant to be a tool for rating and comparing window products and their potential for condensation formation.”
Although the CR method specifies procedures for testing actual windows, most window manufacturers use NFRC-approved computer software to determine CR numbers.
Which window manufacturers report CR values?
While NFRC window labels must include information on the window’s U-factor, solar heat gain coefficient, and visible transmittance, reporting the CR number is optional.
Many major window manufacturers, including Andersen Windows, Marvin Windows, and Pella Windows, have chosen not to report CR numbers on their NFRC labels.
However, many other manufacturers do report CR numbers. Among those who choose to report CR numbers are Amerimax Windows, Climate Guard Products, Coastal Innovative Products, Kolbe & Kolbe Windows, Performance Manufacturing Inc., Sierra Pacific Windows, and Weather Shield.
Reported CR numbers range from a low of about 15 to a high of about 72:
- reports that the CR of their windows ranges from 57 to 64.
- reports that the CR of their windows ranges from 42 to 60.
- reports that the CR of their vinyl casement windows ranges from 40 to 54.
- reports that the CR of their windows range from 15 (for a single-pane window) to 75 (for a triple-pane window).
- reports that the CR of their fiberglass awning windows ranges from 48 to 72.
- reports that the CR of their windows ranges from 49 to 58.
- reports that the CR of their casement windows ranges from 44 to 62.
Comparing the two rating systems
For the window shopper, the existence of two competing methods for rating the condensation resistance of windows is unfortunate. The two methods share many features; as notes, both standards “take into account thermal conductivity, geometry, thermal variation, and airflow resistance.”
However, the two methods are fundamentally different. Tracy Rogers, a technical director at Edgetech I.G., a manufacturer of warm-edge spacers, is the author of a paper on condensation resistance (). Rogers notes, “There is no correlation between AAMA and NFRC condensation resistance rating values.”
, “The differences between the CR and CRF ratings are significant, though their goals are the same. The primary method of determining the CR rating is through simulation, while the CRF is based on measured data. Both should be used primarily as comparative evaluations between windows. Since there is no current data available to compare CR and CRF ratings, determining whether a CRF rated window performs better than a CR rated window, or vice versa, is difficult.”
Sean O’Brien, a senior engineer at Simpson, Gumpertz & Heger Inc., is the author of an article detailing problems with the CRF rating method. In his article, , O’Brien notes, “At the conclusion of the test, the standard does not require the testing agency to report on whether or not condensation was observed on the interior surfaces of the glass and frame – something that would appear to be inherent in the nature of a condensation resistance test.”
Anyone interested in delving deeper into the weaknesses of the CRF metric should check out O’Brien’s article, which provides “an explanation of why the CRF is not an effective measure of condensation resistance, despite years of use and industry-wide acceptance, and why designers need to account for a variety of complicating factors when evaluating condensation resistance.”
Not correlated with U-factor
While it may not be particularly that surprising to discover that there no correlation between CR and CRF, it is surprising to discover that there is no correlation between either rating method and a window’s U-factor. It’s quite possible for a window to have an excellent (that is, low) U-factor, but still be very susceptible to condensation.
Tracy Rogers created a graph to examine the question of whether there was any correlation between window U-factor and CR (see Image #4, below). His conclusion: there is no correlation.
Rogers wrote, “The differences in each of these glazing options are varied. While some had changes to low-e coatings and others had spacer changes, the key point is that there is neither a direct nor a linear relationship between U-factor and condensation resistance. As concerns about condensation on the interior surfaces of fenestration products continue to grow, it is imperative that this lack of direct correlation between these performance characteristics is understood … The relationship between condensation resistance and U-factor performance of fenestration systems is neither direct nor linear and may, at times, be inversely influenced by specific component modifications.”
In other words: this is a thermal bridging issue, not a window U-factor issue.
Finally, it’s worth noting the limitations of both rating systems. Windows can suffer from condensation due to factors that aren’t considered by either rating method.
In that vein, , “Neither the CR nor the CRF take into account factors, other than the window, that can contribute to indoor condensation. Window treatments are a good example. Shades, curtains, and blinds often reduce the amount of airflow across the window, allowing the window to cool and reducing evaporation. This increases the likelihood of condensation. Window detailing and mounting can also affect window condensation. For example, windows mounted towards the outside of the wall plane will receive less interior airflow. In addition, the window frame will remain colder because it is in line with the coldest elements of the wall, such as the sheathing and cladding. Together, these conditions result in a window that is more prone to condensation.”
Does condensation resistance matter?
When I began researching this topic, I assumed that well-informed window shoppers should learn about methods used to rate condensation resistance, and should choose windows with a high CR or CRF. Now, however, I’m not so sure. Chris Mathis told me, “When people do the level of research into condensation resistance that you do, they discover that it’s really not a very useful number. But it is intended to give you a sense of the relative risk that you will have condensation. Your actual mileage may vary.”
Mathis also noted, “AAMA and NFRC have both failed to present condensation resistance in a way that is meaningful to Joe and Mary Window Shopper.”
If you live up north, and are worried about condensation, it can’t hurt to track down the CR or CRF number for the window you are interested in. Higher numbers are better than lower numbers. Mathis notes, “The colder your climate and the higher the indoor relative humidity that you want to maintain, the more attention you have to pay to the edge-of-glass details.”
The MSHI , “Select a window with an NFRC Condensation Resistance (CR) rating greater than 50. If using AAMA’s condensation resistance factor (CRF), some environmental conditions need to be determined first. If the typical indoor temperature in the winter is 70°F with a relative humidity of 40%, a CRF of 64 will prevent most condensation down to an outdoor temperature of 0°F. An A CRF rating of 70 will prevent most condensation down to -15°, and a CRF of 72 will do the same down to -20°F. … As mentioned above, the CRF is a weighted average of window performance across different areas of the window, so some condensation may still occur at specific points on the window.”
It’s an early warning system
Does it matter if the manufacturer of your window doesn’t report CR or CRF values? Not necessarily. For one thing, conditions in your house go a long ways toward determining whether you’ll have condensation on your windows.
If your house has windows that are double-glazed or triple-glazed, and you have frequent condensation on your windows, it’s likely that the indoor relative humidity of your home is too high. In a sense, your windows are an early-warning system to alert you that something is wrong with your house. “When there is condensation on the windows, there are probably some gross moisture problems in the building,” Jim Larsen told me. “What you see on the window should be the least of your concerns. You should be worried about what is happening in your walls.”
Homes with window condensation problems usually have a major source of moisture — a wet basement, dozens of houseplants, or a hobby that introduces a lot of water vapor into the air. It’s important to address these problems directly, either by eliminating the source of moisture or by installing some type of exhaust fan.
If your home is very tight, and you have lots of condensation on your windows, you probably need to install a mechanical ventilation system — and use it.
Finally, remember that curtains or shades keep windows cool at night, thereby increasing the chance of condensation (see Image #5, below). If one of your windows is regularly wet, try opening up the curtains before you go to bed.
Martin Holladay’s previous blog: “Air Leakage Degrades the Thermal Performance of Walls.”