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Musings of an Energy Nerd

Rating Windows for Condensation Resistance

Window shoppers are confused by the two rival methods for measuring condensation resistance

Condensation on your windows is often an early warning sign that your interior humidity is too high. While this window may just need better weatherstripping, persistent condensation problems often occur in homes with wet basements or inadequate ventilation.
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Condensation on your windows is often an early warning sign that your interior humidity is too high. While this window may just need better weatherstripping, persistent condensation problems often occur in homes with wet basements or inadequate ventilation. A scene from Dr. Zhivago: a Russian window in the middle of winter.
Image Credit: Image #2: Dr. Zhivago
The colder the climate, the more important it is to choose a window with a high condensation resistance factor.
Image Credit: Image #3: AAMA
Tracy Rogers produced this graph to show that there is no direct correlation between a window's U-factor and its condensation resistance.
Image Credit: Image #4: Tracy Rogers
Curtains and blinds can worsen window condensation problems. If the interior side of your window gets wet at night, it's a good idea to open your curtains or blinds before you go to bed. [Photo credit: Steve Easley]
Image Credit: Image #5: Steve Easley

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.

Thermal bridging

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 fond of noting, “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 “Condensation Resistance Factor”. 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.”

According to an AAMA document, “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 an online calculator 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 Minnesota Sustainable Housing Initiative website. 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 an NFRC document, “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.”

Another NFRC document 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:

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 the article posted on the Minnesota Sustainable Housing Initiative (MSHI) website 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 (“Considerations for the Condensation Resistance of Fenestration Assemblies,). Rogers notes, “There is no correlation between AAMA and NFRC condensation resistance rating values.”

The MSHI document notes, “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, Finding a Better Measure of Fenestration Performance, 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, the MSHI report concluded, “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 advises Minnesota window buyers, “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.”

Click here to follow Martin Holladay on Twitter.


  1. Aaron Smith | | #1

    condensation on the window exterior
    With good performing windows, doesn't it make sense that you could also get condensation on the exterior of your windows in the Fall when there is a high diurnal temperature range and high outdoor humidity? Basically the outside of the window isn't warmed quickly enough so it drops below the outdoor air dew point.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to John Semmelhack
    You wrote, "Removing the screens also slightly increases solar gain and winter daylight."

    To me, the difference in daylight (and visibility) isn't slight; it's significant. I would never want to go through a northern winter with insect screens on my windows. The world is gray enough without them.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Aaron Smith
    Aaron Smith,
    There are two circumstances that can lead to condensation on the exterior of a window.

    The first circumstance happens during humid weather in the summer, when condensation can form on the exterior of single-glazed windows on an air-conditioned building. I saw this at Independence Hall in Philadelphia. The exterior moisture buildup was so bad that it had stained the bricks under the windows.

    The second circumstance occurs on the exterior of triple-glazed windows that face open patches of sky. Condensation can occur on the exterior of such windows, under the same weather conditions that causes dew on grass. Because of night-sky radiational cooling, the window glass can get colder than the air temperature, leading to the formation of condensation.

    The second type of exterior radiation usually won't happen if your window faces an adjacent building or a patch of woods. It's most likely to happen on a window facing open sky, especially if the house is on a slope and the window faces downhill.

    Fortunately, most windows are designed to handle water on the exterior, so exterior condensation is no more problematic than rain.

  4. David Fay | | #4

    correlation between U-factor and condensation resistance
    I don't buy the supposed lack of correlation between U-factor and Condensation Resistance. If you look at Figure 3, there is a very good correlation (higher CR with lower U-factor) except for one window (Glazing 4). If that were my data, I'd check the measurements for Glazing 4 to see if they were done correctly.


  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to David Fay
    Actually, there are two anomalies in the graph: Glazing #4 and Glazing #7.

    Here's the bottom line: if you build a window with a highly insulated frame, and low-e double glazing with argon fill, but you choose an IGU with an aluminum spacer, the spacer can be a thermal bridge that leads to a low condensation resistance (CR) value -- even if the window's overall U-factor is relatively low.

  6. User avater
    Mike Eliason | | #6

    once again passivhaus, leads
    once again passivhaus, leads the way? the certification requirements from the passive house institute in darmstadt require temp factor for condensation risk (fRSI) - and that goes beyond just window certification as well (e.g. the schoeck isokorb also lists an fRSI factor).

  7. User avater GBA Editor
    Martin Holladay | | #7

    Reponse to Mike Eliason
    While it's true that Passivhaus-certified windows are highly resistant to condensation, the window certification process established by the Passivhaus Institut in Darmstadt, Germany is not yet a useful tool to guide American window shoppers.

    So far, not a single U.S. or Canadian window manufacturer has managed to get any of their windows certified by the Passivhaus Institut. (However, I've heard that at least one U.S. manufacturer is getting close to having some windows certified.)

    The cost of Passivhaus-certified windows (for North American buyers) is so high, and the hurdles of importing windows so daunting (case in point: the difficulties experienced by Roger Normand) that -- for the time being, at least -- the Passivhaus certification process is irrelevant to American window purchasers.

    In the future, if a number of North American window manufacturers get some of their windows certified, and if these certified windows can be sold for a reasonable price, then it might make sense for American buyers to look for Passivhaus-certified windows.

  8. User avater
    John Semmelhack | | #8

    Interior insect screens
    Interior insect screens (for casement and awning windows) have a similar effect as curtains or shades. Removing screens in the winter time lessens the risk of condensation. It's a twice a year mild pain-in-the-butt. Removing the screens also slightly increases solar gain and significantly increases winter daylight.

    At a friend's house, removing the screens in winter made the difference between condensation (and moldy window frames) and no condensation.

  9. User avater
    John Semmelhack | | #9

    You're right, Martin. While the solar gain increase is slight, the increase in visual transmittance is significant.

  10. Greg Smith | | #10

    "Finally, remember


    "Finally, remember that curtains or shades keep windows cool at night, thereby increasing the chance of condensation. If one of your windows is regularly wet, try opening up the curtains before you go to bed."

    I just wanted to reemphasize your closing paragraph because that one factor, blocking or allowing air movment across the glass surface, is often the holy grail of reducing or eliminating surface condensation on windows.

    I cringe when I see people suggesting the use of themal shades (not window quilts, that is a different critter) or other loose coverings for increasing the R value of a window system specifically because of the effect that the coverings will potentially have on window condensation.

    I am a fan of using the stretch films over existing windows, when necessary, for both adding insulating value as well as reducing or eliminating condensation issues - even if I'm not a fan of the "use it once and throw it away" mentality that often is part of using such products.

    I am curious if you have had any thoughts about a follow up blog on window condensation when using surface 4 coatings? That could make for worthwhile discussion point as well.

    At the GANA (Glass Association of North America) conference in June, in Minneapolis, Jim Larsen and Tracy Rogers had an interesting debate on the potential for increased condensation when using surface 4 LowE coatings. The gist was that while surface 4 coatings do lower the interior glasss temperature, what is the potential effect on condensation resistance when using them? There was quite a difference of opinion and a lot of charts and graphs.

    As a quick aside, Joe Lstiburek was the opening presenter of the conference and I'm not sure that all the window guys knew how to take him (personally, I thoroughly enjoyed his presentation. I had a front row seat).

    And back on track, I would suggest that anyone reading Tracy Rogers' full article on condensation resistance take the spacer comparison graphs and statistics with a grain of salt - and unless someone has a specific question about the article or the graphics, I will leave it at that.

    Slipping over to Roger Normand's blog on his passivhaus windows, the spacers used in the picture of the Bieber passivhaus certified triple pane that accompany the latest installment of his blog happen to be aluminum; while as near as I can tell (the picture isn't totally clear to that detail) from the picture of the badly condensating window in this blog, the spacer is not aluminum. So using aluminum spacers can land a passivhaus certification and using non aluminum spacers can result in serious condensation issues due to thermal bridging...both are true.

    When dealing with window condensation, people often quote relative humidity. In fact it seems people always quote relative humidity, yet as you already pointed out, its the dew point that is the real consideration while relative humidity is...relative.

    I have a simulation example of two homes, in a power point presentation that I use, one has an RH of 29% and the other has a RH of 40%, yet it's the home with 29% RH that has a window condensation issue while the home with 40% RH the windows stay clear. While the homes have a significantly different relative humidity as shown, the dew point is the same in both homes. RH is not the significant factor, dew point is.

    As always, great article Martin, I thoroughly enjoyed reading it, and thanks.



  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Oberon / Greg Smith
    Oberon / Greg,
    Thanks for you comments.

    A note to GBA readers about "surface 4 coatings": Greg is referring to a relatively new type of double glazing: one that includes a hard-coat (pyrolitic) low-e coating on surface #4 (that is, the innermost surface of glass facing the interior of the house). The use of a surface 4 coating allows double-glazing to have two low-e coatings, thereby lowering the U-factor of the IGU.

    Somewhat paradoxically (counterintuitively?), a surface 4 coating reflects radiant heat energy back into the room, and IGUs with surface 4 coatings have a colder interior surface than conventional low-e double-glazing without the surface 4 coating. The glazing performs better, but the surface is colder. However, if you are sitting near the window stark naked, the glass will feel warmer (even though it is colder). OK -- enough with the paradoxes.

    IGUs with surface 4 coatings will have a slightly increased risk of condensation compared to other types of double glazing. Will homeowners care? Stay tuned...

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Dewpoint vs humidity
    Dewpoint vs humidity. Sort of misleading leaving out the temperature corollary.

    Dewpoint and humidity are linked via what we all read on our tstats, temperature.

    What ever became of the water filled solar gain windows? I have always intended to build a fish filled window just for the hell of it.

  13. User avater
    James Morgan | | #13

    earlier tech
    Top of the line single-glazed steel casements used to happily deal with condensation by means of tiny gutters at their base with drainage channels leading to the exterior. Ah, life was so much simpler then.

  14. Doug McEvers | | #14

    Window style and condensation
    I have found casement windows to be more prone to condensation than double hung of the same brand and specifications. Has anyone else had a similar finding in a cold climate?

  15. User avater
    Nick Sisler | | #15

    Why no CRF data?
    Why doesn't Rogers share any CRF data? Is it not available?

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Nick Sisler
    I'm not sure why Rogers didn't share any CRF data.

    For residential windows, it's hard to find any published CRF values. That's because manufacturers of residential windows are focused on NFRC labeling requirements. NFRC promotes the CR metric, not the CRF metric.

    As far as I can tell, the only published CRF values are for commercial windows. For example, here is a data sheet for Efco commercial windows that includes CRF information:,3460.pdf

    With the glazing specified in the data sheet, that particular Efco window has the following ratings:
    CRF-Frame: 44
    CRF-Glass: 59

    Window Tech Systems of Malta, New York published CRF values for their windows here:
    These values range from 47 to 63.

  17. Kristen Simmons | | #17

    yep, it's a spacer issue
    The main issue (to me) does seem to be that most American window manufacturers are using a less than ideal spacer. The manufacturers could do better if they wanted to, without a significant cost increase.

    "So far, not a single U.S. or Canadian window manufacturer has managed to get any of their windows certified by the Passivhaus Institut."

    To which I respond, because they haven't tried. Large manufacturers don't see a market yet for high performance components in the U.S. Case in point: Velux manufactures skylights in South Carolina to a much lower standard that what they make and sell in Europe. One cannot even buy their European product from their North American sales representatives.

    Smaller manufacturers, like Thermotech in Canada make windows that are used in Passive Houses in North America - good u-frame, u-glass, and good spacer being the critical components. FYI, there will be a workshop for window manufacturers interested in PHI certification in San Francisco following Greenbuild. Clearly some companies see this as an opportunity in high performance. More information here:

  18. Kristen Simmons | | #18

    Response to Martin: higher cost ≠ higher performance
    I want to address your cost and import comments. My experience purchasing windows this year for a passive house retrofit has been very different from Mr. Normand's. Mostly because the project budget must be significantly smaller than his (Optiwin aluminum-clad wood windows might be the most expensive PHI-certified windows in the US), but partly because I wear multiple hats on this job and can do energy modeling on the fly during design and review.

    I experienced no hurdles importing from Europe. The entire transaction was with an American sales representative and was very similar to other experiences that I have had specifying and reviewing windows. The lead time was longer by about 4 weeks, but I was aware of that up front, so it wasn't a surprise. As for delivery, the sales rep. is taking care of that too. My windows are due on site in about a week, so perhaps i should get back to you then about delivery.

    Regarding window selection, there was a learning curve. Aesthetics, performance and cost vary considerably. The most important thing to take away is that higher cost ≠ higher performance.

    I received quotes from several manufacturers, for at windows with different levels of performance (all would have worked for the project) and different materials (wood, insulated wood, aluminium clad wood, PVC, etc.). The difference between the high and low quotes was about 30k. Since the performance was comparable, I decided to go with the new Schüco ThermoPlus window for the project (my own house, actually). It's very solidly made, and my 12 year old nephew has been unable to break the corner sample yet, despite some serious effort. I loved the more expensive wood windows. but money is an issue. I chose to compromise on aesthetic rather than performance. That 30k can purchase enough PV to make the project net-zero, fyi.

    On a side note, i never did get a quote for the Optiwin wood windows that Mr. Normand used. Beautiful to be sure, but the wide frames significantly reduced glazing area and therefore solar heat gain. This wouldn't be an issue in new construction, but it can be a problem when one is retrofitting using existing rough openings.

  19. Greg Smith | | #19

    Reply to aj Builder
    "Dewpoint vs humidity. Sort of misleading leaving out the temperature corollary.

    Dewpoint and humidity are linked via what we all read on our tstats, temperature.

    What ever became of the water filled solar gain windows? I have always intended to build a fish filled window just for the hell of it."


    I left my comment on dew point open-ended in hopes of more questions/comments in follow up posts. You are absolutely right of course that temperature is what determines the relative humidity.

    Dew point is defined as saturation vapor density or 100% relative humidity and relative humidity is simply the ratio of the level of moisture in the air versus the maximum possible level of moisture that can be in the air prior to evaporation. Dew point is the transition temperature between evaporation and condensation.

    Per the water-filled IG's, I have heard of them, but no clue if anyone (outside of independent inventors) is actually taking the concept seriously.

  20. Greg Smith | | #20

    reply to Doug McEvers
    "I have found casement windows to be more prone to condensation than double hung of the same brand and specifications. Has anyone else had a similar finding in a cold climate?"


    I happen to agree with you and I chalk it up (my opinion) to the fact that casements as installed are outside of the exterior wall which results in colder glass/frame components.

    In addition, because of how casements are installed there is less interior warm air movment over the glass, and the interior screen (if not removed) also contributes to blocking warm interior air from the window.

    The advantage of casements, of course, is that they typically close much tighter than slider windows (vertical or horizontal) and they are less likely to be prone to cold air infiltration that can plague loose fitting sliders resulting in condensation as well.

    I am not a big fan of casements, but that is just personal preference.

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