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Community and Q&A

Could Martin Holladay relaunch his “Windows for Passive House” blog for 2010?

Veronique Leblanc | Posted in PassivHaus on

I am planning to build a Passive House and – although I am European- I would prefer not to import windows from Europe to help the PH idea become truly American.
Martin, your 2009 blog was so great, I would love you to launch a 2010 update to know what progress has been made on the American continent to make windows high-performing while more affordable than those of our German colleagues.

Thank you.


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  1. GBA Editor
    Martin Holladay | | #1

    Il me semble que rien a changé -- les meilleures fenêtres en amérique du nord sont toujours celles de nos amis canadiens: Thermotech, Inline, Accurate Dorwin, Duxton, et Fibertec. Les fenêtres de Serious Materials sont probablement les meilleures fenêtres américaines.

    I'm always open to revisiting a topic that I covered in one of my blogs -- for those who didn't see it, here's a link to my blog on Passivhaus Windows -- and I will certainly keep my eyes open for changes in the industry.

    It seems that more and more American window manufacturers are beginning to offer triple glazing -- albeit the wrong kind of triple glazing, a thin type with insufficient gaps between the panes and lower performance than full-thickness triple glazing -- and a few are offering high-solar gain glazing in such a way that a builder doesn't have to telephone the president of the company to arrange for a special order.

    So far, my recommendations haven't changed. Cold-climate builders should buy Canadian windows with pultruded fiberglass frames and full-thickness triple glazing, with high-solar-gain glazing for the south elevation. As many windows as possible should be fixed, and the remaining windows should be casement or awning units.

  2. J Chesnut | | #2

    I have a Buderus boiler in my house. It was manufactured in Germany but there is a big enough market here that they are readily available. Some of the best performing solar panels are from China. What kind of car do you drive? While I would love to see domestic windows of the quality of the European PassivHaus windows I would also like to see demand grow for the PassivHaus certified windows so that they might become more readily available in our market. I haven't seen the Canadian windows up close but I suspect the hardware and frames don't match the quality of what I have seen from Europe.
    I think currently PH is used as a German import. To make it "american" I believe requires regional institutes that can provide same expertise that PHI offers in Europe. We in the states do not have the same institutional support nor a research facility that can test windows or ventilation equipment to become PassivHaus certified. As I understand it PH is based on an optimized cost value determination where the PH thresholds are determined at the point where the efficiency of the envelope reduces the mechanicals needed to heat and cool to the point of it being competitive in the market. This so far has proving not to be the case in the markets of the cold climates in the states. An 'american' PH would revisit these issues.

  3. VeroEnergy | | #3

    Merci Martin! It would be very useful to have a price per sqft VS performance comparison matrix for these windows (including their frames)with ex factory prices and "landed" prices as well as their warranty contract and after sales service conditions.

    J., there are very efficient PH in the US and we have PHIUS to make the concept "local". In my mind, what is missing to increase the demand for PH certified windows (and R60+ building insulated panels) is some state and federal incentive like there is for solar and geothermal.

  4. VeroEnergy | | #4

    By the way, J., I drive American or made in America cars!

  5. Riversong | | #5



    Take a look at the interstate Fuel Saver Special composite casement windows manufactured in Pittston, Pennsylvania with a U.14, SHGC .21, VT .38, and 0.05 CFM/sf infiltration at a moderate price.

    They also make Vinyl windows at an even better price, but vinyl... My friend Ed Nickles has been using them for a while and speaks very highly of the company and their products.

  7. Riversong | | #7

    SHGC 0.21?

    Sounds like an energy waster unless you're building where the sun don't shine.

  8. user-869687 | | #8

    Or where it shines way too much. At any rate this window is not the best choice for those of us in the blue zone on that Energy Star climate map.

  9. J Chesnut | | #9

    I was hoping I would catch you driving a Mercedes or an Audi : )
    I worked on a PH project that used the Optiwin Alu2Wood window and they are very high quality windows that are worth the price in the context of an architect commissioned costum house. Custom homes tend to have large windows and I have some confidence that the solid wood (larch I believe) frames will hold up well over time. The pultruded fiberglass frames are relatively recent (I believe) so I don't know how to gauge their longevity. Aside from the environmental concerns with the production of vinyl, I'm less confident about the longevity of their frames.
    I believe Martin finds the Canadian windows sufficient for performance reasons and sees little sense in paying a premium for the European imports but I hope he finds the time to include the European models in his discussions. In addition to performance longevity, quality and beauty matters for us architects and we work on projects that can tolerate premium costs.
    All the best with your PH project.

  10. MICHAEL CHANDLER | | #10

    Robert, I don't want to start a flame war but It is hard for me to see a U-.14 as an "energy waster."
    I'm not sure what you're putting in buildings up north but I see the argon 366 glass with U.31 and SHGC .20 as the norm for energy efficient homes around here and it would be an upgrade to go to a window like this on the north and west and an argon 179 glass with a U..28 SHGC .70 on the east and south. Here in the Southeast we don't want west sun and we gain little from the north.
    Regardless, this is a great value in an American-made window. We don't all build in the north east.

    I'm looking at aphoto I took of a label on a Serious Materials high-priced window that has a U-.22 and SHGC.39. the cost per window needs to be considered against the spec in my world.

  11. Riversong | | #11


    I don't understand why you would think it "flaming" to engage in debate about the relative merits of low window conductivity vs. high solar heat gain coefficient. This is not only a reasonable but a vitally important topic of discussion.

    What may be construed as "flaming", however, is to counter my statement by sidestepping its simple message.

    I certainly did not state that a low-U window is an energy waster, but that a very low SHCG window very well may be, regardless of its other qualities. And of course you're correct that the balance point between these two depends on local climate and siting as well as the overall thermal envelope design.

    But you also fail to help this discussion when you compare whole window values to glazing values. There are no windows on the market with SHGC of 0.70 except single-pane clear.

    Cost of window units must also be a factor in anyone's calculus, which is why I'm thrilled that Pella has finally listened to the demands of the passive solar designers and now offers a very competitively priced double-glazed high SHGC option in their wood/clad Proline and their fiberglass Impervia line.

    Martin continues to claim that triple-glazed windows should be the bottom line in cold country design, but I've found that a good quality double-glazed lowE unit with a high SHGC and a good price can be far more cost-effective and sometimes more energy-efficient.

    The only "flame" we should be considering is that limitless free solar energy that is the foundation for all life and should be the foundation of sensible and responsible home design.

  12. Riversong | | #12

    quality and beauty matters for us architects and we work on projects that can tolerate premium costs.

    Warning: flame ahead.

    Importing expensive windows from across the world just because you can and because it satisfies some personal aesthetic urge is the antithesis of "green".

    More than any other quality, "green" should be local, not only avoiding the unsustainable transportation impacts, but also supporting vibrant local or regional communities and economies.

    Custom homes tend to have large windows

    Unfortunately, this is true not for energy- and resource-efficiency reasons, or simply for optimizing the balance between daylighting, view, ventilation, solar gains, energy losses, and aesthetics - but largely to satisfy either the client's or the architect's ego.

    This is the antithesis of "green". It is personal ego and collective hubris which have created every ecological, economic and cultural crisis we now face.

  13. Riversong | | #13

    Warning: flame ahead (reformatted for better burn).

    quality and beauty matters for us architects and we work on projects that can tolerate premium costs.

    Importing expensive windows from across the world just because you can and because it satisfies some personal aesthetic urge is the antithesis of "green".

    More than any other quality, "green" should be local, not only avoiding the unsustainable transportation impacts, but also supporting vibrant local or regional communities and economies.

    Custom homes tend to have large windows

    Unfortunately, this is true not for energy- and resource-efficiency reasons, or simply for optimizing the balance between daylighting, view, ventilation, solar gains, energy losses, and aesthetics - but largely to satisfy either the client's or the architect's ego.

    This is the antithesis of "green". It is personal ego and collective hubris which have created every ecological, economic and cultural crisis we now face.

  14. VeroEnergy | | #14

    I am no Schwarzie, so no Hummer, not even a "regreened" one! I am no perfect green mind,I
    but I try hard to put my lifestyle in line with my philosophy. I would like to come back on the "green"strategy for the US market: I do not believe a second that US homeowners are going to embark massively on energy saving products that are expensive and imported. This is opposite to the local culture and the current depressed economy makes it even tougher. For me, green should be energy efficient, yes, but also affordable and easily accessible locally. I believe Passive Houses can be built at an affordable cost and with quick payback on the extra dollar required for energy efficient systems if they are designed and thought in a clever way.
    This is the reality in Europe so we need to put our brains and ressouces towards this goal: no ther way to achieve volume (if government does not make it "code" standard soon) so that it comes from homeowners' demand because of its obvious cost efficiency and quick payback. There is nothing amazingly expensive in the materials required for superinsulated houses and labor cost has gone down big time with the crisis. If Habitat for Humanity has chosen Passive House as their building standard from now on, we need to try too. Anybody can then decide to put extra trimming and molding to their superinsulated house shell if one wants or can spend more money instead of struggling to pay for good insulation, proper windows and efficient mechanicals.
    Let's try at least!

  15. Graham Irwin | | #15

    As Martin and others can attest, Passive House (PassivHaus) draws directly and opening from earlier efforts in the US and Canada in super-insulated buildings. Generally, most of the components are available domestically, with windows and mechanical systems being the biggest departure from standard construction materials - beyond that, it's just a more rigorous design and construction process. In terms of domestic expertise, it is certainly currently available (if I must say so myself) and more and more available as the Passive House Institute US (PHIUS) continues to train consultants:

    Regarding the cost optimization question, that is a core element of the Passive House standard, and our design process includes thorough cost benefit analysis and optimization as well as performance modeling. I fully expect that our experience and our findings will inform the Passive House standard, as it is based upon a scientific approach.

  16. Graham Irwin | | #16

    Sorry, I meant to say "performance monitoring," not "performance modeling," though that is also an integral part of the process.

  17. VeroEnergy | | #17

    So do you mean that you find your passive house projects cost idem or less than standard construction houses?

  18. Riversong | | #18


    I doubt that any PH builder could claim a below-standard cost. But my modified Larsen Truss house system is cost-competitive with conventional construction because it uses less wood and lower-cost materials than energy-code standard construction.

    However, not every building jurisdiction would approve a house framed from local rough-sawn lumber with no sheathing on a shallow frost-protected or rubble-trench foundation without an engineer's stamp.

  19. Graham Irwin | | #19

    That depends on what your goals are and how you evaluate the "cost." On a first-cost basis, not so far, but the US construction industry has been engaged in a fairly ruthless process of first cost reduction to the point that the life expectancy of most new homes is less than 30 years (the length of a typical mortgage) I am told!

    If it is a life-cycle analysis, I suspect PH looks far better, and if it is a "green" approach, better still, as many such efforts are "so near but yet so far" from Passive House, where a small difference in approach or a small incremental investment would yield radical savings and/or construction cost reductions. For example, if the goal is "net zero" carbon or energy is the goal, one quickly finds that a lot of styrofoam and/or shredded newspaper (cellulose insulation) can be purchased for the cost of a PV panel, and Passive Houses require vastly smaller PV systems for equivalent "net" performance.

    That said, the process I employ determines the optimal levels of insulation by finding the point of "diminished returns" for each envelope measure and utilizing an integrated design approach, where the designer and builder both determine the preferred strategies. The basis of the Passive House design process, the Passive House Planning Package (PHPP) is a detailed and field-tested energy modeling program wherein every aspect of a building's energy performance is determined and analyzed.

  20. GBA Editor
    Martin Holladay | | #20

    Exaggeration alert!

    It's not true that "the life expectancy of most new homes is less than 30 years."

    If it were, most of the homes built in 1980 would now be being bulldozed ... but that clearly isn't happening...

  21. Riversong | | #21

    It is true that most consumer products seem to have a life expectancy roughly equal to the warranty period.

    The median US owner-occupied house in the US in 2007 was 32 years old. I've seen no data on new homes (of course we don't have that data yet because those homes have not yet expired).

    While it's true that the typical commercial building is replaced after 30 years, that's because of change of use, not delapidation. I think it's fair to say that most new homes in the US would have a life expectancy of 50-100 years (with appropriate maintenance).

  22. Lucas Durand | | #22

    In your blog on passivhaus windows, you wrote:

    Although Heat Mirror glazing has been around for decades, many builders are taking a wait-and-see attitude about the product.

    Since the time of writing, have Heat Mirror windows come into more popular use?

    I have an opportunity to buy Heat Mirror glazing units (no frames) from a manufacturer here in Ontario at a very good price. I would have some installed into awning or casement frames locally and install the rest of the units sans frame. Here are the numbers for the models I'm considering (both are triple glazed with argon fill):
    (East and south) TC88 - U-value: 0.147, SHGC: 0.48, VT: 0.63
    (North and west) TC88 w/ Low E - U-value: 0.134, SHGC: 0.34, VT: 0.55
    These numbers were copied from the architect information binder here:

    I wonder what you all think?

  23. user-869687 | | #23

    In the PDF it states that a 10-year warranty applies only with shop drawings approved by the glazing manufacturer. That could be a problem for site-fabricated windows. Consider also that some window manufacturers offer a 20-year warranty on their glazing units.

  24. Riversong | | #24

    Any decent glazing unit should have a 20 year warranty.

    I know that when I used to site-fabricate large fixed windows using patio door replacement units, the 10-year glass would lose its seal and fog in exactly 10 years, while the 20-year glass seemed to last forever.

  25. GBA Editor
    Martin Holladay | | #25

    In addition to my concerns about plastic film durability, I have two other concerns about Serious Materials' low-U-factor windows: their unacceptably low SHGC ratings, and their unacceptably low VT ratings.

    Only cold-climate builders are interested in very low U-factor windows. Yet (ironically) the Serious products have low SHGC ratings, making them inappropriate for northern climates.

    In addition, I have heard several people say they are disappointed by the gray tint that accompanies some of the units. The VT number tells the story. These windows save heat, but they also cut down on light.

    Serious makes a wide variety of glazings, so these criticisms don't apply to every window. But look at the numbers. I have yet to find an operable window from Serious that combines low U-factor, a decent SHGC, and an acceptable VT.

    In one of my blogs, "Choosing Triple-Glazed Windows," I wrote that a cold-climate builder should look for a window with "a U-factor ranging from 0.19 to 0.26 and a SHGC ranging from 0.39 to 0.47." I still think that's good advice.

    In another one of my blogs, "Passivhaus Windows," I pointed out the dangers of a low VT number. Even Robert Clarke, a Serious Materials rep, pointed out that "any window with a VT below 0.40 'would not be ethical to sell as clear glass.' "

  26. VeroEnergy | | #26

    Both warranty duration (and effective performing period of time) and look are very concerning again for Passive House projects were we design Southern facades to be as "glazed" as possible: how green is that to have windows fail so quickly when houses are supposed to last a lifetime at least? Should not a fixed window function and last same as a wall like in commercial buildings' curtain walls?

    Let's not forget that in Europe we try and keep houses for centuries and sometimes we find the good old principles of orientation to the sun and thick walls are already there...

  27. user-626934 | | #27


    The 725 Series (one plastic film, between 2 panes of glass) from Serious is probably the best bet in their lineup for those who are looking for high solar gains and low U-values. It's also less expensive than the 925 and 1125 series. Serious offers the choice of a high solar gain or a low solar gain package. Here are the whole-window performance numbers for the high gain package:

    Casement: U=0.22 / SHGC=0.39 / VT=0.50
    Fixed: U=0.20 / SHGC=0.50 / VT=0.65

    Here's the link to the 725 Series brochure:

    I've seen the 725 Series (both high and low-gain) installed in a house I'm consulting on, and did not notice any difference in "tinting" compared with the Thermotech triple-pane windows (both high and low-gain) I have in my own house.

  28. GBA Editor
    Martin Holladay | | #28

    I agree -- that choice is probably as good as you're going to get with Serious.

    I still prefer Thermotech's high-solar-gain triple-glazed casements. The specs are better: whole window U-factor 0.19, whole window SHGC 0.42. That beats the Serious 725 casement (U=0.22 / SHGC=0.39).
    Here's the Thermotech Web page:

  29. user-626934 | | #29

    I'm with you on Thermotech, Martin. I have no regrets from installing them at my house.

  30. Andrew Henry | | #30


    The hardware on the Thermotech's is pretty damn solid. And it is one of the nicest windows I have put in from an installation perspective. The pultruded fibreglass frames are very solid, and stable. If you go with a hardwood veneer (maple) on the inside you get the look of wood with the stability of fibreglass.

    There is a good reason why my cross-country racing skis are made of materials like fibreglass. It's stable, predictable and handles extremes of moisture and temperature much better than wood (and definitely vinyl). It makes sense to use it as a window frame material. To mee it seems to be the most appropriate material for window frames.

    Robert, I find it odd that you'd be content with double-glazed windows in Vermont. There has been some discussion about ER (Energy rating) in comments to previous window related questions. With a really good triple glazed window with high SHGC the window becomes a net gain with respect to heating. A double glazed window as far as I know will always be a net heat loss. I have a lot of respect for the wall systems you use in building your houses, but why would you squander the gains you made with super-insulation with windows that are a net-loss (ER less than 40, or pre-adjustment less than 0, back when 'zero meant zero'). I seem to recall you not liking HRVs which still has me confused, since it's the same issue, your giving up heat that you could easily retain?

    Martin, could you maybe elaborate on the effects that the width between glazes has on double and triple glazed windows' energy efficiency? And for that matter what type of 'air', for example argon, the space between glazes is filled with, has on energy efficiency. It does seem to be something that is overlooked in the discussion of window performance. I recall Stephen Thwaites bringing it up in comments on another thread and I am only starting to get my head around it's impact.



  31. GBA Editor
    Martin Holladay | | #31

    There is an optimal gap between panes for each gas. For argon gas, an optimal gap (from a thermal performance perspective) results in triple-glazed windows with a glazing thickness of 1 3/8 in., so if you're considering buying triple-glazed windows with a total glazing thickness of only 7/8 in. or 1 in., and it's filled with argon, it's a compromise glazing that was chosen to fit a frame designed for double glazing.

    Krypton, a much more expensive gas, has a smaller optimal gap, so some U.S. manufacturers who make window frames that can't quite accommodate full-thickness triple glazing resort to krypton in skinny glass. That works -- you can get decent performance specs -- but it costs an arm and a leg compared to argon. It makes more economic sense to design the frames for full-thickness triple glazing.

  32. Riversong | | #32


    All my double-glazed south-facing windows provide significant net gains, even in cloudy northern VT with about 50% solar availability.

    I determine this with my own energy modeling spreadsheet, which I know to be accurate since it very closely predicts actual energy consumption on all my houses. I teach energy-efficiency and passive solar design using this spreadsheet.

    I learned how to engineer houses, both structurally and thermally, from my teacher and mentor, Charles Wing, engineer and founder of (now defunct) Cornerstones School of Energy Efficient Building in Brunswick ME. In the 80's, he determined that a clear (lowE wasn't available) double-glazed south window in Portland ME (HDD = 7300) would offer a net gain of about 1.2 therms per square foot of glazing over the heating season, and even provide a net gain on the east and west. A north window would lose 0.6 therms per square foot, or only half as much as the south windows gain.

    A double-glazed lowE window with decent SHGC provides significantly more.

    When high-end triple-glazed windows are twice the price of standard commercial double-glazed lowE units (Pella Proline or Impervia), I find it very hard to justify the expense when I'm designing or building for affordability.

  33. Andrew Henry | | #33


    I am not sure how you have arrived at the conclusion that your "double-glazed south facing windows provide significant net gains".

    The ER system suggests that the best your going to do for a fixed double glazed window is to break even; losses canceling out the gains, an ER of zero.

    See section 6.3...

    As for affordability I'll concede that good triple glazed windows like Thermotech's are going to cost a lot more. But you get what you pay for. Is it good enough if the goal is to reach zero carbon emissions by 2050?

    All that said what your building and teaching is far and away a whole lot better than what is generally built.



  34. Riversong | | #34


    I already answered that. I don't rely on someone else's packaged software or representative energy ratings. I use my own spreadsheet, and its accuracy is attested by the high correlation between modeled and actual energy consumption.

    When I calculate net gain from a window, I don't take into account air leakage, since that's a variable best assigned to the entire house assembly and established with a blower door test. I'm calculating solar gain vs conductive heat loss. And it's a net gain with double-glazed lowE windows - often better than with proportionally lower SHGC triple-glazed windows.

    If I can double the whole house R-value for less cost than upgrading to triple-glazed windows, that's a better place to invest limited financial resources.

    The first super-insulated house I designed and built won an honorable mention (3rd place) in a national energy- and resource-efficient design competition sponsored by NESEA. 1st and 2nd place went to million dollar homes - hardly a contest, since you can do almost anything when money is not an object. My modest home cost only $48,000 complete with site work, well and septic. In spite of having to make compromises in such high-ticket items as windows (we site built all the fixed windows), my little house was one of the most energy efficient in the US at the time and was mostly built with local, rough-sawn lumber that supported the regional economy.

    The potters who own the home are still heating it with 4/5 of a cord of wood that they pick up off their forest floor and cut with a bowsaw (which is exactly what my energy model predicted), and when they have to leave it in February to go to a craft show it never freezes even with no supplemental heat.

    And, by golly, it has only double-glazed lowE glass on all sides.

  35. Andrew Henry | | #35


    I am impressed!


  36. Lucas Durand | | #36

    Thank you all for the advice.
    I hadn't noticed the warranty caveats.
    Will investigate further.
    I love this website.

  37. Jesse Thompson | | #37

    One note of warning about double vs triple glazing in cold climates beyond energy savings, we've had a client call the window technician in because they thought their new double glazed low-e fiberglass / wood framed window (U-0.29 in 7,500 HDD) was leaking, when it was merely cold enough that it was dropping a cold air current down on them in an uncomfortable way.

    Everyone's definition of comfort varies, and some people will be more sensitive than others to issues like this. The inside face of a triple glazed window will always be warmer if this is a concern.

    Jesse Thompson
    Kaplan Thompson Architects

  38. user-716970 | | #38

    When it gets even colder outside (9500 HDD where I live) double glazed units just won't do. Condensation and even frost buildup will occur even with 25% indoor RH.

  39. VeroEnergy | | #39

    Robert, could you share with us your house's specs and budget allocation per sqft and the sqftage?
    I like the idea of playing with performance VS cost in modeling simulations.
    Homeowners look at energy efficiency with a budget and payback requirement so prizes should be distributed with the market price in mind!

  40. Riversong | | #40

    The inside face of a triple glazed window will always be warmer if this is a concern.

    The inside temperature of a window unit has as much to do with the glass spacers, frame and air leakage as with the number of glass panes.

    All but fixed windows leak, and poorly-built or poorly-installed windows leak a lot, particularly sliding units.

    Condensation and even frost buildup will occur even with 25% indoor RH.

    The inside surface temperature of a window is dependent on outdoor vs indoor temperatures, not on HDDs.

    Good quality double-glazed lowE casement or awning windows, with insulated spacers and non-conductive or minimal frames should not condense except in the most extreme outdoor temperatures with indoor humidity at 30% and should never frost.

    Heavy condensation is as much a function of window placement (deep boxes, or near room corners) or interior coverings (drapes, blinds, shutters) which limit air circulation, and frost should never occur unless there is air leakage.

    At zero degrees outdoor temperature, a double-glazed lowE window should be about 55° and a triple-glazed unit about 60° at center of glass. Testing has determined that the probability of discomfort with double lowE is 22% (with only 1% of that due to drafts), while that for triple lowE is 18%. Not a huge difference.

  41. J Chesnut | | #41

    Concerning air leakage, PH European tilt and turn windows install in a manner that the 'outsulation' outboard the window overlaps the insulation of the window frame. This type of install both reduces thermal bridging and coupled with a double stick butyl tape that bridges the gap between window frame and window bucks makes an highly air tight install. The tilt and turn hardware is a multi-lock mechanism that pulls the window casing tight to the frame with pressure on a perimeter gasket.

    Are the Canadian windows with good performance specs typically flange installs? I hope to see some of the Canadian frames at an upcoming AIA convention.

  42. GBA Editor
    Martin Holladay | | #42

    I'm not sure what this sentence means: "Testing has determined that the probability of discomfort with double lowE is 22% (with only 1% of that due to drafts), while that for triple lowE is 18%. Not a huge difference."

    But I'll share my own experience, at the risk of being ridiculed for recounting a mere anecdote: When it's below 0 degrees F, it's uncomfortable to sit next to a double-glazed window. I have never yet been uncomfortable sitting next to a triple-glazed window.

  43. Mark Klein Gimme Shelter | | #43

    You are a good sport for putting your own personal comfort on the line in this discussion.

  44. Jesse Thompson | | #44

    I've seen condensation on double glazed Low-E modern Marvin Integrity and Anderson units (with no window coverings) in Portland Maine in the winter, and we don't even get that cold here (10 degrees is common, below 0 very rare).

  45. Riversong | | #45


    ASHRAE standard 55 incorporates two mathematical measures of comfort, based on the subjective responses of large samples of real people: PPD, or predicted percent or people who feel discomfort, and PMV, or predicted mean vote on a scale of +3 to -3 (+3 hot, +2 warm, +1 slightly warm, 0 neutral, –1 slightly cool, –2 cool, –3 cold).

    Human comfort standards for the interior environment are based on these metrics. This is how we know, for example, that most people like warm floors and cooler ceilings but not cold walls or windows. So human subjective testing has shown that there is not a large difference between double and triple glazed windows in terms of reported discomfort.

    The difference between window types that effects both human comfort and efficiency have less to do with the number of layers of glass as they do with the conductivity of the glass spacers and sash material, and the air-tightness of the frames and weatherstripping.

    These latter factors are also the ones that typically determine whether a window will experience condensation or even frost (in addition to the indoor RH - Jesse, you don't indicate this variable).

    Remember that what determines the temperature difference between the inside air and the inner surface of the window glass is merely a 1/8" layer of still air (R-0.68). So a double-glazed unit will have a delta-T from room air of about 21.6% of total in/out delta-T, while a triple-glazed unit will have a delta-T of approximately 13.6% of total. That means, with 70° inside and 0° outside, the center of glass temperatures would be 55° and 60°, respectively. And that means that for center of glass to reach dewpoint on a double-glazed window would require an interior RH of 60%.

    So the real problem is at glass edges and frames, and when dead air creates a colder or more humid environment at the window (one of the problems with deep window boxes, as they can pool colder air).

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