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Frost on H-windows

shawncal | Posted in Energy, Heating & Insulation on January 14, 2009 04:41am

We are experiencing heavy frost buildup on all the windows in our new guesthouse whenever the temps drop much below 0F (it is currently -30F here in northern MN).  We built this structure very well (SPF envelope, extremely airtight and highly insulated).  The windows are all H-windows, triple paned, the best we could afford.  We are heating the building with electric in-floor hydronic and ventilating with a high-efficiency Venmar HRV.  The humidity sits at about 30%.  The house has been blower door tested twice and is super tight (like .05CFM/ft2). 

Next door to this is our old farmhouse, which we live in.  It is 100+ years old, not very well insulated and relatively leaky.  Heat is wood stove.  windows are relatively new, but lower quality double-paned, double hung style- they don’t even begin to compare to the H-windows in the guesthouse.  I can feel air coming in around these windows, yet there is not a bit of frost buildup on the coldest of cold days.  Humidity is the same as the guesthouse, and we have no ventilation system other than the natural draft from the leaks and what the wood stove is pulling into the building.

Should I expect window frost in a super tight house, or is there another explanation here?  Even if i run the HRV on high, continuously, it doesn’t seem to help…

Shawn
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Replies

  1. ted | Jan 14, 2009 04:57pm | #1

    When I designed and built my parents house in the Twin Cities back in 1990 we spec'd out H-Window. In hindsight, if it hadn't been for the wide range of cladding colors available back then I probably would have chosen something else.
    During the first winter the windows frosted up like you mention. The house was built tight just as you mention and has an HRV. The first year I just thought it was probably due to excess moisture while the house was drying out. But as it happened every winter thereafter I concluded there must be something wrong with the gasketing around the glazing or something. After the first year we called in the factory reps but they insisted nothing was wrong and that the house wasn't getting enough fresh air. We went ahead and boosted the air intake to no avail. In retrospect I should have been more persistent about pursuing the issue the following year. They just kept coming back at me with the same old B.S. about how their windows are the tightest in the industry. Which I definitely think is balony. If I take my hand up near any of the several windows I can literally feel a draft coming off the window on a very cold day.
    Four years ago I finally got tired of helping my parents put up 3M plastic every year and had them purchase and install an interior type of storm window. I think it cost them over $4k to do all the windows in their house but it definitely put the brakes on freezing condensation on the windows.
    I'd be interested in following this thread to see if ant other H-Window users have had similar issues.

  2. DanH | Jan 14, 2009 06:37pm | #2

    30% humidity is too high when the temps are -30F, even with triple pane glass. You need to get it below 25%, and even then a little frost is likely. (Riversong is going to disagree with me, but he doesn't live in Minnesota.)

    You don't see it in the old house because with the cold air intrusion the humidity near the windows is nowhere near 30%. In fact, you'd have to humidify pretty aggressively to get a structure that leaky up to 30% in subzero weather.

    Also, any new structure will be abnormally humid for the first year or two.

    God is REAL, unless explicitly declared INTEGER
    1. Riversong | Jan 16, 2009 09:43pm | #21

      30% humidity is too high when the temps are -30F, even with triple pane glass. You need to get it below 25%, and even then a little frost is likely. (Riversong is going to disagree with me, but he doesn't live in Minnesota.)

      Maybe this ain't MN, but we had temperatures last night ranging from -20° to -42°.

      Last night at my cabin was perhaps a mild -15°, and with 28% indoor RH (at 60°) I had no more than 3/4" of frost on the bottom of one of my single-glazed windows and almost none on the other.

      I guess the laws of physics are different in different parts of the country. Doesn't the Midwest have a "mystery spot" where water flows uphill? 

      Riversong HouseWright

      Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

      1. DanH | Jan 16, 2009 10:01pm | #22

        "and almost none"
        The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

        1. Riversong | Jan 16, 2009 11:09pm | #24

          "and almost none"

          And your cryptic point is.............................? 

          Riversong HouseWright

          Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

  3. User avater
    BillHartmann | Jan 14, 2009 09:45pm | #3

    H-windows? Is that Hurd?

    "The humidity sits at about 30%" At what temperature?

    And do you have any covering or window treatments that you keep closed?

    .
    William the Geezer, the sequel to Billy the Kid - Shoe
  4. Riversong | Jan 15, 2009 01:27am | #4

    Should I expect window frost in a super tight house, or is there another explanation here?  Even if i run the HRV on high, continuously, it doesn't seem to help...

    First, I would suggest getting a good quality digital hygrometer, as I doubt that your readings are accurate. With 30% RH in the new house you should not be seeing that kind of frost on good quality triple-glazed windows, and with 30% RH in the old house you should be seing frost on all the windows.

    Second, as someone else suggested, if you have heavy curtains or shades on the new windows, that will keep them significantly colder and more susceptible to condensation and frost. Without convective heat, there is nothing to help warm the air next to the inner glazing on cold days, which would also reduce condensation.

    What is the whole house air exhange rate of the HRV? Are you getting a minimum of 0.35 ACH?

    Any new construction is going to have high indoor humidity for the first year, as the building materials can contain thousands of gallons of excess moisture that needs to be eliminated.

    If those H-windows have a whole unit U-value of 0.25 (as their website suggests), they're not particularly efficient for triple glazed windows.

     
    Riversong HouseWright
    Design *  * Build *  * Renovate *  * Consult
    Solar & Super-Insulated Healthy Homes
    1. shawncal | Jan 15, 2009 06:48am | #6

      To answer some questions-

      I am using a digital hygrometer, so the readings should be accurate.  The humidity has been around 30% and the temp at 62-65F.  Today, I ran the HRV on high continuously, which should provide about 0.8 ACH.  This brought the humidity to 27%, but the windows frosted up once it got dark and outside temp started dropping (its -20F already...). 

      There are no interior coverings on any of the windows.

      I do not have all of the ducting installed for the HRV, so some areas of the building (although it is an open floor plan) are not getting as much airflow as others.  All of the windows are frosting, but those in the remote areas are definitely worse, so the convective heat theory may explain my problem.

      I guess this leads me to two questions:

      1) What is the appropriate humidity levels we should be trying to achieve in this climate?

      2) If the H-windows are not that great (apparently, i didn't do enough homework on this one, and regret having spent so much $$ on these), what are the better choices out there?

      thanks all!Shawn

      1. DanH | Jan 15, 2009 07:02am | #7

        25% during this cold wave. 30% is probably fine for temps above zero. 35% above 20F. But even at 25% you're apt to get some frost in the bottom corners. How much frost are you getting?
        God is REAL, unless explicitly declared INTEGER

        1. shawncal | Jan 15, 2009 03:05pm | #13

          Dan-

          Just took this photo, at 29%RH, 63F inside temp, -22F outside...why did we decide to build a house at the NORTH POLE!?

           Shawn

          1. ted | Jan 15, 2009 05:48pm | #15

            Hey those windows look a lot better than the ice build up problem I had at my parents house.
            I did a google on H-Window frost problems. Nothing much came up. But there was a link on a legal forum pertaining to the original H-Window company going belly up and selling out to another company. And the previous company not being responsible for a bunch of crummy windows with faulty seals.

          2. ted | Jan 15, 2009 05:53pm | #16

            I also notice the frost buildup at the seal between the sash and the frame. This is exactly like one of the problems my parents house had. In some cases large portions of the sash around the seal would frost up like that. Run your hand around the seals. I'm sure you can feel the cold draft coming off the window, especially at the gaskets.
            Someday I plan to go over there and take one of their windows apart and try replacing some of the seals to see if that helps.

          3. DanH | Jan 15, 2009 05:53pm | #17

            That's about what I'd expect for those conditions.
            God is REAL, unless explicitly declared INTEGER

          4. ted | Jan 15, 2009 07:35pm | #18

            Of newer windows I think that is pretty bad. I've installed windows from all the major companies and many from the not so major ones. H-Window is the only one I've seen that has frost buildup on the sash itself.

          5. DanH | Jan 15, 2009 07:46pm | #19

            Maybe it could be better, but the example shown was minor, and nothing to worry about.
            The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

          6. Riversong | Jan 15, 2009 09:11pm | #20

            The frost at the frame edge is due to poor air sealing. The rest of the frost is due to conductivity at the glass spacer (poor thermal break).

            A better window would not have these problems. The Canadian pul-truded fiberglass windows, like Thermotech, are about the best on the market.

            But that is minimal frosting and shouldn't be a problem if all the wood is well-sealed. I would not drop the indoor RH any lower than 30% or run the HRV more than necessary, if you want to maintain efficiency and a healthy indoor environment.

            ASHRAE recommends indoor RH between 30% and 60%, but it should not be higher than 40% in the winter and 30% is safer for the house. 

            Riversong HouseWright

            Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

          7. peteshlagor | Jan 16, 2009 10:06pm | #23

            That same window - would it perform better if it had been properly finished?

             

          8. Riversong | Jan 16, 2009 11:12pm | #25

            That same window - would it perform better if it had been properly finished?

            I don't know what you mean by "properly finished".

            The frosting at the glass edge is due to overly conductive spacers, and the frost at the sash edge is due to poorly-functioning weatherstripping.

            It's possible that the weatherstrip leakage could be due to poor installation (out of square or out of plane), or even getting wood finish (paint, urethane, e.g.) on the rubber seals. 

            Riversong HouseWright

            Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

          9. peteshlagor | Jan 16, 2009 11:29pm | #26

            By "properly finished,"  I mean the entire window has been disassembled, all wood surfaces have been preped, primed, whatever and then coated with an high quality coating.  In which case, the weatherstripping becomes tighter and the small gaps which allow some air passage become significantly lessened.

            Howevr, I rarely see homes where this has been done.  It's usually quick and dirty spray of the exposed surfaces and they're gone.

             

          10. user-455887 | Jan 20, 2009 05:37am | #27

            I'd like to, respectfully, suggest another possible causes of frost on the sash

            If you have a moment, look at the cross sectional detail of the frame;

            http://www.hwindow.com/pages/details.html

            The frost shows up first on the lower portion of the frame.  I suspect that this is because there is natural convection in the largish vertical hollow cavity between the sash and frame.  This convection means the bottom of the hollow is colder than the top of the hollow - so the frame is coldest where the jamb meets the sill.

            Vinyl and fiberglass frames with large single hollow sections can also experience a similar condensation/frost pattern.  I suspect the fact that in this case the hollow is substantially alum'n isn't helping matters....

          11. Riversong | Jan 20, 2009 06:37am | #28

            I'd like to, respectfully, suggest another possible causes of frost on the sash

            The frost shows up first on the lower portion of the frame.  I suspect that this is because there is natural convection in the largish vertical hollow cavity between the sash and frame. 

            Possible, but unlikely. That channel is in the exterior frame. The frost in the picture is on the interior solid wood frame right next to the weather seal. Almost certainly air leakage at the seal. 

            Riversong HouseWright

            Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

      2. MHolladay | Jan 15, 2009 01:20pm | #11

        Shawn,
        I've never seen an H-window. I recently installed Thermotech Windows in my house (fiberglass-framed casement windows, argon-filled triple glazing, double low-e, U-0.17) and I've very happy with them. It's -14° right now and of course there is no condensation or frost. You can sit beside them and you don't get that cold feeling you get sitting beside double-glazed windows.

        1. mickeyhickey | Jan 15, 2009 01:54pm | #12

          I have double pane windows upstairs and down. Upstairs there are honeycomb cell shades about 3/4 inch in from the glass, downstairs there are single sheet shades 2 1/2 inches in. Upstairs we get condensation on the bottom 1/4 of each pane when temperatures approach -4 F (-20 C). Downstairs we never get condensation. The insulation effect of the shades upstairs and the closeness to the glass means that the airflow across the glass upstairs is so slow that the glass cools to below the dewpoint which appears as condensation on the glass where the glass is coldest. Cold air flows down the glass and is coolest at or near the bottom. I have never had problems with condensation downstairs because the gap is wide and the shades have little insulating value. There are a number of cures including lower the RH by increasing air changes, raise the shades to 1/4 or 1/3 from the bottom, increase the gap between shades and glass, use inefficient shades, raise the temperature to 75 F. I have gone as far as spending the winter in Argentina but that is a little extreme.

  5. oberon476 | Jan 15, 2009 05:45am | #5

    I would agree that with everything being up to specification, triples with LowE and argon/krypton should never frost, not even at -30. 

    I would also ask RH at what temperature?

     

     

     



    Edited 1/14/2009 9:53 pm ET by Oberon

  6. RedfordHenry | Jan 15, 2009 07:02am | #8

    What's an H window?  I'm not familiar with that term.

    1. DanH | Jan 15, 2009 07:04am | #9

      It's a brand.
      God is REAL, unless explicitly declared INTEGER

  7. Clewless1 | Jan 15, 2009 09:52am | #10

    What are "H" windows??

    It is a super tight house ... hence ... you can get frost on any window if it is cold enough outside. Condensation on windows is a simple balance of humidity and temperature. You've done everything to trap moisture. You could calc the inside surface temp of the window and determine if your 30% RH will condense at that temp. Obviously it does. The reason your other house doesn't do it is because it ventilates so well that it doesn't allow the humidity to rise high enough. It's simple physics  albeit can be difficult to sleuth out the 'problem' ... intuitively, it doesn't make sense.

     

  8. User avater
    jocobe | Jan 15, 2009 03:43pm | #14

    May be of interest......

    http://forums.taunton.com/tp-breaktime/messages?msg=113887.1

    View Image

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