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Thread: Window Condensation 101

  1. #1
    DIY Senior Member chefwong's Avatar
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    Default Window Condensation 101

    This is probably a question for Dana, our resident insulation guru it seems.
    When there is condensation on the interior of a insulated windows, does it mean that the window has lost it's insulation...

    I bought a weekend project house and it's got a bunch of older insulated windows and a couple of brand new Kolbe windows in them.

    Moisture on windows I thought was part of the humidity created when cooking, etc and it's cold outside.
    What I'm noticing as I pay closer and *plan* is that the newer windows have nary any moisture on them but the olders ones have quite a bit.

    I still need to decide on a plan on opening and insulating these walls if I choose. These old plaster walls have seen better days. I'm just avoiding the wire and lath I suppose. The interior walls are great and are concrete hard. The walls facing the exterior are crumbling in some areas, and in others, I can see moisture or dew is getting to it. It's a brownstone, so I'm not sure if the moisture issues are just uninsulated walls and the temperamental difference to the plaster that is causing the moisture...

  2. #2
    Plumber jimbo's Avatar
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    If the seal on a double pain window fails, the window always fogs up internally. You may just have too much cold air AROUND the window, allowing the inside surface to be cold, hence condensation.

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    DIY Senior Member chefwong's Avatar
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    The condenstation I'm referring to is not inside the double pane, but just the glass facing the interior of the home

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    In the trades Dana's Avatar
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    The tighter the house, the higher the interior air dew points are, and the more likely you'll get window condensation. Newer-better low-E windows have higher interior temperatures than older double-panes, so the new ones might only be below the dew point of the interior air a few hours/day, whereas the older ones could dwell there for many hours, getting wet. Double-pane windows with 3/8" or less between them are less insulating than double panes with bigger gaps between the inner & outer glazing (and are even less insulating than tight single-pane + storm windows.)

    Window shades and curtains add to the problem. By putting even a modest amount of insulation between the window and the room, the temperature of the glass is lower, and unless those shades are air-tight, room air convects by the window depositing moisture on the cool pane.


    If the weepy-windows are in otherwise good shape and don't leak air, a tight-fitting exterior low-E storm window effectively gives them triple-pane performance at less than replacement-window cost, and the temperature of the interior face of the glass will be higher.


    As for the exterior walls, I'd need more information on the wall stackup to advise. If you're lucky it's a brick-veneer cavity wall with a sheathed structural studwall and a gap between the sheathing and the brick facing. If it's solid brick, no-cavity with the scratch coat plaster hanging on the interior brick your options are limited and potentially expensive.

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    DIY Senior Member chefwong's Avatar
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    The walls are wood stud framed.
    In the kitchen, I knocked down, reframed with sheetrock but this is more because I used tile for the backsplash...

    The walls definitely need attention. I'm just putting it off.....as the few patches I have done, scratch coat, brown coat, etc - to keep in line with the house, is a chore. Doing whole walls



    Quote Originally Posted by Dana View Post
    The tighter the house, the higher the interior air dew points are, and the more likely you'll get window condensation. Newer-better low-E windows have higher interior temperatures than older double-panes, so the new ones might only be below the dew point of the interior air a few hours/day, whereas the older ones could dwell there for many hours, getting wet. Double-pane windows with 3/8" or less between them are less insulating than double panes with bigger gaps between the inner & outer glazing (and are even less insulating than tight single-pane + storm windows.)

    Window shades and curtains add to the problem. By putting even a modest amount of insulation between the window and the room, the temperature of the glass is lower, and unless those shades are air-tight, room air convects by the window depositing moisture on the cool pane.


    If the weepy-windows are in otherwise good shape and don't leak air, a tight-fitting exterior low-E storm window effectively gives them triple-pane performance at less than replacement-window cost, and the temperature of the interior face of the glass will be higher.


    As for the exterior walls, I'd need more information on the wall stackup to advise. If you're lucky it's a brick-veneer cavity wall with a sheathed structural studwall and a gap between the sheathing and the brick facing. If it's solid brick, no-cavity with the scratch coat plaster hanging on the interior brick your options are limited and potentially expensive.

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    DIY Senior Member Runs with bison's Avatar
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    What is the humidity level and temperature in the home? (Measured, not the setpoints.) Are you seeing the condensation at a specific outdoor temperature? (As in "there is always some condensation at 20 degrees, but not at 25-30 or some such.)

    I've not seen condensation with double panes and I tend to leave the humidistat at the same set point even when it hits -14 F outside. Once the thermometer drops to around 0 F it struggles to hold 30% humidity indoors at 69 F. The only place I'm seeing condensation and frost is the only single pane window in the home...in an uninsulated/unconditioned space, and only then when it is bitter cold.

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    DIY Senior Member chefwong's Avatar
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    No Central AC here nor do have I own one of those digital thermometers that show humidity.
    The only room gut I have done is the kitchen but slowly expanding areas on the weekends.

    The exteriors walls - obvious location where they are....the majority of the walls when you touch them are cold to the touch.
    It's odd as there are 2 single pane windows and nary a condensation in sight when cold.
    The majority of the older double pane do have more and less. Less sometimes is *condensate* over the entire picture window but generally we're talking cold temps and condensate on the very bottom pane of the window. In looking at the plaster above the baseboards ....which alot of them are somewhat soft right between the windowwill and baseboards, I suspect over the years, the water condensate has dripped and softned her up.

  8. #8
    In the trades Dana's Avatar
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    If there is at least an inch of air-space between the exterior masonry and the studwall sheathing you can retrofit blown-cellulose or high density (1.8lb, not 1.0lb) Spider or Optima fiberglass in there without having to fully gut the wall, and it will outperform any batt solution. Making sure the gap has at least some vent holes top & bottom is key- in summer the moisture drives of sun on dew/rain wetted brick are quite intense and could cause summertime condensation on the sheet-rock/plaster inside the cavity, which is often a larger factor for deteriorating plaster than wintertime issues in a D.C. climate.

    If the studs are right up against the masonry with no wood-sheathing & no gap a spray-foam for foam/fiber combi solution can get you there without putting the exterior edge of the studs at risk, but without a ventilated gap you can't use fiber (of any type) without putting at least some vapor retarder between the masonry and the sheetrock/plaster. An inch of closed cell on masonry (0.8-1.5 perms), filling the rest with high density fiber works, or a mid-density all-foam solution that is between 0.75-5 perms at the depth sprayed.

    Dry wintertime interior air is a symptom of high ventilation rates (or very low occupancy rates- a one person household with 9-5 job + a social life doesn't emit much water into the house). It's generally better to tighten up the house to where it usually stays above 30% RH using minimal active ventilation rather than run humidifiers. The exceptions would be if there are smokers in the house, or folks with a propensity for using noxious cleaning agents, etc, where higher ventilation rates are mandatory for indoor air quality reasons. In a D.C. climate this won't take nearly as tight a house as it would in the upper midwest.

    The dew point of 30%RH/70F air is ~37F, and it takes a fairly cold day for the interior pane of any decent double-glazed window to hit that temp unless you're using interior blinds curtains. But at 40% RH/7F the dew point is ~44-45F, and even a U0.34 window doesn't need 0F outside to see some condensation.

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