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Kathleen J | Posted in Energy Efficiency and Durability on

We have a new construction home, less than one year old. Windows are the type that slide inward to clean, etc, vinyl, double pane. We have had temps drop below zero a few days in the past couple weeks. Whenever it gets below 0 degrees, we get condensation. Not the entire window, but along the bottom edge of the interior window, the space in between the bottom fixed window and the top slider, and in just a few windows it will frost in the bottom corner of the frame. Have checked humidty, and we run about 25%. Also have a call into the window rep, but haven’t heard back yet. Asked the window guy who installed, stated he used fiberglass insulation around the windows, which doesn’t make me particularly happy. The basement windows, however, are insulated with foam (trim not finished yet), and those also condensate the same way, so not sure if that’s it.
What other avenues would you explore?

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  1. Brian P | | #1

    Your windows may not be installed perfectly, but I would say this is to be expected with double pane windows in the type of weather you describe (that we've been having in the Northeast recently). As long as you're keeping your humidity in check, which it sounds like you are, just wipe them down in the AM.

    We have nice Alpen fiberglass frame windows in our house in NH. We'll see some condensation at the very bottom the interior glass when it gets around 0F and some frozen condensation below 0F. It's partially because the windows are installed on the exterior of a 12" double stud wall.

    I think a house would need higher end triple pane windows to avoid any condensation in this weather.

  2. Kathleen J | | #2

    Thank you, Brian. Don’t remember a winter we have been this cold. Not too typical and we should be back to our normal 20-30 degrees bu this weekend. Hopefully.

  3. User avater
    Dana Dorsett | | #3

    With metal spacers & edge seals on the insulated glass units you'll often get condensation along the edges during cold/very-cold weather due to the thermal bridging of the metal.

    For WAY too much information on the topic of edge seal types and window performance see:

    Insulating around the window with fiberglass is fine as long as they pack it tight enough, the skinny cavities are completely full with no gaps, and they air seal it. (Backer rod is OK if done right, but latex foam or low expansion polyurethane sealants are better.)

    Many window installers are under the impression that one has to be sure to not pack the fiberglass to tight, but this is in error- the fiberglass is more air retardent and has a higher R/inch when packed a bit tighter than it would be when installed in a standard stud bay. It would almost have to be hammered into place to be packed "too tight". It just has to fill all the available space, and have a decent density with some spring or push-back to it's loft.

  4. Kathleen J | | #4

    Dana, would you recommend adding some type of additional sealant to the fiberglass? Almost certain he didn’t pack it tight enough. Not sure if it would help stop the condensation since metal components will always be a part of the window. You’re right, didn’t think about that until you mentioned it.

  5. User avater
    Dana Dorsett | | #5

    Fiberglass is not an air barrier, and needs an air barrier on at least one, but preferably both sides of the assembly. If it's easy to pop the trim and get in there to re-pack the fiberglass packing it in a bit with appropriately sized backer rod never hurts.

    If there are bigger gaps or you want to yard out the fiberglass and use something else, latex can-foam sealant (eg DAPtex Plus) trimmed flush before re-installing the trim has the least chance of distorting vinyl windows.

  6. Kathleen J | | #6

    It’s not easy to pop the trim. Hard to explain, it would likely cause damage to the wall. Have you ever heard of companies that inject foam? Trying to read up on it now.

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

    You're focusing on the wrong issue. Condensation on the interior side of windows when the temperature drops below zero has nothing to do with air leakage at the perimeter of the window. It's just physics. Your interior air is warm and humid, and the window panes are cold. The result is condensation.

    For more information on the phenomenon, see Rating Windows for Condensation Resistance.

    There are only two ways to reduce this kind of condensation: either (1) lower the indoor relative humidity (not a good solution in your case), or (2) raise the temperature of the glass (in other words, switch from double glazing to triple glazing -- a switch which makes the interior of the glass warmer).

    It's cold, so there is ice in the corners of your windows. That's what happens. It usually isn't a big deal. When I went to the post office yesterday morning, ice covered the entire interior of the doorknob of the post office door, and about 1/3 of the glass on the interior of the window lite, and there was ice along the entire vertical stile of the door nearest the latch. It's cold out.

    My neighbor yesterday was joking about the ice building up on the windows in her children's bedrooms. She said, "It's good when the ice finally gets thick enough that it blocks the air leakage."

  8. Kathleen J | | #8

    Thank you Martin. Was mostly concerned with the frost on the bottom corner of the vinyl frame. Do you feel even in that space, not on the glass, is going to be typical for this drastic cold weather? Will have to research adding more heat also.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Different windows have different types of frames; certainly an expensive Passivhaus-certified window will have a frame with a lower U-factor than a run-of-the-mill vinyl window. That said, a little ice on a vinyl window frame won't do any damage, as long as you don't get puddles of melted condensation on a wood window stool for weeks at a time.

    In my house, my run-of-the-mill wood windows (Weather Shield) have recently had some ice on the frames, especially in the coldest rooms of the house.

    If you don't want to look at the ice, you can try turning up your thermostat. Also, make sure you don't have any curtains or blinds, since the use of curtains or blinds increases the chance of condensation.

  10. Andrew G | | #10

    I have a friend at work in Central Maine who has been having this issue on a new build also. Vinyl double pane windows. I think it is unfortunate that we are building new houses that have issues like this. I wish the prices of triple pane windows with Passivhaus type frames could become more affordable.

    I have an older house with replacement windows. Ice has been forming on them also during the nights when it has been -20. It used to be worse before I installed a bathroom fan to control moisture. Make sure all of your exhaust vents are air sealed and actually vent to the outside.

    I find that keeping the temps warmer in the house cuts down on the condensation on the windows because of increased relative humidity and this raises the temperature of the glass. Also as long as there is caulk between the window and the window frame the water should not be able to drip down into the drywall.

  11. Malcolm Taylor | | #11


    It may not be the cause, but I'm struck by your description that it occurs below the top slider. Sliders don't seal anywhere as tightly as awnings or casements. It my be air leaking between the operable portion and the frame.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Malcolm makes a good point. With some window types, it's possible to reduce air leakage by improving the weatherstripping or installing a shim under the sash lock. On other window types, such modifications are more difficult.

  13. Carrie Sturrock | | #13

    Hi Kathleen,
    Full disclosure: I work for a small company in Portland, Oregon that makes interior window inserts to help with condensation. We see what you describe a lot - even with new double-pane windows. Condensation occurs when the interior surface of your windows falls below the dewpoint and the moist interior air near it condenses onto the glass. Your case is interesting because the relative humidity of your house is below where the U.S. Energy Star program says is optimal - which is 30-50 percent. You wouldn't want to lower it anymore. One cost-effective solution would be to create a near airtight seal over your windows by blow drying plastic wrap over the interior frame. This will keep the warmer, more humid interior air from condensing on the window pane. You could also use window inserts.

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