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Community and Q&A

Condensation on New Double-Hung Windows

user-6811390 | Posted in Energy Efficiency and Durability on

We built our home in 2017. I researched and researched windows and settled with Simonton which had an average rating of third on most websites. I did build the house as energy efficient as possible (within budget) including spray foam walls, roof deck and floor. I live in western NC. I used 2×4 walls with the spray foam. The Simonton windows are double hung. The problem is moisture covers about 2 inches of the bottom of each sash in the winter time. Oh, and a door that is almost completely full of windows. The windows in the door drip water every winter day to the point it’s already buckled the floor which needs replacing after 3 years. The Simonton windows has mold and mildew each year which has stained the felt and looks terrible. Anyway, I’ve had an HVAC guy tell me my house is too tight; no air flow. I understand air flow, but the energy-efficient appears to be creating mold on my windows. Anyway, I just wanted to get your thoughts before I spend 3k on “whatever” the HVAC guy told me to get. I think it may have been a whole-house dehumidifier.
Thank you in advance,

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  1. Expert Member


    Tight, well insulated houses need mechanical ventilation, both for indoor air quality and to control humidity. Whether that ventilation also needs some form of dehumidification depends on your climate. Often the ventilation alone is enough.

  2. onslow | | #2


    The door problem sounds so severe that one wonders if it is single pane glass. A top quality storm door that is set and sealed properly should help get the door back into better thermal range for your humidity levels. The double hung windows may suffer both from poor air sealing and if metal clad on the outside, excessive conduction of the cold via the seals between the panes. I had similar issues of air leakage at the bottoms of my original 1940 double hung units which could only be controlled by sealing the window perimeters with a clear peel-able sealant that resembled that goo mailers use to anchor AARP cards and such to inserts.

    Sadly, I replaced all the wood units with spiffy new metal clad double hungs from a major brand that proved almost as bad. The metal spacers between the panes were thermally awful. The exterior aluminum cladding contacted the IGU exactly at the spacer line. Together they conducted the cold very efficiently, which for Illinois winter conditions meant ice forming. The painted wood interior parts molded and rotted. It did get to zero pretty often when I lived there.

    I think (perhaps incorrectly) of NC having milder temperatures during the winter, which infers a pretty high interior humidity level. Even average double pane windows should hold a surface temp of 50F or better unless you are seeing temps near zero. If you don't have fish tanks galore or make cauldrons of soup, then the big question is where all the humidity is coming from. As Malcolm notes, moving air through the house is the quickest way to get levels down. You could try leaving a bath fan on overnight and see if there is any discernable difference. The energy penalty of throwing some of your interior air out may well be less than the energy penalty of running a dehumidifier for extended periods. Also cheaper on the front end.

    You might also be aggravating the window condensation with too well fitted cellular shades. My darkening cellular shade has to be propped up 3-4" off the sill or the window chills down to the point where condensation will form. Not 2" worth though. Do you perhaps have a crawl space issue feeding moisture into the home? Increasing the ventilation with a bath fan in that case might be counterproductive as you could be pulling up the humidity even more from a damp crawl space.

    I would recommend some detective work before throwing in a dehumidifier. My own house is at or above PGH values and only the cellular shades give me grief. We also dip down below zero pretty often, so look closely at your humidity sources before committing to treating symptoms.

  3. Expert Member
    NICK KEENAN | | #3

    I'd start off by getting an indoor thermometer with hydrometer so you can measure indoor humidity. The first question to answer is whether you have a humidity problem or a cold window problem. During heating season ventilation should be enough to keep the humidity at acceptable levels. It's when the weather is mild that humidity becomes harder to manage.

  4. user-6811390 | | #4

    Thank you all for your responses. To answer some questions you all posted, no crawl space just an unconditioned basement but it stays fairly mild with no heat or air movement. No cellular shades, we have the typical slat blinds. From what I'm gathering, I need to check/monitor the humidity and test with leaving our bath fans running overnight. I'm okay with running them to help eliminate the moisture on the windows and door. And Roger, no fish tanks or soup making here. :-) Unfortunately, we do hang clothes in the laundry room to dry by the ceiling fan - only items we don't want in the dryer. So, once again, thank you for the information provided. I will work towards understanding our humidity levels and attempt to post back my findings. Fortunately, it's getting spring time so no more cold nights for moisture buildup, but I will still monitor. Thanks again ALL!

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