the furnace came on last week and now there is water sitting in the bottom corner of your windows every morning. this comes up every fall, almost always from somebody who has already started looking at replacement windows.
i clean houses in western new york for a living. i have wiped that corner in a lot of kitchens. the windows are almost never the problem.
and i am not the best authority on that. a window company is. andersen publishes a whole brochure about condensation, and andersen sells windows, which means they have every commercial reason on earth to let you believe your windows are bad. their brochure says the opposite. in their words: “windows do not cause condensation and, at the same time, can’t eliminate condensation,” and interior condensation “is really an indication of excess humidity in the home.”
when the people selling the thing tell you not to buy the thing, believe them.
what the water actually is
condensation shows up, same brochure, “when warm, moist air comes into contact with cooler surfaces.” in october the glass is the coldest surface in the room by a wide margin. it is not leaking. it is just the first place in your house cold enough to pull water back out of your air.
which makes it a gauge. you own a humidity meter and you have been wiping it off every morning without reading it.
the epa puts the line at 60 percent relative humidity for mold and says indoor humidity “should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible.” university of minnesota extension is tighter for winter: 25 percent in winter up to 50 percent in summer, and “when the relative humidity is more than 50%, moisture problems may occur.”
here is the part that nobody mentions. the number is not fixed. andersen’s own table slides it down as it gets colder outside. up to 40 percent when it is 20 to 40 degrees out. 35 percent at 10 to 20. 30 percent at 0 to 10. 25 percent below zero.
so the humidity that was completely fine in september is wrong in january, and you did not change anything. the outside did.
where all that water is coming from
you, mostly. extension’s list is bathing, cooking, dishwashing and breathing, plus houseplants, firewood stored inside, and humidifiers. andersen puts numbers on two of them: “cooking three meals a day adds eight or ten cups of water into the air,” and every shower adds about another cup.
eight cups of water a day, poured invisibly into a house you have sealed up for winter. it has to land somewhere, and it lands on the coldest thing.
the one that catches people out is laundry. extension names unvented appliances specifically. if your dryer vent is packed, or it dumps somewhere that is not outdoors, your dryer is a humidifier that also dries clothes. i have written about why the lint trap is not the dryer vent, and the wet windows are the other half of that bill.
the thing you plugged in three weeks ago
the humidifier. cold season starts, somebody in the house gets a cough, the humidifier goes on, and then it simply never goes off again. both the epa and extension name humidifiers in their own lists of moisture sources, which is a strange thing to read in october, when plenty of the houses i walk into have one going.
i am not telling you to get rid of it. there are real reasons to run one and i am not going to pretend otherwise. but a humidifier is clean for about three days, and it is the one moisture source in your house with a switch on it.
so use the windows. if the glass is running by seven in the morning, the humidifier is winning. turn it down a setting, move it out of the smallest room, or put it on a timer so it is not going all night.
if you are buying one anyway, a cool-mist humidifier is the shape i would pick, because there is no hot water in it and the tank actually opens wide enough to scrub. flagging that plainly: that is a case about a feature set, not a brand i have lived with.
how i deal with the glass itself
pull the water off before you try to clean anything. streaks are not a cleaner problem, they are a drying problem, and a wet window in november is that same problem with the difficulty turned up.
on a window that streams every single morning, a squeegee with a hook on it is the entire job. one pull down, one wipe along the bottom, ten seconds, no bottle involved. i have not used this exact one. the hook is the reason i would choose it, because a squeegee that hangs on the window frame gets used every day and one that lives in a cupboard gets used in april.
for the real clean, the stuff i buy for work: aerosol glass cleaner and microfiber cloths out of the auto aisle. the auto cloths are better than the ones in the cleaning aisle and nobody can explain to me why. a sheet of heavy paper towel for the last pass on the glass if you are fussy about it, and i am.
then the sill and the sash, which is the part everybody skips and the only part that actually matters. a multi-surface cleaner and a dry cloth, and get into the corner where the water pools.
the part that stops being cosmetic
epa again: “mold is frequently found on walls in cold corners behind furniture where condensation forms.” a wooden sill that sits wet from november to march goes dark, then chalky, then soft. by then you are not cleaning, you are replacing trim.
if there is already a black spot on a painted sill, the answer is not a specialty product. it is a dilute bleach solution and a cloth, and i am deliberately not putting a link on that, because it is the most boring bottle in the store and you almost certainly have it under the sink already. wear nitrile gloves for it. the listing defaults to small, so change the size before you check out. i am recommending the glove, not that particular box, which is a category i would buy and not a brand i have tested against others.
bleach and acid never meet. not in a bucket, not on a rag, not one after the other on the same surface. that goes for any acidic bathroom cleaner you own, and it is the one rule i will not let anybody fudge.
and if the black is down in the caulk line rather than sitting on the surface, stop scrubbing right now. you cannot scrub mold out of a caulk line, and you will spend an hour proving it.
what i would actually do this week
run the bathroom fan and leave it running until the mirror is clear, not until you are out of the shower. run the range hood when you cook, every time, including for the pasta water.
open a window a crack for a few minutes a day. i know how that sounds in a cold month. it is extension’s own advice: open windows slightly “so warm, moist air can leave and cold, dry air can enter.” you are not heating the outdoors, you are swapping a few cubic feet of wet air for dry air your furnace can bring up again without much effort.
get a humidity gauge. extension says to measure it rather than guess, and they are right. i do not have one to point you at and i am not going to send you to a random listing just so there is a link in this paragraph.
then watch the glass for a week. it will tell you whether any of it worked before your heating bill does.
your windows are fine. your air is wet. that is a far easier problem to fix than the one you were about to pay for.
some of the product links above are affiliate links, which means i may earn a commission if you buy through them at no extra cost to you. #ad #WalmartPartner. four of the seven products here are things i buy for work and i have said which. the other three are framed as a category call with the reason attached, in the sentence that links them.
the professional cleaner’s checklist is a free two page printable, and it lives in the resource library with everything else i have made.
put your email in and i will send you the link.
