Two companies told you the same thing and both of them answered a different question than the one you asked. There is a version of “condensation means the windows are doing their job” that is completely true. It applies to water on the outside of the glass. On a good low E unit, so little heat escapes through the window that the outer pane never gets warmed by the house, and on a clear night it drops below the dew point of the outdoor air and fogs up. Homeowners call the installer in a panic and the installer, correctly, tells them it is proof the unit is working.
Water on the inside is the opposite finding. It means the interior surface of that glass got colder than the dew point of the air in your room, and interior glass only gets that cold when it is losing heat fast. Same phenomenon, different side, reversed meaning. I would like to think both companies knew that and were sloppy rather than the alternative.
Three Places It Shows Up, Three Different Verdicts
The location is the whole diagnosis, so be precise about it.
Outside The Glass
The unit is performing. Nothing to do, nothing to buy, and this is the only case where the line you were given is correct.
Between The Panes
The perimeter seal has failed and the fill gas is gone. That sealed unit is finished and this one is a genuine replacement.
Inside The Glass, Running Down To The Frame
This is yours, and the pattern says more than the location alone. The water is heaviest along the bottom and streaking toward the frame. That is edge of glass condensation, and it shows there first because the perimeter of a sealed unit is the coldest part of it.
Older units used aluminum spacer bars, and aluminum conducts cold straight around the edge of the glass. Warm edge spacers use stainless, tin coated steel or foam polymer for exactly that reason. If the bottom edge is soaking while the center of the pane stays clearer, you are looking at the spacer.
The Numbers Your Installers Left Out
This is where it stops being a matter of opinion.
Whether a window condenses is a race between two temperatures. The interior surface of the glass on one side, the dew point of your room air on the other. Windows lose that race in most houses because they are the worst insulated surface in any exterior wall, so they run colder than everything else in the room.
What changes dramatically is by how much. Engineering figures on this lay it out plainly:
- Outside at 30°F, a single glazed window starts running with water once indoor humidity passes roughly 32%.
- Same night, same 30°F, a double glazed unit holds out to about 58%.
- Outside at 10°F, single glazing gives up above 17% indoor humidity.
- Same 10°F, triple glazing needs the room over 62% before anything forms.
Sit with that 32% for a second. That is a normal, healthy, mildly dry house, sitting right inside the 30 to 40% band recommended for cold weather. A poor window will still stream at that level, all night, every cold night, no matter what you do about moisture.
The honest framing of your situation is that the glass sets a ceiling on how much humidity your house is allowed to carry, and yours has set it very low.
Your Old House Already Ran The Experiment
The strongest piece of evidence in your whole message is the one both companies walked past.
Same family. Same breathing. Same cooking, same showers, same laundry. Different building, and in the newer one this never happened. You ran a controlled trial with the occupants held constant and the building swapped, and the result came back pointing at the building.
That is worth more than any humidity reading, because it removes the explanation everyone reaches for first. It is not your lifestyle. Something about the newer house kept the interior glass warmer, and newer usually means some combination of a lower U factor unit, a warm edge spacer, frames installed closer to the insulated line of the wall, and often some background ventilation the old house does not have.
The neighbors’ houses looking dry is the same evidence a second time.
Two Fixes You Are Already Running That Cannot Work
The Dehumidifier
It is probably doing very little on the nights you need it. Residential compressor dehumidifiers are not built to run cold. Most are designed for rooms above roughly 60 to 65°F, and below that the coil ices and the unit either cuts the compressor or spends its time defrosting itself. On a cold night in a cool house, a compressor unit is close to an expensive fan.
Desiccant machines are the ones that keep pulling water down to about 33°F, which is why they end up in unheated garages and cold stores. If yours is a compressor model sitting in the hallway while the wet windows are upstairs, it is fighting the wrong room at the wrong temperature.
The Vent Latches
They are open at the wrong time of day. The moisture load in a house is generated overnight, in the rooms people sleep in. A resting adult puts out somewhere around 30 to 70 grams of water an hour through breathing and skin, and across 8 hours behind a closed door that adds up with nowhere to go.
By the time the latches go on at 9am, the water has been on the glass for 3 hours and is evaporating back into the room. Venting through the day dries the house at the hour it needed it least.
Pull The Curtains Off The Glass
Costs nothing to test. Curtains and blinds pulled tight across a window cut off the room air that was warming the pane, so the glass runs colder than it otherwise would and condenses more.
If your worst windows are also the ones with heavy drapes hanging right against them, leave a gap at the bottom for a week and watch whether that window starts behaving differently from the others.
What To Ask For If You Get Quotes
There is a specification for exactly this and almost no salesperson volunteers it. Condensation Resistance is an NFRC rating on a scale of 1 to 100, higher meaning more resistant, driven mostly by the U factor and the spacer. Consumer Reports covers where it sits on the label. It is optional, so plenty of manufacturers skip publishing it, and NFRC is careful to say the number is a comparison tool rather than a prediction of whether a given window fogs at a given humidity.
Ask anyway. A company that can produce a CR figure and talk about the spacer understands what you are describing. A company that tells you wet interior glass is a sign of good windows has told you what it knows.
Before spending anything, put a cheap hygrometer in the worst room and read it at 6am rather than in the evening. If that reading comes back in the 30s and the glass is still streaming, the humidity was never your problem and you have the answer ready for the next salesperson who walks in.