· by Michael Blair, Nashville's Windows & Doors
Condensation on the room side of the glass is not a failed window. It is moisture in the house reaching a surface cold enough to condense it, and the window is simply the coldest surface available.
Where does window condensation come from?
Cooking, showering, laundry and breathing, plus in many Nashville houses a vented crawl space contributing to the whole building.
A tightly built or recently air-sealed house holds that moisture rather than leaking it away, which is why condensation sometimes appears after an efficiency upgrade.
What the level should be
Roughly 30 to 40 percent relative humidity in winter suits most houses. Above that, condensation on glass becomes likely on cold nights, particularly on north elevations.
A hygrometer costs very little and turns the question from an argument into a number.
How do you reduce condensation on windows?
Ventilate at source: bathroom fans that run long enough and vent outside, a range hood ducted outside rather than recirculating, and a dryer vented properly.
Then look at the crawl space, which in Middle Tennessee is frequently the largest single moisture contributor to the house above it.
- Run bathroom fans twenty minutes after use
- Duct the range hood outside rather than recirculating
- Check the dryer vent is clear and terminates outside
- Seal and dehumidify the crawl space
- Keep curtains from trapping cold air against glass
When it is the window
Condensation between the panes is a failed seal and is a window problem. Condensation on the frame rather than the glass suggests a thermally poor frame, common on older aluminum.
Persistent condensation in one room only usually indicates a local moisture source or poor air circulation rather than a whole-house humidity issue.
FAQ
Condensation on the Inside of Your Windows — questions
Do new windows stop condensation?
They reduce it, because a warmer inner glass surface condenses less readily. They do not remove the cause. A house with high indoor humidity will still condense on the next-coldest surface, which may be a window frame, an exterior wall corner or inside a closet on an outside wall.
Is window condensation harmful?
Water running onto a wooden sill repeatedly will eventually damage the finish and then the timber, and persistent damp encourages mold at the frame. Occasional condensation on a very cold morning is normal; water present most mornings through winter is telling you the humidity is too high.
What humidity should my house be in winter?
Around 30 to 40 percent is a reasonable target in a Middle Tennessee winter — comfortable, protective of wood floors and furniture, and generally below the point where windows stream. If you need to run above that for comfort, expect condensation on the coldest glass in the house.
Why do the windows in only one room get condensation?
Usually a local moisture source or poor air circulation rather than a whole-house humidity problem. A bathroom fan that is not run long enough or does not vent outside is a common one. North-facing rooms run colder, and curtains drawn tight against the glass trap cold air there, so condensation shows up first in those rooms.