· Michael Blair
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 the moisture comes 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.
Reducing it
Ventilate at source: bathroom fans that run long enough and vent outside, a cooker 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 cooker 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 aluminium.
Persistent condensation in one room only usually indicates a local moisture source or poor air circulation rather than a whole-house humidity issue.