Why this window gets wet first
Window condensation is a surface-temperature problem and a moisture problem at the same time. Indoor vapor reaches the glass, frame edge or reveal; if that surface is cold enough, local RH rises toward saturation and droplets can form.
Three inputs matter most
Indoor temperature and RH determine dew point. Outdoor temperature affects how cold the inside surface becomes. Glazing and frame performance determine how much of that outdoor cold reaches the room-facing surface.
Center glass is not always the coldest place
Edges, spacers, frames, reveals and thermal bridges can be colder than the center of the pane. Curtains and blinds can also reduce warm room air reaching the glass. That is why the Lab lets you override its glazing estimate with a measured cold-surface temperature.
At what outdoor temperature does the modeled window become risky?
The calculator now solves the preset window model backwards. With your current indoor temperature and RH held constant, it estimates the outdoor temperature where the selected interior surface would reach about 80% RH, and the colder breakpoint where it reaches saturation. This is useful for testing a coming cold snap, but it remains a model: real glass edges, frames and reveals may be colder than the preset surface.
What to compare
Test the same window under current RH and a lower target RH. Then compare a second room at the same outdoor temperature. This helps distinguish a whole-home moisture load from one unusually cold assembly.
Measurement protocol for one troublesome window
- Measure room temperature and RH away from the glass.
- Record outdoor temperature at the same time.
- If possible, measure the coldest accessible interior point: edge, frame or reveal, not only center glass.
- Repeat on a drier or colder day and save both snapshots.
The whole-home tool also reports the approximate room RH corresponding to 80% surface RH and saturation for the selected surface, so the result changes with both weather and construction.
Condensation is a moisture-control signal, not a diagnosis by itself. See the primary moisture guidance.