Direct answer
Start by identifying exactly where the moisture is.
Water on the room-side surface of the glass is usually condensation caused by humid indoor air meeting a cold window. Fog or moisture trapped between panes points to a failed insulated-glass seal. A cold draft suggests air leakage. Water appearing around the frame during rain may be a flashing or exterior-drainage problem.
Minnesota winters make this diagnosis especially important because very cold outdoor temperatures lower interior glass temperatures and increase the chance of condensation even on relatively efficient windows.
Interior condensation: usually an indoor humidity and surface-temperature issue
ENERGY STAR explains that interior condensation forms when the window surface temperature drops below the dew point of the indoor air. In plain English: warm, moist indoor air touches cold glass, and the moisture turns into liquid water.
This does not automatically mean the window is defective. Even ENERGY STAR certified windows can experience condensation in cold weather if indoor humidity is high enough or air circulation over the glass is restricted.
What increases interior window condensation?
Common factors include high indoor humidity, long showers, cooking, drying clothes indoors, unvented or poorly vented exhaust sources, tightly closed blinds, blocked heat registers, and very cold outdoor weather. Newer homes can also hold more moisture if they are relatively airtight and ventilation is limited.
ENERGY STAR recommends managing indoor humidity and making sure kitchen and bath exhaust fans vent outdoors. It also notes that window treatments should allow airflow over the glass and that the space between the window frame and rough opening should be insulated during installation.
Why condensation can damage more than the glass
Occasional light fogging may be mostly a comfort issue. Repeated water running onto the sill is different. Persistent moisture can damage wood trim, paint, drywall and adjacent finishes and may encourage mold growth if surfaces stay wet.
If you wipe the glass every cold morning and the sill is still becoming saturated, reduce the moisture source and investigate whether the window has poor thermal performance, air leakage or installation gaps.
Fog between the panes: a different problem
Double- and triple-pane insulating glass units are assembled with sealed spaces between the panes. If the edge seal fails, moisture can enter that sealed cavity and create fogging or haze that cannot be wiped from either exposed surface.
That is not ordinary indoor condensation. It is a glazing-unit problem. Depending on the window design, the remedy may be replacement of the insulated glass unit, sash, or entire window.
A draft is not the same as condensation
You can have a draft without visible water. ENERGY STAR notes that air leakage can occur around windows and other openings. The leak may be through weatherstripping, an operable sash, a frame joint, or the gap between the window unit and the rough opening.
Drafts matter because moving cold air lowers nearby surface temperatures and can make condensation more likely. They also affect comfort even when the glass itself performs reasonably well.
Water during rain: think flashing and drainage
If water appears around the window only during wind-driven rain, especially at the top or side of the opening, the problem may be exterior flashing rather than glass condensation. The U.S. Department of Energy's building-science guidance treats windows as interruptions in the wall drainage plane that need properly lapped flashing.
Correctly installed sill, side and top flashing directs water out of the wall cavity. If those layers are missing or reversed, water can get behind the siding and appear at the interior trim even when the window unit itself is not defective.
How to diagnose the four common moisture patterns
1. Water on the inside glass on cold mornings
Most likely: interior condensation. Check indoor humidity, exhaust-fan use, airflow over the window and whether condensation affects many windows or only one.
2. Fog trapped between panes
Most likely: failed insulated-glass seal. Because the moisture is inside the sealed unit, wiping the room-side glass will not remove it.
3. Cold air movement around the sash or trim
Most likely: air leakage. Weatherstripping, sash alignment, frame joints or installation gaps may be involved.
4. Water around the frame during rain or snowmelt
Most likely: an exterior water-management issue. Inspect flashing, siding integration, head details and the surrounding wall assembly.
What window ratings can tell you
The National Fenestration Rating Council (NFRC) provides independent ratings for window energy performance. U-factor describes heat transfer; lower values indicate better resistance to heat flow. NFRC also publishes condensation-resistance ratings, where a higher number indicates better resistance to condensation under the test conditions.
These ratings are useful when comparing replacement windows, but they do not override indoor humidity. A high-performing window can still condense during extreme cold if the indoor dew point is high enough.
When should you reduce indoor humidity?
If many windows across the house are wet on the room side, the problem is more likely to be whole-home humidity than simultaneous failure of every window. Use bath and kitchen exhaust fans, make sure the dryer vents outdoors, and avoid running humidifiers aggressively during severe cold.
The appropriate indoor humidity changes with outdoor conditions and the house. The goal is not to make the home uncomfortably dry; it is to prevent sustained condensation on cold surfaces.
When replacement makes sense
Replacement may be reasonable when insulated-glass seals have failed across multiple units, frames are deteriorated, windows are difficult to operate, air leakage is persistent, water damage has affected the opening, or the existing windows have poor thermal performance and are already near the end of service life.
If one sash has a failed glass unit but the frame and installation are otherwise sound, a smaller repair may be possible. Good diagnosis prevents a homeowner from replacing an entire window for a humidity issue—or trying to solve bad flashing with a dehumidifier.
Minnesota-specific considerations
Cold winters create a large temperature difference between indoors and outdoors. That makes window surface temperature, air leakage and indoor humidity more visible than they may be in fall or spring. Homes in Burnsville, Lakeville, Apple Valley, Eagan, Minneapolis, Saint Paul and Northfield should expect condensation risk to increase during the coldest weather.
Solaris provides replacement-window installation and can coordinate windows with siding and exterior flashing work when the opening and wall need to be addressed together. See completed work in the Work Near You hub.
Bottom line
Where the moisture appears tells you which problem you are solving. Room-side condensation points toward humidity and cold surfaces. Between-pane fogging points toward a failed sealed glass unit. Drafts point toward air leakage. Water during rain points toward flashing and wall drainage. Diagnose first, then choose the repair.
For more homeowner guidance, visit the Solaris Advice hub.
Research & Sources
- ENERGY STAR — Residential Windows, Doors & Skylights. Explains window condensation, air leakage and practical steps for managing indoor humidity and installation gaps.
- National Fenestration Rating Council. Defines window performance ratings including U-factor, air leakage and condensation resistance.
- U.S. Department of Energy Building Science Education — Window Flashing. Explains why window openings are vulnerable to water leakage and how flashing integrates with the wall drainage plane.
General homeowner education only. Individual homes vary and may require a site-specific exterior, moisture, drainage, or energy assessment.