Why Your Windows Are Sweating Inside: Causes and Solutions

You walk into your living room on a January morning and your windows are dripping. Water beads cover the glass, streaks run down to the sill, and there is a small puddle forming where the glass meets the frame. Your first thought: something is wrong with my windows.

Maybe. But probably not — at least, not in the way you think. Interior window sweating is overwhelmingly a humidity and temperature problem, not a window manufacturing defect. Understanding the actual cause saves you from either ignoring a problem that will eventually grow mold or spending thousands replacing windows that are working exactly as designed.

The Science Behind Window Sweating

Window sweating — condensation on the interior surface of your glass — happens because of a fundamental physical principle called the dew point.

Every air mass holds a certain amount of water vapor. The warmer the air, the more moisture it can hold. The dew point is the temperature at which air becomes saturated and can no longer hold its moisture in vapor form. When air contacts a surface at or below the dew point, the water vapor condenses into liquid droplets on that surface.

Your windows are the coldest surfaces in your home's building envelope. Even well-insulated double-pane windows have an interior glass surface that is significantly cooler than the rest of your room. When the outdoor temperature drops enough to pull the glass surface below the indoor air's dew point, condensation appears.

Here is what that looks like with real numbers. If your home is 70 degrees F with 40% relative humidity, the dew point is approximately 44 degrees F. If the interior surface of your window glass drops below 44 degrees, it will condense. On a single-pane window when it is 10 degrees outside, the glass surface might be 25-30 degrees — well below the dew point, guaranteed condensation. On a quality double-pane low-E window under the same conditions, the glass surface stays around 50-55 degrees — safely above the dew point, no condensation.

This is why the window itself matters, but indoor humidity is the factor you can control most directly and quickly.

Common Causes of Interior Window Condensation

When homeowners along the Wasatch Front ask "why are my windows sweating," the answer is almost always one or more of these sources adding excess moisture to the indoor air.

Whole-House Humidifiers Set Too High

This is the single most common cause of widespread window condensation in Utah homes. Because Utah's outdoor air is extremely dry in winter, many furnace-mounted or standalone humidifiers run continuously. A setting that feels comfortable for your skin (40-45% humidity) may be far too high for your windows when outdoor temperatures drop below 20 degrees.

The fix is straightforward: reduce your humidifier setting as the outdoor temperature drops. At 20 degrees F outside, keep humidity below 35%. At 10 degrees F, below 30%. At 0 degrees F, below 25%.

Cooking and Dishwashing

Boiling water on the stove, running the dishwasher, and baking all release significant moisture. A pot of boiling water can release a quart of moisture per hour into your kitchen air. Without adequate exhaust ventilation, this moisture migrates to nearby windows and condenses.

Showers and Baths

A typical 10-minute hot shower generates roughly half a pint of airborne moisture. In a bathroom without an exhaust fan — or with one that is undersized or that vents into the attic rather than outside — this moisture saturates the bathroom air and migrates to windows.

Laundry and Dryer Venting

If your dryer vent hose is disconnected, damaged, or clogged, some or all of the moisture from drying clothes is being released into your home instead of outside. Even a partially blocked vent can contribute meaningful moisture. Hanging clothes to dry indoors has the same effect.

Breathing and Perspiration

A family of four generates approximately 2-3 gallons of moisture per day just from breathing, perspiring, and normal bodily functions. In a tightly sealed home, this adds up quickly.

New Construction and Renovation

Newly built homes or recently renovated rooms have elevated moisture levels for 12-18 months as concrete, drywall mud, paint, and wood framing materials cure and release trapped moisture. If you moved into a new home and are experiencing widespread condensation, this moisture load is likely a significant contributor.

Aquariums, Houseplants, and Firewood

An uncovered aquarium is essentially a humidifier running 24/7. A large collection of houseplants adds moisture through transpiration. Freshly cut or green firewood stored indoors releases significant moisture as it seasons. Any of these can contribute enough moisture to tip windows into condensation.

Is Your Condensation Normal or a Warning Sign?

Not all window sweating requires the same response. Here is how to gauge whether your condensation is routine or signaling a deeper issue.

Normal and manageable: Light condensation on one or two windows in the morning that clears by midday. Condensation only in the bathroom during or after showers. Condensation only in the kitchen while cooking. These are all humidity-specific situations that respond well to improved ventilation.

Worth investigating: Condensation appearing on most windows in the house every morning. Condensation that does not clear until afternoon or persists all day. Water pooling on sills by the time you wipe it. Condensation appearing even when you have your humidifier turned off. These patterns suggest either a whole-house humidity issue, inadequate ventilation, or windows that are underperforming.

Action required: Condensation that you cannot wipe away because it is between the panes — this is seal failure, not an indoor humidity issue. Ice forming on the interior glass surface. Mold visible on sills or frames. Water staining on walls below windows. Any of these require more than humidity adjustment.

How Utah's Climate Amplifies the Problem

Utah homeowners deal with a combination of factors that makes interior condensation more common than in many other parts of the country.

The outdoor climate is exceptionally dry — Salt Lake City averages about 30% relative humidity annually and drops well below 20% on cold winter days. This extreme dryness makes indoor humidification feel necessary for comfort, but it also means any humidity you add to your home has nowhere to escape through the building envelope except through air leaks and ventilation.

Temperature extremes compound the issue. When outdoor temperatures drop to single digits — which happens regularly in the Salt Lake Valley and is routine in Park City, Heber, and other mountain communities — the window glass surface temperature drops dramatically. Even quality double-pane windows see their interior surface temperature fall to the mid-40s or low 50s, which is right at the dew point threshold for typical indoor conditions.

The prevalence of furnace-mounted humidifiers in Utah homes adds another layer. These systems are designed to be set and forgotten, but they should actually be adjusted frequently based on outdoor conditions. A humidity level that is perfect at 35 degrees outside is too high at 5 degrees outside.

And then there is the age of Utah's housing stock. Many homes in the Salt Lake Valley, Utah County, and Davis County were built in the 1990s and early 2000s with builder-grade double-pane windows that are now reaching the end of their effective life. As IGU seals degrade and insulating gas leaks out, these windows condense at lower and lower humidity levels — making it seem like the humidity problem is getting worse when it is actually the window performance declining.

Room-by-Room Troubleshooting

Different rooms condense for different reasons. Targeting your investigation by room can help you find the cause faster.

Kitchen windows: Usually a ventilation issue. Verify your range hood exhausts to the outside (not just recirculates with a charcoal filter). Use the exhaust fan on high when boiling, steaming, or running the dishwasher. Run it for 10-15 minutes after cooking stops.

Bathroom windows: Same principle. Confirm the exhaust fan is functioning, adequately sized (80+ CFM for a standard bathroom), and venting to the exterior. Replace the switch with a timer or humidity-sensing switch so the fan runs long enough to clear moisture after showers.

Bedroom windows: Breathing is the primary moisture source. Two adults sleeping in a closed bedroom with the door shut generate enough moisture to raise humidity significantly by morning. Keep the bedroom door slightly open at night to allow air exchange, or run a small fan to improve circulation.

Basement windows: Basements are naturally cooler and often have higher humidity from foundation moisture. A basement dehumidifier may be needed. Check for any water intrusion through the foundation walls — this is a separate issue from window condensation but contributes to it.

Living room and family room: If these central rooms condense, it is usually a whole-house humidity issue. Check your humidifier settings first. Then verify air circulation — heavy curtains, closed blinds, and furniture pushed against windows all trap stagnant air against the cold glass.

Quick Fixes You Can Try Today

Before investing in any equipment or services, try these no-cost and low-cost steps:

1. Turn down your humidifier: If you have a furnace-mounted humidifier, lower the set point by 5-10%. Wait two days and check if condensation improves.
2. Open window treatments in the morning: Pull back curtains and raise blinds to let warm room air reach the glass.
3. Run exhaust fans longer: After cooking or showering, leave fans running for 15-20 minutes to clear moisture.
4. Crack a window on dry days: When outdoor humidity is low (which is most winter days in Utah), cracking a window for even 10-15 minutes flushes humid indoor air.
5. Run ceiling fans clockwise on low: This pushes warm air from the ceiling down toward windows, raising the glass surface temperature slightly.
6. Move furniture away from windows: Bookcases, couches, and desks placed against windows block air circulation and create moisture traps.

Long-Term Solutions for Persistent Sweating

If quick fixes reduce but do not eliminate the problem, consider these longer-term investments:

Install a hygrometer ($15-30) in the worst-affected room. This gives you objective data instead of guessing. Target 30-35% humidity during the coldest weeks.

Upgrade bathroom exhaust fans to higher-CFM models with humidity-sensing switches. These automatically activate when moisture levels rise and run until the humidity normalizes. Budget $150-300 per bathroom, installed.

Install an HRV or ERV system. A heat recovery ventilator brings in dry outdoor air while recovering heat from outgoing stale air. This is the gold standard for managing humidity in tight, well-insulated homes. Installation runs $2,000-$5,000 but pays back through both moisture control and improved indoor air quality.

Replace failing windows. If your windows are over 15 years old and condensation has worsened year over year despite good humidity management, the windows themselves may be degrading. Failing IGU seals, worn weatherstripping, and deteriorated gaskets all reduce the window's ability to maintain a warm interior glass surface. Modern replacement windows virtually eliminate condensation at normal humidity levels. See the comprehensive condensation and mold guide for a complete assessment framework.

For a step-by-step implementation plan covering all seven of the most effective condensation prevention methods for Utah homes, read the prevention methods guide.

Frequently Asked Questions

Why do my windows only sweat on the bottom half?

Condensation is heavier at the bottom of windows because cold air sinks. The lower portion of the glass is exposed to the coldest air pooling at floor level, making its surface temperature lower than the upper glass. Gravity also pulls condensation droplets downward, so moisture that forms higher on the glass runs down and accumulates at the bottom.

My neighbor's house was built the same year as mine but their windows do not sweat. Why?

Indoor humidity, ventilation habits, and window maintenance all vary dramatically between households. Your neighbor may run their humidifier lower, have better exhaust ventilation, keep curtains open more often, or simply have fewer moisture-generating activities. Their windows may also be a different brand or model with better-performing IGU seals.

Do storm windows help with interior condensation?

Yes. Both interior and exterior storm windows add an additional air space that keeps the primary window's interior glass surface warmer. Interior storm windows are particularly effective and cost $50-150 per window. They are a good intermediate step if full window replacement is not in the budget.

Can I put a space heater near a sweating window to stop condensation?

While warming the air near the glass can raise the surface temperature above the dew point, this is energy-intensive and can be a fire hazard if the heater is too close to curtains or furniture. A small fan directing warm room air toward the window is safer and more effective.

Your windows are telling you something. Whether the message is "manage your humidity" or "I need to be replaced," understanding the cause is the first step toward dry sills, clear glass, and a healthier home.