Window Condensation — Why It Happens and How to Fix It

You wake up and your windows are dripping wet. Water pools on the windowsill and runs down the wall. This is condensation — and in the Philippines, it is almost always caused by air conditioning meeting humid tropical air. Here is why it happens and how to stop it permanently.

Why Condensation Forms

Condensation forms when warm, humid air contacts a cold surface. In the Philippines, this happens when your air conditioner cools the glass surface below the dew point of the ambient air. The moisture in the surrounding air condenses into water droplets on the cold glass — exactly like a cold glass of water "sweating" on a hot day.

Key numbers: Philippine ambient humidity averages 75 to 85 percent year-round. At 80 percent relative humidity and 32°C outside temperature, the dew point is approximately 28°C. If your air conditioner cools your room to 22-24°C, the interior glass surface drops to 20-24°C — well below the dew point of the outdoor air leaking in through window gaps.

The Three Causes

Cause 1: Air leaks around the window frame — humid outdoor air enters through gaps between the window frame and the wall, or through worn weatherstripping on sliding windows. This humid air contacts the cold glass surface and condenses. This is the most common cause and the easiest to fix.
Cause 2: Single-pane glass — single-pane glass is an excellent heat conductor. The exterior surface absorbs solar heat, but the interior surface is cooled by the air conditioner. The interior surface temperature drops below the dew point, and condensation forms.
Cause 3: Over-cooling the room — setting the air conditioner below 22°C in a poorly sealed room creates a larger temperature differential, increasing condensation. Every degree cooler increases condensation risk.

How to Fix It

Fix 1: Seal Air Leaks (Most Effective, Lowest Cost)

Re-seal the frame-to-wall junction with neutral-cure structural silicone (Dow Corning or equivalent). Replace worn wool-pile weatherstripping on sliding windows. Check and replace door sweeps on glass doors. Cost: ₱500 to ₱2,000 per window. This single step resolves 60 to 70 percent of condensation problems.

Fix 2: Upgrade to Double Glazing

Insulated glass units (IGUs) keep the interior glass surface warmer because the air gap between the two panes acts as thermal insulation. The interior pane stays closer to room temperature rather than being chilled by contact with outdoor conditions. This eliminates condensation in most cases. Cost: ₱4,000 to ₱8,000 per sqm premium over single glazing.

Fix 3: Improve Room Ventilation

Running the AC with the exhaust fan off traps humid air inside the room. If your bathroom or kitchen is connected to the bedroom, humidity from showers and cooking migrates into the bedroom and condenses on the cold window glass. Use exhaust fans to remove humid air at the source.

When Condensation Is Dangerous

Occasional light condensation (a thin film that evaporates within an hour of turning off the AC) is cosmetic and harmless. But chronic condensation that pools and runs down the wall causes:

Mold growth — behind the window frame and on adjacent wall surfaces. Black mold is a health hazard, particularly for people with respiratory conditions.
Wall damage — repeated wetting and drying cycles degrade paint, plaster, and drywall. The wall behind the window frame can develop soft spots and crumbling.
Frame corrosion — standing water in aluminum window tracks accelerates corrosion, particularly on mill-finish and aging anodized frames.
Floor damage — water running off the sill onto wooden or laminate flooring causes warping, staining, and delamination.

Prevention Summary