Natatorium Condensation on Room-Side Glass
A community aquatic center has 1-inch insulating glass units in thermally broken aluminum frames. The units have low-e coating on surface 2, argon fill, and warm-edge spacers. Design conditions are 82°F indoor air at 60% relative humidity and 10°F outdoor air. During the first winter, heavy condensation streams down the room-side glass, ice forms at the frame, and staff report fogging between the panes in two units. The owner says the windows are defective and wants all units replaced under warranty. The glazing contractor says the condensation is normal and points to the pool’s humidity. Maintenance has been running exhaust fans intermittently and has kept the pool area at 60% RH because swimmers complain about dry air. The project team needs a technical basis for deciding whether this is a product failure, a design condition, or both.
1. Distinguish the significance of room-side condensation versus fogging between panes.
Read the worked answer
Room-side condensation means the interior glass or frame surface has dropped below the dew point of the indoor air. It is a surface-temperature and humidity problem, not by itself evidence that the sealed unit has failed. Fogging between the panes means moisture has entered the sealed air space, usually through a failed edge seal or saturated desiccant; that is a sealed-unit warranty issue. In this project, the two fogged units should be evaluated as probable seal failures, while the streaming condensation on the other units requires a psychrometric and surface-temperature explanation.
2. Using the design conditions, explain whether condensation should be expected even in a properly functioning unit.
Read the worked answer
At 82°F and 60% RH, the dew point is roughly 67°F. A rough interior surface temperature can be estimated from the indoor-outdoor temperature difference and the assembly U-factor: surface temperature ≈ indoor temperature − U × ΔT / interior film coefficient. With ΔT = 72°F and an interior film coefficient near 1.46 Btu/h·ft²·°F, a U-factor of 0.30 drops the surface about 15°F, to roughly 67°F. That is at or below the dew point, and the bottom sightline near the spacer will be colder. Condensation is therefore expected at 60% RH even with a reasonably good unit. The failed between-pane units are different, but the room-side condensation is primarily a design-condition issue.
3. Recommend a corrective sequence and identify what would justify warranty replacement.
Read the worked answer
First, verify each unit for between-pane fogging and document the results. Second, reduce indoor humidity with continuous dehumidification, targeting a level the fenestration and envelope can tolerate, and improve air circulation across the glass. Third, inspect the thermal break, warm-edge spacer, and sill details for cold bridges. If the owner insists on 60% RH, the assembly may need triple glazing, low-e on surface 3, or a better frame to raise interior surface temperatures. Warranty replacement is justified for units with failed edge seals and internal fogging, not for room-side condensation caused by high humidity unless the unit’s certified performance was misrepresented.