Condensation and Water Staining at a Low-Slope Atrium Skylight
A 6.0 m by 2.5 m aluminum-framed skylight sits over a three-storey atrium at a 15° slope, facing north. The glazing is double-glazed, low-e, argon-filled insulating glass units with warm-edge spacers. Within one heating season, maintenance reports water dripping from the glazing at the low end and dark staining on the exposed aluminum rafters. The interior relative humidity in the atrium is maintained at 45% in winter. Inspection finds the sill weep holes sealed with exterior sealant, no continuous interior air seal between the glazing and the rafter cavity, and fiberglass insulation packed tight against the aluminum rafters. The building owner wants to replace the glass with triple glazing and add more insulation. The glazing contractor is asked whether that will solve the problem.
1. Diagnose the likely source of the water and staining. Is it glass failure, condensation, or leakage? Explain the evidence.
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The pattern points primarily to interior condensation and trapped moisture, not necessarily an insulating-glass unit failure. The water appears at the low end, where the frame is coldest and where drainage should occur. Staining on the exposed aluminum rafters indicates prolonged wetting of the frame cavity, not just occasional surface condensation. The atrium humidity is 45% in winter, which provides a substantial moisture source. Warm, moist interior air is leaking into the rafter cavity because there is no continuous interior air seal. That air reaches the cold aluminum, condenses, and the water cannot escape because the weep holes are sealed. Packing insulation tight against the rafters makes the problem worse by creating a cold bridge and by preventing ventilation and drainage. A failed IG unit would usually show fogging or visible moisture inside the sealed cavity; the reported staining and low-end dripping are more consistent with condensation on the frame and in the glazing pocket. A definitive diagnosis should still include checking the IG unit seals and the exterior weather seal.
2. Evaluate the owner's proposal to install triple glazing and add more insulation. What else must be done?
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Triple glazing and more insulation may reduce the cold-surface effect, but they will not correct the root causes. Improved glass U-value can raise the interior surface temperature and reduce condensation on the glass, but the aluminum rafter and glazing pocket remain cold bridges unless thermally broken. Adding insulation tight against the frame can make the situation worse by trapping moisture and blocking ventilation. The critical work is to control air, water, and vapour:
- Establish a continuous interior air seal between the glazing and the interior, while allowing the glazing pocket to drain and vent to the exterior.
- Restore the intended two-stage drainage path: an interior gutter or collection channel, weep holes at the low end and at each glazing pocket, and baffles to prevent wind-driven water entry.
- Keep insulation out of contact with the aluminum frame or use a thermal break and a proper air/vapour barrier detail.
- Verify the skylight slope and the low-end curb height; at 15°, water must drain freely and not stand against seals.
The owner should be told that triple glazing alone is an expensive partial fix and could conceal the moisture problem rather than solve it.
3. Describe the correct drainage and ventilation details for this skylight.
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A sloped aluminum skylight needs a deliberate path for any water that enters the glazing pocket, plus a way for the pocket to breathe. The detail should include:
- Interior air seal: a continuous seal on the warm side, separate from the exterior water seal, so moist interior air does not enter the frame cavity.
- Two-stage water management: an exterior seal or flashing at the glazing, an internal gutter or sill channel that collects any leakage or condensation, and weep holes or slots at the low end that drain to the exterior.
- Ventilation: small vents or open joints at the high end and baffled weeps at the low end so the pocket can dry. The weeps must not be sealed during exterior caulking.
- Thermal break and isolation: a thermal break in the aluminum frame and setting blocks or glazing tape that keep the glass off bare metal where possible.
- Slope and curb: maintain the designed slope and a raised curb at the low end; do not allow water to stand against the glazing seal.
The contractor should also confirm that the weep path is not blocked by insulation, setting blocks, or sealant and that the interior gutter can be inspected and cleaned.