Windows That Sweat and a Cold Second Floor
A 1970s house has double-pane windows with aluminum frames and no low-E coating. In winter, the homeowner reports heavy condensation on the inside of the glass and frames, especially in bedrooms, and that the second floor is 5°F colder than the first. The windows are original and have visible gaps at the frames. The house has a forced-air furnace with a central return and no mechanical ventilation. A blower door reads 4,800 CFM50 on a 2,400 ft² house, and a pressure pan test shows strong leakage at the window frames. The homeowner asks whether replacing the windows will fix the condensation and the cold second floor.
1. Evaluate whether new windows alone will solve the condensation and comfort complaints.
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New windows will reduce surface condensation because a warmer interior glass temperature raises the surface above the dew point, and low-E argon-filled units with insulated frames perform far better than the existing aluminum units. But windows alone will not fix the cold second floor or the underlying cause. The blower door and pressure pan results show substantial envelope leakage at the frames and elsewhere; that air leakage drives both comfort complaints and moisture problems. The house also lacks mechanical ventilation, so indoor humidity from showers, cooking, and occupancy has no controlled exhaust path. The correct approach is a package: air-seal the envelope (frames, top plates, rim joists, penetrations), add mechanical ventilation to control indoor humidity, then replace windows if budget allows. Windows are part of the solution, not the whole solution.
2. The homeowner wants to prioritize spending. Given the diagnostics, rank the improvements and justify the order.
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First, air-seal the building envelope. The 4,800 CFM50 and pressure pan leakage indicate large uncontrolled air exchange that causes drafts, uneven temperatures, and moisture transport. Sealing top plates, rim joists, window frames, and penetrations is inexpensive per unit of benefit and addresses the cold second floor directly. Second, add mechanical ventilation - a balanced ERV or HRV, or at minimum a properly sized bath and kitchen exhaust with makeup air - to control indoor humidity and dilute pollutants. Third, address insulation where gaps exist, especially in the attic and over the second floor, to reduce the temperature stratification. Fourth, replace windows with high-performance low-E units. Windows have the longest payback and the smallest effect on the comfort complaint relative to air sealing and ventilation. Re-test the blower door after each phase to verify improvement.