Potential for Condensation on Windows
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New build in western Massachusetts (zone 5) – we are using Loewen double pane windows, with 6” wall cavity fluffy insulation and hi R (r13) exterior continuous foam. Other pertinent details: windows installed at exterior (wrb (typar?)) plane, ~r60 unvented roof, central ac/erv ducting, radiant floor heat. I am concerned by a few other similarly detailed local examples where cold temperature condensation issues have come up. Is there a way to ensure we won’t have the same problems? Do we need an engineer to look at all this and give a report?
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https://www.payette.com/glazing-and-winter-comfort-tool/
For a given window and outdoor temp, condensation is about controlling humidity.
Regardless of the wall structure, condensation on the inner window glass surface is a matter of interior humidity vs temperature of the inner glass pane. If the house is PGH level, meaning it also has a very low air leakage rate, then continuous ventilation is required to keep interior RH down low enough, even in winter, to avoid moisture issues. Ventilation is then best done via an HRV to recover heat from the outgoing air. In most cases, winter RH in the range 30-35% is a reasonable balance between human health/comfort and structure health.
Even with RH kept from rising too high due to human activity inside, bitter cold exterior air still could result in some condensation on the inside surface of double glazing. Raising the interior surface temperature above the air dew point avoids that, and triple-pane window glass is the means of achieving that higher inside surface temperature.
My own house is in the PGH category, and it has triple-pane windows everywhere but in the doors. Interestingly, when the inside sheetrock was being skim-coated with plaster during the 2011 winter, interior humidity during the work day was greatly increased. I'd vent the house for a few hours after the crew had left to flush out much of the moisture. During the day, however, the open field of each triple pane window was clear of condensation. Only the edges of the glass had any condensation, due to thermal conduction where the panes joined. In contrast, the whole glass surface of the doors was covered in condensation. Several times each day I would wipe water from the edges of the windows, to protect the wood trim, and from the whole glass surfaces of the doors.
In addition to avoiding condensation, I have to say that those triple-pane windows do avoid discomfort next to them in winter.
Thanks that’s great information
You can look up Cardinals technical guide where there is a table that shows the indoor relative humidity that will cause condensation at different outdoor temperatures by glass package. This will show you the risk at the center of glass, under different conditions.
If you want to read about condensation resistance, check out this article:
Buying Windows .
You should also be aware that drapery can change the surface temperature of the glass b/c the air space between drapes and the glass acts as an extra layer of insulation, dropping the surface temperature of the glass. Draperies are generally not at all vapor-tight, though so they allow interior humidity to reach the glass surface at the same time the surface temperature decreases.