Humidity dripping windows outside - Zone 6

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Conde

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I have noticed that my house is dripping from the window vents on the exterior during these recent cold snaps (-10°F and colder) in Minnesota.

Build: 2009
Location: Minneapolis MN
Walls: 2x6 cellulose insulation
Windows: Some kind of double pane
HVAC: Forced air, natural gas. ERV. Whole home humidifier (35% RH setting).

There seems to be a vent at the bottom of the windows, how is indoor air reaching this point? Do I need to do anything to correct this?

Window_1.jpg



Thanks

Jim
 

Dana

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There's probably an air leak in the window unit framing, or between the window unit and the rough framing around it. The forced air heating is creating sufficient room to room pressure differences that it's driving air out that leak when the air handler is running, pulling air in from some other location, using "the great outdoors" as part of the return path. (It's probably coming back in via the ERV.)

If it's a vinyl window the dimensions of the window can change quite a bit with weather, and it may leak more during negative double-digit weather than when it's in positive digits territory, or conversely. Finding and fixing the leak might be possible with a blower door and infra-red imaging, but the ice-drip location is doing at least some of that locating for you. You might be able to fix it with a 3/8" drill hole and some non-expanding latex foam sealant (eg DAPtex Plus) into any hollow points of the window unit framing, or between the window unit and other framing.

It's remotely possible the air leak from the interior side is at the kickboards near the floor (that can be sealed with polyurethane caulk) or an electrical outlet box in a framing bay just under the window (sealable with can-foam or caulk), but cellulose is pretty air-retardent, making this less likely.

Adjusting the return paths for the heating supply air to reduce the room to room pressure differences to Energy Star standard, which is under 3 pascals (~0.012 water-inches) when the air handler is running, whether or not the door to that room (or other rooms) is closed or open should help too. Cheap hand held dual port manometers with a 0.01" resolution are good enough to find the biggest offending rooms, and the direction of pressurization/depressurization. Any reading greater than 0.01" would indicate a duct imbalance issue. For this room it's apparent that it's being pressurized- it's only a matter of how much. It's unlikely that mere stack-effect pressures are big enough to move that much air/water into a leak around a window, unless that's a tall house, and the ERV intake is at ground level. (At -10F outdoors, 7oF indoors you're looking at a stack effect of about 4 pascals per story, less at lower temperature differences.)
 

Conde

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I will definitely get a hand-held manometer, I finished my basement a little while ago so I should check those rooms as well. The house is a full 2 stores and this is occurring on the second floor guest bedroom. I know the room does have a return vent.

Thanks for the quick and informative reply, I love this site!
 
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