Window Condensation 101

[chefwong]

This is probably a question for Dana, our resident insulation guru it seems.
When there is condensation on the interior of a insulated windows, does it mean that the window has lost it's insulation...

I bought a weekend project house and it's got a bunch of older insulated windows and a couple of brand new Kolbe windows in them.

Moisture on windows I thought was part of the humidity created when cooking, etc and it's cold outside.
What I'm noticing as I pay closer and *plan* is that the newer windows have nary any moisture on them but the olders ones have quite a bit.

I still need to decide on a plan on opening and insulating these walls if I choose. These old plaster walls have seen better days. I'm just avoiding the wire and lath I suppose. The interior walls are great and are concrete hard. The walls facing the exterior are crumbling in some areas, and in others, I can see moisture or dew is getting to it. It's a brownstone, so I'm not sure if the moisture issues are just uninsulated walls and the temperamental difference to the plaster that is causing the moisture...

[jimbo]

If the seal on a double pain window fails, the window always fogs up internally. You may just have too much cold air AROUND the window, allowing the inside surface to be cold, hence condensation.

[chefwong]

The condenstation I'm referring to is not inside the double pane, but just the glass facing the interior of the home

[Dana]

The tighter the house, the higher the interior air dew points are, and the more likely you'll get window condensation. Newer-better low-E windows have higher interior temperatures than older double-panes, so the new ones might only be below the dew point of the interior air a few hours/day, whereas the older ones could dwell there for many hours, getting wet. Double-pane windows with 3/8" or less between them are less insulating than double panes with bigger gaps between the inner & outer glazing (and are even less insulating than tight single-pane + storm windows.)

Window shades and curtains add to the problem. By putting even a modest amount of insulation between the window and the room, the temperature of the glass is lower, and unless those shades are air-tight, room air convects by the window depositing moisture on the cool pane.


If the weepy-windows are in otherwise good shape and don't leak air, a tight-fitting exterior low-E storm window effectively gives them triple-pane performance at less than replacement-window cost, and the temperature of the interior face of the glass will be higher.


As for the exterior walls, I'd need more information on the wall stackup to advise. If you're lucky it's a brick-veneer cavity wall with a sheathed structural studwall and a gap between the sheathing and the brick facing. If it's solid brick, no-cavity with the scratch coat plaster hanging on the interior brick your options are limited and potentially expensive.

[chefwong]

The walls are wood stud framed.
In the kitchen, I knocked down, reframed with sheetrock but this is more because I used tile for the backsplash...

The walls definitely need attention. I'm just putting it off.....as the few patches I have done, scratch coat, brown coat, etc - to keep in line with the house, is a chore. Doing whole walls

The tighter the house, the higher the interior air dew points are, and the more likely you'll get window condensation. Newer-better low-E windows have higher interior temperatures than older double-panes, so the new ones might only be below the dew point of the interior air a few hours/day, whereas the older ones could dwell there for many hours, getting wet. Double-pane windows with 3/8" or less between them are less insulating than double panes with bigger gaps between the inner & outer glazing (and are even less insulating than tight single-pane + storm windows.)

Window shades and curtains add to the problem. By putting even a modest amount of insulation between the window and the room, the temperature of the glass is lower, and unless those shades are air-tight, room air convects by the window depositing moisture on the cool pane.


If the weepy-windows are in otherwise good shape and don't leak air, a tight-fitting exterior low-E storm window effectively gives them triple-pane performance at less than replacement-window cost, and the temperature of the interior face of the glass will be higher.


As for the exterior walls, I'd need more information on the wall stackup to advise. If you're lucky it's a brick-veneer cavity wall with a sheathed structural studwall and a gap between the sheathing and the brick facing. If it's solid brick, no-cavity with the scratch coat plaster hanging on the interior brick your options are limited and potentially expensive.

[Runs with bison]

What is the humidity level and temperature in the home? (Measured, not the setpoints.) Are you seeing the condensation at a specific outdoor temperature? (As in "there is always some condensation at 20 degrees, but not at 25-30 or some such.)

I've not seen condensation with double panes and I tend to leave the humidistat at the same set point even when it hits -14 F outside. Once the thermometer drops to around 0 F it struggles to hold 30% humidity indoors at 69 F. The only place I'm seeing condensation and frost is the only single pane window in the home...in an uninsulated/unconditioned space, and only then when it is bitter cold.