Historic HomeWorks Forum

[J. Dunson Todd]

I was looking over the discussion on interior storm windows and the usefulness of insulating the pockets for the counterweights on older windows, and realized this might be the place to ask about exterior window condensation. I belive this condensation is created by warm moist interior air escaping and encountering the cold dry exterior air. If my wooden windows are sealed over with an "interior storm" plastic sheet, where is the warm air coming from? Could it be traveling between the trim and the old plaster wall into the window cavity? Would the interior storm create enough of a barrier on its own? I have seen condensation form on old windows with traditional wooden storms (or with none) where the glazing has deteriorated. The second part of this is how to address the condensation. The aluminum storms do not have moisture drain holes in the bottom of the tracks which i belive newer storms now have. Does there need to be a minimal amount of exterior air between the storm and the window, sort of a balance?
Thanks. J. Dunson Todd

[johnleeke]

You are correct, condensation on the interior side of the exterior storms is due to warm moisture laden air from the living space. The "interior storm" plastic sheet may allow a small amount moisture to pass right through the plastic. The plastic is better thought of as a "vapor retarder" than a "vapor barrior. Usually most of the moisture is carried in moving air that may very well travel to the space between the primary sash and the exterior storm via the "stud space" within the wall (if the building is wood framed).

We usually install exterior stroms leaving a 1/4" wide gap between the bottom edge of the storm and the sill to allow an occasional puff of exterior air to blow into the space and dry it out. Most (perhaps 80 to 90%) of the energy saving and comfort benefit of exterior storms is due to the way they prevent the windy blast of cold air from flowing across the glass of the primary sash. The prevention of air infiltration make only a minor contribution. At a minimum there should be 3 or 4 1/4" holes along the bottom edge of the exterior storm to allow for drainage. The two 1/16" wide slots or slight dimples provided by the manufacturer can get clogged up with debris to easily.

The whole window system should be very resistant to air exfiltration, and the exterior of the window system should allow a small amount of air exchange with the exterior air. Think of it more as a ratio than a balance. The interior element (the primary sash or your plastic sheet, or the combination of the two) should be about 10 times more resistant to the passage of moisture than the exterior element (exterior storm).

[J. Dunson Todd]

Thanks John-
given that the exterior storms are aluminum (which is an excelent conductor) do you feel that a traditional wooden storm would be just as if not more effective at blocking out the blasts of cold air? If you suggest leaving a gap at the base of the storm would the thermal transfer of cold from the metal storm reach the inside? I may be spitting hairs here since i have no insulation in the wall cavity.

[johnleeke]

Wood storms would be just as effective at blocking the wind as aluminum. I like the idea of wood storms for another reason too. The wood is more environmentally friendly than aluminum.