I live in a building with wooden double-glazed window frames and have this tiny "ventilation" outlet installed into one of the larger windows in our apartment (see below).

What is its purpose, and most importantly, should it stay in its current position during the cold months of the year? I feel like it defeats the purpose of the insulated glazing, but I am a mere layman when it comes to windows, I am afraid.

enter image description here

  • It is a great picture of the vent in question, could there be another one so we can what context it is used? Is the glass at the bottom of the picture, so it is upside down for example....
    – Jack
    Jan 12, 2019 at 17:44
  • The vent is situated at the very top of the exterior window frame's inside pane, facing the outside surface of the interior pane.
    – gnzg
    Jan 12, 2019 at 17:51
  • So this ventilation port allows air exchange between the interior air and the space between the two panes? Jan 12, 2019 at 17:56
  • @JimStewart, I would guess so, but does this not defeat the purpose of the air chamber between the panes?
    – gnzg
    Jan 12, 2019 at 17:58
  • 1
    It could be a trickle vent. Seems they are more common in Europe.
    – Jason
    Jan 12, 2019 at 22:22

1 Answer 1


Is the ventilation port going to a covered port on the outside wall? If so, the purpose would be to allow ventilation that rain could not get through. In the past it was important to have a route for outside fresh air to get into the living space because natural gas space heaters drew their combustion air from the living space and the combustion gasses were vented into the living space. The fresh air prevented the formation of carbon monoxide.

If the ventilation port is a route into the space between the two panes of the double pane window, then the purpose would be to dry out the space between the two panes. This would prevent fogging between the two panes.

EDIT Having lower humidity interior air flow through the space between the panes both keeps the window clear from fogging and heats the window. So even on a very cold day it should be comfortable to sit next to the window and not be chilled. This might slightly increase your heating demand, but this is an ingenious feature which shows the designers of your building really thought about making the apartments comfortable for the occupants.

And it could even decrease your heating demand as follows. Consider a person sitting by the window to read, do a crossword, or sew. If the window is cold, the person sitting by the window might turn up the heat in the whole apartment.

  • The second option seems legitimate, but wouldn't the trade-off here be no insulation during the cold months?
    – gnzg
    Jan 12, 2019 at 18:02
  • Having some lower humidity interior air flow through the space between the two panes will cost a little more in heating than if you could have a completely sealed and dry space between the two panes, but it is not possible to maintain a seal between the two panes even with modern double pane windows. In some cases the seal fails after little more than 10 years and the windows fog. I would think that there should be means to renew the dry gas charge between the panes, but most people just replace the windows! The designer of your windows has a implemented an ingenious feature to stop fogging. Jan 12, 2019 at 18:19
  • Is there an inlet vent on the bottom of the window? (It might be very inconspicuous and be without the control feature that the top one has.) The reason I ask about this is that I would expect that the air flow between the panes of the window would best be driven by convection--denser air near the floor would displace warmer and less dense air near the ceiling. Jan 12, 2019 at 20:49
  • 1
    in fact there is one, however I just now discovered it, as it was buried under layers and layers of laquer. Apparently whoever renovated the apartment did not bother to research why it was there to begin with...
    – gnzg
    Jun 6, 2021 at 20:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.