insulation
My apartment bedroom window has a thin vertical strip along the right side that allows warm air from the room to escape. Even with the bedroom thermostat set to 80 degrees, the room hasn't gone above 70, mostly due to this.
You can clearly hear that it is not sealed, it sounds as if the window is open an inch or two, but it is actually closed and locked.
How can I seal this strip to prevent warm air from leaking out? It is a plastic material.
You put the vapor barrier on the inside of insulation to prevent cold air from getting near the vapor barrier. If warm, humid air is touching a cold surface, the water will condense out, just like you see on a glass of ice water on a hot, humid day. This condensation is a bad thing, since the presence of water over a long period of time will cause mold to grow. The moisture may reduce the effectiveness of your insulation. It may cause dry rot in your home. It may cause other problems, attracting bugs who like moist places.
How can you fix this? Turning the insulation to lie in the proper orientation is clearly the best solution.
I suppose one could put an additional layer of vapor barrier on the inside. That will stop the inside, moisture laden air from getting to that middle layer. The problem is, this will create a layer of insulation that does not breath. Any moisture in there will never escape, and you will have the same problems as I mentioned above. So this could potentially be a bad solution.
Better seems to add more insulation to the outside, with NO vapor barrier on it. That will prevent the cold air from getting near the vapor barrier. It may not be possible, or even terribly easy. But this is why you try to do things the right way the first time.
For a quick and dirty fix, I have had some success with a Register Booster Fan.
This has alleviated most of the problem in my house. I still plan to install more insulation (and upgrade to a multi-zone system), but this is a lot less work up front. You can use it as a stop gap until you figure out what permanent fixes you need to make.
Best Answer
If you rent you can always ask the apartment manger the reason for the vent and if it can be closed. Also look on the side of the window to see if there is any switches, levers or slides to open/close it. If you don't own, you can always just cover it with 2 inch wide masking tape which will drastically cut down on the airflow and be easy enough to remove at the end of the lease.