insulationwindows
Our windows in the back of our house receive a lot of wind. When it's windy or very cold, the air is coming in through various crevices: window track and space between window. The windows are about 7 years old and the fuzzy insulation looks okay. I've tried to install insulation, but it didn't seem to resolve it.
The pictures you added, were edited with info to help you decide which way you want to go.
There are 3 options to choose
1) Out right replacement of the whole window sash and all.
This is the most expensive way to go about it.
Its benefits are the ability to check the condition of the framing. Windows leak over time and unseen damage will be hidden.
The other considerations are you may need to remove your siding and reset some of it to get you new windows in with their preapplied trim in place. Then again, it is a newer style of window versus sash weights, and the trim or nailing flange will fit very nicely. There is a chance you will need finish it off by covering it with a PVC brick mold.
2) Renovation kit with sash and jamb. These fit a new jamb with sash directly into the existing jamb, saving the existing trim and labor removing the jamb and trim. You only need to remove the sash and track.
3) Use a jamb reliner kit
Air leakage is the primary reason folks apply window film in the winter. By sealing against airflow through the unit as a whole, the volume of air moving through those gaps is dramatically reduced. Obviously this isn't a great solution for the cooling season.
Otherwise, modern window sashes are sealed either with vinyl tubing or brush seals. You could saw slots into your sashes and install them that way.
That may not be a practical solution in a rental home. Various types of sash seals are available on the market. Most involve a flange and a vinyl tube that's installed against the sash. Because of your flush frame situation, this could be a bit challenging. Most are designed to fasten to the protruding jamb. I suggest a trip to your home improvement store for a selection.
Best Answer
Looking at your pic and arrows, it looks like the bottom sash does not fit well. If all the factory weather seals are in place, not damaged or worn, I think they may be poor quality. That is rare since most windows in the last 10 years or so are normally fairly tight. The area that really concerns me are the sides where you show the arrow. This area usually depends on a snug mechanical fit, not weather stripping to seal the unit. Does this lower unit seem loose? Leaking at the joint of the two sashes is common and can be fixed with new seals most of the time, but the sides do not. Also, some windows need to be locked for the sash to sash seal to fully engage.
Caulking is not a good way to seal these kind of leaks if you ever want to open them again!
Maybe you can more fully describe the areas in question and the fit of the windows.