I lived in an old apartment building with horribly drafty single pane windows. They were all 3x5 except the kitchen window over the sink. Using the Duck Brand window shrink film kit saved me a LOT of money on my winter utility bills. I lived in Eastern NC at the time. My electric bill would hit $300 for a 750 square foot apartment in the winter time, not to mention there was no wall-to-wall carpeting. The landlord was cheap and so they used stone/ceramic tiles in all rooms except the bedrooms.
There were two bedrooms and they had linoleum flooring. I lived in the bottom of two floors and so my heat would escape through the floor and above into the apt above me, as well as out the windows. The $300 bill was before the shrink film was put up. There was also no central a/c. I had a window unit which allowed more escape of air. I took care to completely close off the a/c unit and cover the entire window with shrink film.
My bills for winter time never went above $100 after that. It was only me and my toddler daughter living there.
I'm in another old apartment in mid-state SC. My a/c has been cranking all summer because I have large bay windows and the building is from 1949. I can feel hot air coming in around the sill. I ordered a window kit today to apply because my last bill ending 8/31 was $117. There is no way I can afford these high costs and also it will get far worse in winter.
I left my window film up year round in my old apt and I'm telling you, it saved me hundreds. Like I said, $100 bill total or less in winter (Nov-Mar) and $65 or less in spring and summer. Here we have to run a/c until mid October in SC as it is still too hot. My windows don't open in this apt but I do have a back door so I can open that to swap out air.
In short I'm saying you can leave it up year-round. I did until I moved and I just moved here to this other place but once this film goes up this weekend, it will stay up until I move out. I tried to get my neighbor at my last building in NC to do her windows but she was too lazy. I even offered to do them for her because she kept complaining about the $300+ electric bill. My parents live in a 2536 square foot home and their bill isn't even $300 in winter or summer! No way I'm going to pay that for a 950 square foot place.
Put carpeting down if you can, or at least area rugs, which I did at my old place with the ceramic tile. In winter that tile was like a block of ice under bare feet. And make sure the weather stripping around the doors is good. I can't do anything about the walls or the attic, as I don't own this place (though the landlord offered to sell me this condo)...not sure I want such a money pit, though it is nicely remodeled inside. These things I can tell you truly help.
Put up the window film. Replace the film as necessary. I promise you it works year-round. I'm cold-natured, and so I notice a huge difference in winter and also summer...my a/c would be on every 5 minutes and at times it wouldn't ever cut off at 74 degrees. My condo is in full sun and facing the south.
I keep the a/c thermostat at 74 at night in summer unless it's 95 at night. Then I will put it on 73. In winter I usually keep it at 73 all the time unless I leave on vacation. Then I put it on 65. I can't wait to move out of this place and be done with apts and condos for good.
They are such rip-offs with the cheap flooring, and old sub-par or cheap windows and insulation. I literally lived in a $1200 a month apt in a gated community once, and one of the windows had a huge gaping area along one sill. How they charge that kind of money and skimp on materials is beyond me. I will never willingly live in anything but a brick and mortar singke family home after my lease is up here.
I use light-blocking and energy saving curtains and blinds on all windows. There are only 3 in this condo, but 2 large bay windows (65 inches wide) are facing the sun. If I didn't have the blinds and curtains up, the sun will blind you by midday. I keep the curtains and blinds closed all the time in summer and I open them in winter to allow natural sunlight to warm the rooms. Buy white vinyl or bamboo blinds. White reflects light and heat.
These curtains, blinds, and shrink film should add up to about a 50% savings if not more. I definitely saved 30-60% on bills at my last place.
I take care to cut the film large enough to fit over the molding (about 1" from the outer edge before it meets the wall), and I do use the blow dryer to reduce the plastic whipping in the wind when strong drafts come through. I also place it beneath the blinds so I can open them if needed.