I live in a rented apartment and I am trying to find solutions to keep the heat in. The apartment has several big windows (the 'living room' has one wall that is 80% windows) and I have to run the heat a lot to keep it warm inside.
Do curtains really make a significant difference? I read through other posts, including about insulation film, but I'd like to avoid that if possible. If it is the best solution in your eyes though I'll take it into account.
Curtains or drapes can help insulate a room. One of the main methods is to reduce the amount of air exchange between a cold window and the rest of the room.
Air in a room is always trying to equalize itself. The cold air at a window mixes with the warm air in the rest of the room. If you trap the cold air behind a poor conductor (such as an airtight drape) the rest of the room air remains warmer.
Most cloth is a poor conductor (not good at transferring heat). This is a good thing. The effectiveness is largely based on how airtight the cloth is and how tightly it fits to the window on all sides - the top and floor as well as the lateral sides.
Many curtains are advertised as thermal. In general, they are fairly thick and have an air barrier to block flow through them. Fitting is as important as the material. In addition to going floor to ceiling, try to tightly fit the curtain to the walls on either side.
Further to the other answers, I found Evaluating Window Insulation for Cold Climates by Robbin Garber-Slaght and Colin Craven which appeared in the Journal of Green Building. They test various window treatments and attempt to quantify the improvement by using a two-dimensional heat transfer model. Based on this model, they report a non-trivial (17%) improvement over a bare window when using interior curtains. Their "curtain" for the test was a fleece blanket "firmly attached to the window head and side trim" and hanging over the sill, so not a standard curtain. You can read the paper to get all the details in context. They also mention that curtains used this way can be problematic for collecting water.
It sounds like you want a temporary solution to use while the weather is cold and you don't mind covering the windows if it will keep your apt warmer. Price is important as it is a rental.
The most improvement you can get for your money is to buy some 1 (or 2) inch thick Styrofoam sheets and cut it to fit your window tightly. This will give you far more insulation than curtains would as well as blocking airflow which is also a big source of heat loss. If you want to let some sunlight in, you don't have to cover all the windows with Styrofoam, you can leave one window uncovered.
Lowes calls it "Expanded Polystyrene Insulated Sheathing"
Curtains certainly do help as they block drafts. However, another option is a product called rope caulk. It comes in small strands wrapped on a roll and has the consistency of thick putty. It is non-damaging, and may easily be removed. By pulling a few strands at a time and tucking them into the seams around your window, you can help block drafts. This technique, in addition to curtains, can help make a room a little warmer.
One thing to be careful of: in my home, the radiators are underneath the windows. If you use a full length drape, you're not only blocking drafts, but you are also preventing heat from entering the room. We switched from full length drapes to a heavy "black out drape liner" which is thick enough to stop drafts, but ends above the radiator.
I find the best bet is the shrink wrap plastic kits, because on a sunny day the warmth from the sun is way more efficient than simply keeping cold air out (which the plastic does anyway). Unless of course the window is facing north, then the sun won't benefit you but you will still stop the draft from coming in. I installed them throughout my house and it made a huge difference, and my house was built in 97 so its not that old. All windows exchange heat.
I can't believe nobody has mentioned plastic window kits. As mentioned in another reply, drafts are typically the biggest problem. Yes, curtains will help, but they don't prevent drafts. The basic plastic sheeting kits that they sell at Wally or Target will do a lot more to keep you warm because they form a pretty decent seal. You tape them to the window and heat them with a hair dryer to stretch them taught. They are nearly invisble and they form a very good seal. Then put your curtains up after that.
I had leftover insulation blue board with silver backing and used this as an insert for my curtain. The curtain I have is doubled so in essence I used it as a pillow case and then stapled the bottom after inserting the board. WOW! What a difference with the draft I had.
We are currently re doing a trailer for winter use. From what I have read, you want to block the air coming in, keep the heat from going out and retain whatever sunlight heat you can, south facing windows will be your best friends.(Sun rises in the east...a quick way to tell which way is south.) What we will be doing in our trailer is this...Black velvet or cotton curtain material to face the outside...which will draw in the sun heat, and is a heavy material for insulation. We are going with the highest thread count so it's a good insulating material, also, we are doing two layers, the black on the outside facing side and a lighter color on the inside to brighten things up. (More layers mean more insulation.) Now the best part....we will be putting Velcro all around the windows, then we will be attaching Velcro to a heavy duty plastic wrap, similar to the wrap used in home renos to cover insulation.....it might be called vapor barrier, which we will then attach to the window for a close seal. Top it off with the curtains. This way as oppose to getting the material that seals to the window is better if you want to be able to open your window only....otherwise completely sealing the window is the best way to go...with curtains. Also, white and light colors repel the sun's heat, so if you are able to absorb the heat during the day, consider changing the outside color of your curtains. Cheers.
