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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.

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Curtains can (I think) increase the convection if they are not going from floor to ceiling. – La Raison Jan 27 '13 at 19:52
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Yes, curtains make a big difference, especially if you have old/drafty windows. – Alex Feinman Jan 29 '13 at 2:15
    
You mean like tapestries? Yes. – Mazura Feb 24 at 5:07

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.

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Yes but do you have quantifiable results to support your hypothesis? My gut feeling is that curtains still let air circulate, therefore you still lose most of the heat saving... – rogerdpack Feb 23 at 18:00
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@rogerdpack The US Dept of Energy reports that careful use of drapes can reduce heat loss by 25%. See this website. – bib Feb 23 at 19:55
    
OK that link mentions "typical" drapes reducing head loss by 10%. If your drape is carefully attached/velcroed to the sides and doesn't have an escape at the top and bottom, then "up to 25%" thank you for the link. – rogerdpack Feb 23 at 23:16
    
@rogerdpack When you sleep at night, do you use a comforter and sheets made from fiberglass insulation? Or are they made of cloth (e.g. cotton) like curtains and drapes are? Does sleeping without any layers make you warmer or colder? – Snowman Feb 24 at 23:13
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@rogerdpack fair point, a sheet in front of the glass by itself is not effective unless the air gaps are closed. – Snowman Feb 25 at 5:23

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.

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The greatest reduction in heat transfer was with the interior foam shutters, which is what I proposed. – Philip Ngai Jan 30 '13 at 21:11

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"

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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.

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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.

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I love those heat shrink storm windows! Best feature: when you're done installing, you've spent an hour running a hair drier in the room, so it feels much warmer and you feel good about what you've done. – Daniel Griscom Jan 20 at 3:03
    
Or bubble wrap attached/sealed at the sides, I've heard that's good :) – rogerdpack Feb 23 at 23:18

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.

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@rogerdpack - this post Feb 22 2013, user48073's post, Jan 20 2016. – AndyT Feb 24 at 11:12

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.

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Interesting, but a bit hard to visualize. Would you be able to post a picture or diagram showing just what you did? – Daniel Griscom Feb 23 at 17:51

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