How effective are the inside stretch plastic window covers?

Are you talking about those covers you cut to fit, adhere and use a blow dryer on to make taut? If so, they work very well. We have a 112 year old house that we are slowly replacing windows on, and the ones we have not replaced yet we use that film on, and it keeps out drafts well.

https://www.angieslist.com/articles/does-plastic-window-film-really-help-drafts.htm

The double glass windows are the best. You can also make a quilted window cover, to put over these windows. I made ours, and they save a lot of money blowing out into the snowy winters!! from not caring for them. At a material store, get a quilted material, then cover this with a pretty colored material, and hang this up over the window, under the drapes. Keep this closed all winter, or you can make the window quilts so you can lift them up, 1/2 way, and place hangers there, until you want to lower them for night time. Tho, in rooms that you do not use all day, lower the drapes, and save the heat loss. With the drapes over these, you will truly SAVE MONEY!!!! In the spring time, you can place more hangers on the window frame, or you can use the spring rods, to hold the quilt up while you are in the room. Best wishes, Judy.

I highly recommend them. We used them in an 80 year old house that still had the original windows. They kept out the draft.

PS. With the plastic window covers, you asked about, they may help, tho not as good as what I did share with you. The plastic must be lined with felt, and nailed in, and this depends on how many nail marks you want on your window frame. . . I personally do not like this. It will also make noise, as the winds blow, and you are trying to sleep. This area, will also be cold, and miserable. Take care, best wishes, J.