We've been ones to seal our home due to on-and-off year-round allergies and didn't realize that our doing this caused CO2 levels to get quite high (we now use a CO2 monitor). Now when it's cool enough we can leave a couple of windows open to get fresh air into the house with minimal allergy symptoms. An added bonus is the privacy from its design. Bought 5 - haven't used them long and seldom all five at a time - but it's useful to have them near the windows they're for. One was a bit warped but easily straightened by hand. Two are for side-open windows. It's best to hold it open to its fullest as you pull the window shut on it. The round gaskets and friction keep it open in the vertical orientation. Obviously there's less breeze through these than standard screens, but opening two or more windows encourages air flow. For more rapid airing out of the home, leave a fan on in front of one window, either aimed toward the window to pull air from another window through the home, or away from it to bring air in from that window and push it out another. Bonus: Mission Allergy had a very nice price when I purchased them!