Hi, tld. You can buy a plastic flapper valve at home centers that fits you dryer vent line. The stated purpose of these is to route the hot air from the dryer into the basement or whereever. The problem is that this air is also very humid and this can cause problems. By installing the valve backwards in the line, you should be able to block the cold air from coming in. You would have to remember to open it every time you start the dryer and close it when the load is done. It also will collect lint pretty easily and this can be a definate fire hazard. You will have to be the best judge of whether you and other members of your family will remember to open and close it and your feelings about the fire potential.