Exhaust and rangehood fans are designed to remove excess moisture and heat. If your fan is not self-closing, however, heated air can escape, even when the fan isn't being used. This can be a problem, especially in winter when we want to retain heat in our homes.
While many exhaust fans have louvres that shut when the fan is turned off, if your fan doesn't self-close you have two options:
You might like to take a picture of the existing fan to your retail outlet, to ensure that the cover you purchase is suitable.
It is possible to install an exhaust fan cover with a self-closing damper over your existing ceiling exhaust fan. The air pressure created by the fan opens the damper when the exhaust fan is operating, the damper closes when the exhaust fan is switched off.
Install insulation in your ceiling, walls and floors to create a sealed envelope which acts like a thermos in winter and an esky in summer.