Double-glazing film - any use?

In our previous house we fitted Perspex sheets on magnetic strips over single-glazed windows that we then removed and set aside so we could open the windows in the summer. Thought I would try the same thing on one window that we never open in an unheated small room using Stormguard ‘Double Glazing Film’ (purchased from Homebase). Easy to fit and the room feels warmer but sadly my MIN MAX thermometer does not record any significant increase in the room’s temperature. Fitted some of the film also on the single-glazed panel above the inner front door, and over the back door. Cost – about £10.