The pillow trick works for me, flexible and not entirely sealing. Don't intend lighting a fire ever again! trouble is, I'll probably forget to remove it and suffocate Santa.....
Just read through the above posts but still unsure. I have 2 never used fireplaces in my 2 childrens bedrooms. Most mornings they wake up and complain that their throats are dry. We have trickle vents open in their bedroom windows but I'm assuming (perhaps wrongly) that all the moist air is escaping through the open unused chimneys. These fires will never be used and I was thinking of climbing on to the roof and closing the chimneys off at the pots (with a few drill holes for ventilation). Is this a good idea?
Why don't you test your hypothesis by getting a hygrometer to check the air humidity before climbing onto the roof!? Seems a bit premature to pin the blame for the sort throats on one specific possibility without more investigation.